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This is to certify that the

thesis entitled

SHELF LIFE OF FREEZE DRIED Y(X§URT

presented by

SHARONS. MINNIE

has been accepted towards fulfillment
of the requirements for

M.S. degree in PACKAGING

 

 

Woe/W

Bruce R. Harte, Ph.D.

 

Major professor

Date 4/28/87

 

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SHELF LIFE OF FREEZE DRIED YOGURT

BY

Sharon Minnie

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

School of Packaging

1987

ABSTRACT

SHELF LIFE OF FREEZE DRIED YOGURT

BY

Sharon Minnie

This study investigates the shelf life of freeze dried
yogurt. The freeze drying process has gained popularity in
recent years because of minimal structural change in the
food system. Low-fat fresh yogurt was freeze dried and
packaged in pouches. Three differnt materials, Low Density
Polyethylene, Laminate (PET/Al/PP), and Kraft paper were
evaluated. The shelf life studies were done at both ambient
conditions (70 F, 50% RH) and accelerated conditions (100 F,
85% RH). Analytical methods used to monitor the quality of
yogurt were culture viability, package head space analysis,
color change and change in pH. Also sensory analysis was
done to compare flavor changes between fresh yogurt and
reconstituted yogurt after storage. Results indicate that a
good quality freeze dried yogurt powder with viable
organisms can be obtained. The laminate pouch showed the

best results for extended shelf life of freeze dried yogurt.

ACKNOWLEDGEMENTS

I would like to thank my advisor, Dr. Bruce Harte,
Associate Professor, School of Packaging, for his help and
guidance throughout my graduate program. Appreciation is
also extended to my committee members, Dr. Mike Richmond,
Kraft Foods Inc. and Dr. Charles M. Stine, Department of
Food Science and Human Nutrition for their continuing
support. Financial assistance during my program provided
by Dr. C. J. Mackson and Mrs. E. Anderson, School of

Packaging is acknowledged.

Finally, I would like to thank Paul Singh, a very special
friend for all his assistance and support throughout my

graduate studies.

ii

TABLE OF CONTENTS

Page
LIST OF TABLES
LIST OF FIGURES
1.0 INTRODUCTION 1
2.0 LITERATURE REVIEW 3
2.1 Yogurt 3
2.1.1 History
2.1.2 Yogurt Trends
2.1.3 Packaging Trends
2.2 Freeze Dehydration 13
2.2.1 Product Quality
2.2.2 Re-hydration
2.3 Color Analysis 19
2.4 Deteriorative Reactions 20
2.4.1 Bacteria
2.4.2 Non-Enzymatic Browning
2.4.3 Rancidity
2.4.4 Staleness
2.4.5 Oxidation
2.4.6 Role of Moisture and
Storage Temperature
2.5 Culture Viability 28
2.6 Packaging Dried Products 34
2.6.1 Vacuum Packaging
2.6.2 Gas Packaging
2.6.3 Polyethylene
2.6.4 Aluminum Foil
2.6.5 Polyester
2.6.6 Polypropylene
2.7 Yogurt Products and Uses 40

2.7.1 Freeze Drying

iii

3.0 MATERIAL AND METHODS 46

3.1 Yogurt Preparation 46
3.1.1 Bulk Starter Culture
3.1.2 Uncultured Mix
3.1.3 Homogenization
3.1.4 Yogurt Mix
3.1.5 Freeze Dehydration

3.2 Packaging 49
3.2.1 Dried Yogurt

3.3 Sampling 51
3.3.1 Fresh Yogurt
3.3.2 Dried Yogurt
3.3.3 Stored Dried Yogurt

3.4 Material Permeability Tests 54
3.4.1 Water Vapor Transmission Rate (WVTR)
3.4.2 Oxygen Permeability
3.4.3 Carbon Dioxide Permeability

5.0 CONCLUSIONS

BIBLIOGRAPHY

APPENDICES

APPENDIX A

APPENDIX B

APPENDIX C
APPENDIX D

APPENDIX E

56
Culture Viability
Headspace Gas
Color Analysis
pH
Moisture Content
Flavor
72
74
Low Fat Yogurt Formulation 81
Milk Standardization for Low-fat 82
Yogurt
Flavor Scoring Guide for Yogurt 83

Composition of Lee Agar and Plating 84

Reconstitution of Dried Yogurt 85
Techniques

iv

APPENDIX F

APPENDIX G

APPENDIX H

APPENDIX I

Yogurt Score Card
Water Vapor Permeability Constant
Permeability Constant for Oxygen

Permeability Constant for Carbon
Dioxide

86
87
88
89

Table

10

11

LIST OF TABLES

Sales of Yogurt

Growth of the Yogurt Market

Yogurt Production by Container Size
Yogurt Sales by Package Size

Average Values of S; thermophilus and
‘E; bul aricus present in Fresh Yogurt,
InitiaI Freeze Dried and Packaged

Freeze Dried Yogurt Plated on Lee Agar

 

Average Values of Gas Present in the
Package Head Space of Freeze Dried
Yogurt Stored at Room and Accelerated
Conditions

Average Hunter Color Cell Values of
Fresh Yogurt, Initial Freeze Dried and
Dried Yogurt stored in Polyethylene and
Laminate Pouches

pH of Fresh Yogurt, Initial Freeze Dried
and Reconstituted Yogurt

Average Values of Moisture Content for
Freeze Dried Yogurt

Permeability Constants for WVTR, Oxygen
and Carbon-dioxide Transmission of LDPE

Flavor Ratings of Reconstituted Freeze
Dried Yogurt Based on the Collegiate
Dairy Products Scorecard for Swiss Style
Yogurt

vi

Page

11
12
57

59

61

65

66

68

70

Figure

LIST OF FIGURES

Phase Diagram for Water

L a b System for Color Description
and Qualification

Flow Diagram for Yogurt Preparation

Flow Diagram for Yogurt Dehydration
and Packaging

Whiteness Index for Yogurt

Moisture Content for Freeze Dried
Yogurt

vii

Page

15
21

47
50

63
67

1.0 INTRODUCTION

Since its introduction in the United States less than
fifty years ago, yogurt consumption has increased
immensely. Per capita consumption of yogurt has increased
from .90 lbs in 1974 to 2.7 lbs in 1980 (Mullins, 1983).
Much of the success of yogurt can be attributed to an
overall concern with diet. Forty percent of yogurt is
consumed as a snack and another twenty percent each,
account for breakfast and lunch (Anonymous, 1984). In
addition, innovative packaging has complemented yogurt and
yogurt products. The role of plastic packaging in recent
years has been favored over paper and further enhanced
yogurt consumption (Keck, 1983). Multi-color

graphics and package design are reasons for this

dominance.

Freeze drying, a novel dehydration technique, developed
in the mid forties, is gaining popularity in food
preservation because the process involves a minimal
structural change in the food (Anonymous, 1980). At
present freeze drying has become a standard practice in
the food industry for the preservation of fruit juices,
instant coffee, eggs, meat, mushrooms and many other
products (Zaidi et-al, 1979).

The purpose of this investigation was to evaluate the

influence of different packaging systems on the shelf life

of freeze dried yogurt. The quality of reconstituted
freeze dried yogurt was evaluated using both sensory and
analytical tests. Analytical methods included: culture
viability, package head space analysis, color change and
change in pH. Sensory analysis was done to compare
flavor change between fresh yogurt and reconstituted
packaged freeze dried yogurt subjected to storage at room
conditions (72 F, 50% RH) and accelerated conditions (100

F, 85% RH).

2.0 LITERATURE REVIEW

2.1 Yogurt
Yogurt is a coagulated milk product obtained by lactic

acid fermentation of milk by Lactobacillus

 

bulgaricus and Streptococcus thermophilus.

 

 

The micro-organisms in the final product must be viable
and abundant (Tamime & Deeth, 1977). Yogurt is a
relatively tart, high acid product with a characteristic
flavor different from other fermented milk products
(Vedamuthu, 1976). Sellers (1978) defines yogurt as a
fermented milk with a custard-like consistency. This
semi-solid characteristic differentiates yogurt from
other fermented milks. Reed (1981) described yogurt as
having a smooth, shiny appearance and a fresh, sour
aromatic flavor. Definitions for yogurt are many and
remain open and sometimes controversial. What cannot be

disputed is yogurt's obvious popularity.

Recognized by various spellings and names, yogurt is known
around the world (DeHaast et-al, 1979). Not just an
established dairy product, yogurt is a perfect alternative

to junk and snack foods (Helferich and Westhoff, 1980).

2.1.1 History:
The fermented milk product yogurt as it is called in the
United States, derives its name from the Turkish word

jugurt (Tamime and Deeth, 1980).

Although there is no precise record of its introduction,
yogurt is recognized as a product of one of the oldest
methods of food preservation (Anonymous, 1984). Many

have called yogurt the most famous food product known to
man for over 4500 years (Steinberg, 1982). The actual
discovery of yogurt is unclear but many theories have

been suggested and all areas of the world lay claim to

its origin. In actuality the discovery of yogurt (as with
most fermented foods), was probably accidental since the
phenomenon of bacterial fermentation was not realized nor
understood at the time of discovery. The earliest form of
yogurt was accepted because unlike milk, it could be stored
and consumed for several days. The earliest scientific
knowledge of yogurt and lactic acid bacteria was around 1900
(Helferich and Westhoff, 1980). It wasn't until 1939 that
yogurt was introduced into the United States from France.
Plain yogurt was the only flavor produced. In the late
forties, fruit preserves were added on the bottom (sundae

style). Twelve flavors including plain were now available.

Around 1951 a fluid yogurt was introduced that was
initially quite promising. Sales of regular yogurt soon
overpowered the liquid product due in part to the
introduction of Swiss Style (fruit suspended throughout),
which entered the market a few years later. It is quite
possible that in 1951 liquid yogurt was ahead of its time

(Steinberg, 1979). In 1960 frozen yogurt was introduced

in 4 oz. cups and was readily accepted. Demand
continued to grow as frozen yogurt was sold in cones and
marketed as a soft serve treat. In addition, a variety of

frozen yogurt novelties entered the market place.

Yogurt and yogurt products have sparked new interest in
the dairy industry. Yogurt drinks, bars and even yogurt
liqueurs have been introduced in the past few years.
Still, the market for yogurt and yogurt products is
merely a fraction of what is seen in European countries.
The eighties should provide ample opportunity for the
yogurt market to grow, both in sales and in the

development of new products and uses for yogurt.

2.1.2 Yogurt Trends:

Yogurt which was somewhat of a wallflower food category
several decades ago, has been steadily blossoming during
the past twenty years (Przybyla, 1983). Yogurt, although
the smallest segment of the dairy industry is growing at
the fastest rate (Mullins, 1983). Continuing the growth
pattern of the past decade yogurt sales are expected to
rise at a rate more than twice the U.S. inflation rate
throughout the rest of the 80's and 90's (Anonymous,
1982). Annual sales growth of yogurt was greater than 16%
during the 1960's, more than 14% during the 1970's and
real sales growth of yogurt products is expected to be 8%
throughout the 1980's (Anonymous, 1982). Yogurt sales

since 1970 are shown in Table 1.

TABLE 1: Sales of Yogurt

Per Capita
Yogurt Sales Annual Yogurt Sales
($mils) % Change (Dollars)

1970 43.4 0.22
1971 61.2 41.1 0.30
1972 75.3 23.1 0.37
1973 92.2 22.4 0.44
1974 110.3 19.6 0.53
1975 150.1 36.1 0.71
1976 189.7 26.4 0.89
1977 215.9 13.8 1.01
1978 247.6 14.7 1.14
1979 278.6 12.5 1.27
1980 316.5 13.6 1.39
1981 362.7 14.6 1.63
1982 402.6 11.0 1.78
1992F 1,307.2 12.5 * 5.22

F - forecast by Business Trend Analysis
* - compound annual growth

Source: Dairy Field, 165(12):26,30

Yogurt consumption in the U.S. is now more than 2.7 lbs.
per capita and has increased about 5% a year, which is
higher than the average for all food products (Anonymous,
1985). The $622 million 1984 market will grow 22% by

1988 reaching $757 million (Anonymous, 1985). By 1990
total sales should reach $1.4 billion (Horwich, 1983).
Others predict total sales volume will be slightly lower at
$1.3 billion with 1.3 billion pounds being consumed by 1990
(Anonymous, 1982). Growth of the yogurt market from the

1960's to 1990 estimates are outlined in Table 2.

Enhanced by its image as a health food, yogurt should be
one of the top selling dairy products in 1988 (Anonymous,
1985). Yogurt consumption has been helped by the fitness
trend and is now a symbol of todays health conscious
consumer (Anonymous, 1985). As popular as yogurt has
become across the United States, per-capita consumption in
Europe is thirty times as high. There clearly is plenty of
room for total market expansion (Banner, 1985). Yogurt
acceptance in different forms presents a real opportunity
for the dairy industry to boost consumption by catering to

unfulfilled market segments (Steinberg, 1983).

As manufacturers create line extension and develop new
products with yogurt they should also concentrate on new
outlets for its use. The food service industry is
virtually an untapped market. Restaurants, fast food

chains and vending machines are possible selling places.

TABLE 2: Growth of the Yogurt Market

Yogurt Sales Annual Per Capita Sales

(Mil. lbs.) % Change (lbs.)
1960 44.1 -- .26
1961 49.0 11.1 .28
1962 45.0 -8.2 .25
1963 50.4 12.0 .28
1964 53.0 5.2 .29
1965 60.6 14.3 .32
1966 70.0 15.5 .37
1967 90.2 28.9 .47
1968 123.5 36.9 .63
1969 169.0 36.8 .85
1970 171.6 1.5 .86
1971 236.3 37.7 1.16
1972 284.0 20.2 1.38
1973 317.9 11.9 1.53
1974 343.2 8.0 1.64
1975 445.4 29.8 2.11
1976 481.7 8.1 2.27
1977 533.5 10.8 2.49
1978 565.4 6.0 2.61
1979 566.7 0.1 2.59
1980 588.8 3.9 2.67
1981 635.9 8.0 2.86
1982 683.6 7.5 3.02
1992F 1,223.6 6.0 4.89

F - forecast by Business Trend Analysis
* - compound annual growth

Source: Dairy Field, 165(12):26,30

The potential marketing of yogurt to the food service
industry is great. It can be used as a light protein
source, as an additive or component in ethnic foods, in
garnishes and dips or as an element in salads or salad bars
(Anonymous, 1983). The use of yogurt as a cooking
ingredient or as a food additive is still relatively
undeveloped. The challenge is to appeal to the non-users
by creating new and exciting forms of yogurt as well as
developing new uses for yogurt. Yogurt is cultured dairy
products' glamorous star and the opportunities are great

(Krier, 1985).

2.1.3 Packaging Trends:

Yogurt packaging has gone through many changes since
neolithic times when it was first discovered in goat skin
bags and later fermented in clay pots (Anonymous, 1983).

In the 1930's yogurt was a staple food product throughout
most of Europe. Yogurt was sold in Europe in small
crockery pots or jars (Steinberg, 1979). A sheet of
printed cellophane covered each container, held in place by
a rubber band. Jars held as little as 90 grams (3.5 oz.)
and as much as 225 grams (8 oz.) of yogurt. In Switzerland
during the same time period yogurt was sold in glass jars

with metallic foil covers.

When yogurt was introduced to the United States it was sold
in thick returnable glass jars that held 8 oz. In the late
forties waxed paper cups were used. Today nearly 90% of

yogurt packaging is in plastic containers (Dryer, 1985).

10

Briston (1980) claims growth of the yogurt market owes a
great deal to the plastic container. Slack resistant
polystyrene containers and TFFS (thermoform, fill, seal)
systems with peelable foil-laminate lid stock are popular
yogurt packages (Harte et-al, 1984). Spin welded
containers, which allow shipment of bottoms and tops

separately are increasing in popularity.

Yogurt packages exist in a variety of sizes. Multi—packs
which are making a strong showing in the marketplace
represent less than 5% of total yogurt container
production (Table 3). The eight-ounce cup remains the
favorite with the six-ounce size a popular second. In

Table 4 yogurt sales by package size are presented.

Many companies now realize the importance that packaging
plays in marketing. Creative packaging and vivid graphics
have become a marketing tool that yogurt manufacturers use
to secure shelf space and attract consumer attention. Keck
(1983) stresses that the package shape, the artwork or
graphic design and the use of color are areas to
concentrate on when upgrading the container for better

visibility and efficiency.

Innovative yogurt containers are very familiar items in the
dairy case (Keck, 1983). Manufacturers strive for
differentiation and identity through packaging. Future
changes in yogurt packaging may include tamper evident

containers and an increase in aseptics. Liquid yogurt is

TABLE 3: Yogurt Production by Container Size

% of Total

Size Production
8 oz. 51
6 oz. 33
32 oz. (2 1b.) 11
16 oz. (1 lb.) 3
18 oz. 1
Other * 2

* Includes institutional and multi-pack sizes.

Source: Dryer, J. 1985. Dairy Record

11

TABLE 4: Yogurt Sales by Package Size

Percentage of Sales Package Size
84.4 8 oz.
9.6 32 oz.
4.4 64 oz.
1.4 16 oz.
0.1 5 oz.
0 1 5 lb.

Source: Dryer, J. 1985. Dairy Record.

12

13

most suited for aseptic containers but recently General
Nutrition Centers introduced "heat—processed yogurt after
culturing" in aseptic containers (Anonymous, 1986).
Tetra—King is also promoting a package they feel is ideal
for liquid and semi liquid yogurt. Product is filled into
polystyrene containers made from roll-stock in a continuous
process. If the product is sensitive to light and oxygen
polystyrene can be coated with aluminum foil. Packaging
has made a substantial contribution to the dairy industry.
Yogurt is helping to improve the packaging for dairy
products by its use of sophisticated shapes and decorating

techniques (Keck, 1983).

2.2 Freeze Dehydration
Freeze dehydration is the process by which water molecules
are removed from a solid product to the vapor phase without
going through the liquid state, known as sublimation.
Freeze drying consists (Hall, 1966) of
a. Freezing the product

b. Supplying heat so moisture is removed without
passing through the liquid phase, by,

c. Maintaining a vacuum in the vaporizing chamber.

Ice in the product is vaporized by the addition of heat to
the shelves of the drying unit, while refrigerated coils
condense the vapor which is held under vacuum. Sublimation
can only be accomplished when the vapor pressure and
temperature of the substance being dried is held below the

triple point temperature of the water or aqueous solution

14

in the product (King, 1970). In Figure 1 is shown the

phase diagram for water.

Heat is applied to the product under controlled conditions.
Key components of the freeze drying system are the vacuum
pump and condenser. The energy needed to sublime the water
may be provided by heating fluid through coils beneath
trays on which frozen product has been placed (Lingle,
1986). During the beginning of the drying process,
sublimation of the ice occurs at the surface. As drying
proceeds the ice Zone withdraws into the center of the
product and the evolving vapor must be conducted through

the previously dried outer layers (Anonymous, 1984).

An important step in freeze dehydration is pre-freezing.
The crystalline structure, solubility, activity, viability
and color of the product are influenced by the freezing
process (Anonymous, 1984). The freezing phase is of
extreme importance since it determines the success of the
drying process and quality of the dried product (Anonymous,
1984). Generally, rapid pre-freezing results in smaller
ice crystals, slower drying rates and a final product of

overall better quality (Mellor, 1978 and Karel, 1975).

The water present in products prior to freezing exists as
free and bound. During pre-freezing ice will easily
crystallize out of free water. Any remaining free water is
usually eliminated by sublimation during primary drying

(Mellor, 1978).

Water vapour pressure [mbar]

- 5
10«100

'80

, submnguon

-60 ‘-40

FIGURE 1:

15

-20 0 20 40

Temperature [°C]

Phase Diagram for Water

60

80

 

100

16

Primary drying continues while there are still areas in the
product containing ice which can be sublimed (Anonymous,
1984). The ice crystals formed by freezing are sublimed by
vigorous and then gentle heating under vacuum. Bound water
which may be linked to weak hydrogen bonds is evaporated
during secondary drying. Secondary drying begins with the
disappearance of the ice crystals in the product. Moisture
in the product will then be desorbed under high vacuum and
at a temperature that is safe for the product being dried.
Bound water linked by the much stronger electrostatic
forces will remain in the product, resulting in a moisture
content of at least 1 percent after freeze drying. A third
of the total drying time needed to evaporate the majority

of bound water is during secondary drying (Mellor, 1978).

2.2.1 Product Quality

Freeze drying for food preservation produces the highest
quality product obtained by any drying method (King, 1970).
Freeze drying is used for foods more than any other product
category (Litchfield, 1981). Because of low processing
temperatures, near absence of liquid water and minimal
thermal degradation, reactions such as nonenzymatic
(Maillard) browning, protein denaturation, lipid

autoxidation and other enzymatic reactions are minimized.

Freeze dried products suffer little or no thermal
degradation because of low drying temperatures (less than
130 F, 54.4 C). Freeze drying as compared to other drying

methods may offer better preservation of vitamins and other

17

nutrients (King, 1970). It can also preserve the
appearance and texture of many food products (Zaide et-al,

1979).

Most freeze dried products have good retention of volatile
flavor and aroma compounds. Volatiles are generally lost
during the early stages of drying. (Mellor, 1978).
Generally, freeze dried products display outstanding
retention of flavor, color, nutrient and aroma attributes
and offer rapid product re-hydration (Lingle, 1986).
Packaging is important in maintaining the quality of freeze
dried products. Vacuum packages or packages containing
protective gas atmospheres which are impermeable to oxygen
and moisture will maintain a shelf stable product for
extended time periods. Flexible pouches which are often
used for freeze dried food products are light weight and
take up little volume during shipping, handling and

storage.

Currently only a limited number of products can be
economically produced by freeze dehydration (Litchfield,
1981). Although products are usually high quality, it is by
far the most costly of all dehydrating techniques (Mans,
1985). The freeze dehydration process requires specialized
equipment, is very energy intensive and involves long

drying times (Liapis, 1979 and Litchfield, 1982). It may
require 12 to 30 hours vacuum drying time to obtain a
moisture level of 2 % in the product. Lipid oxidation

because of a large internal surface area and low moisture

18
contents can be a problem in freeze dried foods, as in other
dehydrated foods. Complete oxygen removal as well as
packaging in an oxygen and moisture impermeable container
are needed to insure storage stability. Other deteriorative
reactions associated with unstable pre-freezing conditions
can cause color and texture changes, off flavors, and
nutrient losses. If proteins are denatured during drying
the texture can again be affected, resulting in poor
re-hydration (King, 1970).
Products which have been successfully freeze dried are
coffee, strawberries, cooked shrimp, soups, mushrooms,
cooked beef, peas ice cream, cultured cream and natural
cheese. Much research and process understanding is needed
to develop uniform products. With better processing
techniques and energy utilization it may be possible to
overcome the high economic factors.
2.2.2 Re-hydration:
Re-hydration may be considered the last stage of the freeze
drying process. For freeze dried foods, re-hydration rate
is a good measure of the products quality (Mellor, 1978).
Product quality is related to the structure rigidity at the
surface where sublimation occurs. This rigidity prevents
collapse of the solid matrix which remains after drying. A
porous non-shrunken structure is left, which readily
re-hydrates (King, 1970 and Liapis, 1979 and Zaidi, 1979).
To determine product damage encountered during drying the
re-hydration ratio can be employed. This is the ratio of
the weight of water regained to the weight of water lost

(King, 1970).

2.3 Color Analysis
Changes in color of freeze dried products can be a result
of poor prefreezing techniques. Deteriorative reactions
will also alter the color of freeze dried products, as seen
with browning. Generally, browning is associated with off
odors and flavors which render the product unacceptable.
Monitoring color changes is important, since it is a
visible sign that represents an important quality attribute

to the consumer (Szczesniak, 1983).

Sensory perception of color by the eye is responsive to a
tri-stimulus band of wavelengths corresponding to red,
green, and blue. Numerical representations are denoted as
X, Y and Z which are international standards of the
International Committee on Illumination (CIE).
Spectrophotometers such as the Hunter color cell use the
color scales L, a and b to indicate different degrees of
color. Values can be converted to the international
standard by means of established equations.

- 'L' defines lightness, 0 for black to 100 for
perfect white.

- 'a' measures redness when positive, gray when 0 and
green when negative.

- 'b' indicates yellowness when values are positive,
gray when 0 and blue when negative.

The values obtained from a spectrophotometer correspond to
the observers perception of color after the responses of
the eye have been processed by the optic nerve from the
brain (Hunterlab Associates Laboratory, 1976). The optical

sensors in a spectrophotometer respond to the different

19

20

wavelengths of light similar to the three color responses
of the eye. In Figure 2 is shown how the three dimensional
coordinate system of L, a and b values relate to perceived

color.

2.4 Deteriorative Reactions

Dried products with a moisture content below the point
where microbial growth is inhibited may still deteriorate
by chemical, bio-chemical and physical means (McWeeny,
1980). Low-fat yogurt initially frozen and then freeze
dried can undergo deteriorative reactions. The majority of
problems related to dried foods can be prevented with
proper handling, packaging and control of storage

conditions.

2.4.1 Bacteria:

Powders with a moisture content of 5% or lower provide a
poor medium for bacterial growth. If stored properly and
protected from moisture, dry foods will not support

proliferation of microorganisms (Gravini, 1985).

2.4.2 Non-Enzymatic Browning:

Non-enzymatic browning (Maillard reaction) is a major
deteriorative factor in the storage of dehydrated dairy
food products (Kim et-al, 1981). The Maillard reaction is
influenced by time and temperature of storage and moisture
content. Considerable damage to the nutritional quality of

the product will occur if conditions of storage are such to

21

“flflTE

YELLOW GREEN YELLOW YELLOW RED

 

   

 

 

BLUE GREEN RED BLUE

BLACK

FIGURE 2: L a b System for Color Description and
Qualification.

22

promote Maillard reactions.

Non-enzymatic browning in dried dairy products can be
attributed to two possible reactions. Caramelization of
lactose may occur or reaction between free amino groups of
milk protein and the carbonyl group of reducing sugars
(lactose) (King, 1970). Lactose and casein are the two
principle reactants in the browning of dairy products.

Whey proteins are also involved in some circumstances.
Sugar-amino of Maillard browning is most prevalent because
it requires a low energy of activation and is autocatalytic
(Nickerson, 1974). Direct caramelization has a high energy
of activation and is of less importance unless high
preheating treatments are used. Deterioration results in
color changes, off flavors, vitamin destruction, losses in
protein quality and eventually nutritional value of the

product.

Other physical changes associated with non-enzymatic
browning occur through secondary reactions. The primary
reaction between the protein and reducing sugar initiates
these secondary changes. Although not fully understood it
is known that the sugar becomes irreversibly bound and
through a series of degradative changes yields brown
pigments (Parry Jr., 1974). These reactions are also
responsible (Gordon, 1974) for:

- poor palatability and appearance

- loss of nutritional value destruction of essential
amino acids and vitamins

- loss of biological value and digestibility of the
protein

23

- solubility losses

- carbon dioxide production

Product discoloration is normally slow during storage.
However, in unfavorable storage conditions the amino-sugar
reaction is accelerated. The rate of discoloration due to
non-enzymatic browning may increase 2-fold with a
temperature increase of 3-4 C (37.4-39.2 F) (McWeeny,
1980). This type of deterioration is also dependent on the

moisture content of the product.

Warburton (1978) suggests that the maximum safe storage
conditions for dried milk are 40% relative humidity or a
moisture content of 4-6% at 25 C (77 F). Quality may also
be maintained better at temperatures lower than 34 F (1.1
C) but the most important criteria is storage in a dry

environment.

Manufacturing conditions can also contribute to product
browning. It is essential to avoid excessively high
temperatures, long exposure to heat and high moisture

processing conditions.

2.4.3 Rancidity:

Hydrolytic rancidity may initiate off flavor development in
dry milk products. Hydrolysis is catalyzed by lipase
enzymes (lipolyses) which produce free fatty acids and
partial glycerides. Hydrolytic rancidity can also cause
deterioration of milk or milk products as a result of pre

or post manufacturer contamination by microorganisms

24

(Deeth, 1983). Lipase is activated during homogenization
but pasteurization and high heat treatments lesson the
chance of occurrence by inactivating the enzymes. Less
available fat in low fat milk products reduce the

possibility of hydrolytic rancidity.

2.4.4 Staleness:

Another defect associated with dry milk products is known
as stale off-flavor. There have been various suggestions
as to what may cause staleness. Some associate staleness
with the milk fat while others attribute it to
protein-lactose reactions. Still others believe that
staleness is a conglomeration of off-flavor compounds
occurring simultaneously in the lipids and the
protein-lactose complexes (Hall, 1966). Stale flavors
develop rapidly in milk powders under conditions of high
humidity and elevated storage temperatures (Parry Jr.,

1974).

2.4.5 Oxidation:

The most common type of deterioration in lipid containing
dry foods in the presence of oxygen is through oxidative
rancidity. Lipid oxidation is prominent in freeze dried
foods because of large internal surface areas and low
moisture contents. Dehydrated and freeze dried foods are

particularly sensitive to molecular oxygen.

Oxygen can combine with unsaturated fatty acids by free

radical mechanisms. Free radicals and peroxides produced

25

from this reaction react further to yield degradation
products such as volatile aldehydes, ketones and fatty
acids (Warmbier, 1976). These produce characteristic off
flavors, odors and oxidative rancidity. Volatile carbonyl
compounds which result from lipid oxidation are
organoleptically detectable at levels of parts per billion

(Patton, 1962).

The focal point for lipid oxidation in milk is the fat
globule membrane (Richardson, 1983). In dried milk
products triglycerides are relatively susceptible to
oxidation whereas the phospholipids are more stable. When
phospholipids are present in the triglycerides they serve
as an antioxidant to a limited extent. When water is
present the reverse is true, the triglycerides are
relatively stable and the phospholipids are oxidized

(Patton, 1962).

Oxidation varies with light intensity, relative humidity
and temperature. Sunlight and ultraviolet light can
accelerate fat oxidation, as can trace metal catalysts. In
food products many vitamins, pigments, some amino acids and

proteins are oxygen sensitive (Karel, 1974).

Levels of oxygen greater than 1% can cause rancidity
problems. For products packaged in air detectable off
odors can be noticed within a week. Non-fat dry milks are
much more stable than dried whole milks. Non-fat dry milk
can be stored for over one year in air at room temperature

because the susceptible lipids have been largely removed

26

during processing (Labuza, 1971).

2.4.6 Role of Moisture and Storage Temperature:

The best way to minimize unacceptable changes in dehydrated
foods is to keep them cool and dry. Both temperature and
moisture content should be kept low (McWeeny, 1980).

Freeze dried foods are very moisture sensitive because of
their open structure. Even the slightest intake of
moisture by the product is undesirable. Although freeze
dried products are generally superior to other dehydrated
foods, they are highly susceptible to oxidation (Sacharow,

1980).

Chemical reactions that take place in dried foods are
related to the interaction of water with the various
components. The water activity (Aw) is a measure of the
available water in a food which can be used in chemical
reactions and for microbial growth. Water activity
measures the relative vapor pressure above the food

(Saltmarch, 1980).

When milk products are dried, lactose forms an amorphous
glass which is relatively stable. As moisture is absorbed
the glass portion dilutes and collapses into a stage of
crystal structure. Crystallization can begin at an Aw of
0.42. Warburton (1978) suggests the maximum safe storage
moisture content is 6% with an equilibrium relative
humidity of 40% at 25 C (77 F). The first indication that

the moisture content has reached the upper limit is

27

crystallization. Moisture is released and the powder
becomes sticky, forms lumps and may eventually form a solid

cake.

The optimum Aw where dehydrated foods are most stable and
have the longest shelf is near the BET monolayer. The Aw
in this region is around 0.2 - 0.3 for most foods. For
non-fat dry milk 0.3 is the critical maximum water activity
(Labuza, 1981). In terms of percent moisture 4% is near
the calculated monolayer value of 3.96%. Below 4%
oxidative deterioration occurs and above this value

non-enzymatic browning is accelerated.

As Aw increases above the monolayer many deteriorative
reactions increase exponentially. For some reactions rates
level off at high Aw's and may even decrease. This is true
of non-enzymatic browning. When the water content of
dehydrated products is above the critical monolayer Aw,
non-enzymatic browning will double or triple for every 0.1
Aw increase. As the moisture content continues to increase
to a water activity of 0.6 - 0.8 the reaction rate reaches
a maximum because mobilized reactant species become dilute

(Kim et-al, 1981).

If storage conditions are such, to promote Maillard
reactions in dehydrated milk products, other unfavorable
changes may also occur. Some of the major problems are:

- destruction of essential amino acids (lysine and
histidine)

- loss of solubility

28

- development of stale and caramelized flavors

- damage to the nutritional quality (loss of biological
value of the protein)

- protein becomes insoluble especially upon
reconstitution

- losses of several vitamins

- caking, lumping, loss of volatiles

An increase in moisture content in dehydrated products also
provides an opportunity for growth of molds, yeasts or
bacteria. The overall quality of dehydrated milk products
is better at lower storage temperatures and most

importantly powder must be protected from moisture.

2.5 Culture Viability
Yogurt and any products labeled as such should contain
viable cultures (Anonymous, 1981). Bacterial cultures
have been successfully preserved by freeze dehydration for
extended periods of time. To increase the shelf life of
freeze dried cultures, various work has been done with
suspending media and protective additives during culture
preparation. The suspending media is the most important
factor to increase the survival rate of bacteria after
freeze drying and in minimizing the death rate of dried
organisms during storage (Morichi, 1974). Protective
agents in the suspending medium can reduce the death rate
of microorganisms in frozen storage and increase the
survival rate of microorganisms during and after

freeze-drying (deValdez et-al, 1983). Preservatives or

29

protective substances are added to the cell suspension to
protect portions of cells which may normally be destroyed

during the freeze drying process.

A general term for freeze dehydration in all types of
solvents is cryo-sublimation. In the strictest sense,
freeze drying involves aqueous solutions only and
lyophilization pertains to solvents other than water

(Zaidi et-al, 1979). The process of freeze drying has been
widely employed for the preservation of biological
materials in situations where inherent characteristics of
the material must be unaltered (Robinson, 1981). For this
reason, great success has been made with bacterial

cultures.

Freeze dried cultures from Miles Laboratories are packaged
in laminate pouches. Suggested storage for maximum
cultural activity is -18 C (0 F) or colder. Cultures can
be kept at room temperature 22 C (72 F) for up to 24 hours
with negligible loss in activity. Cultures can also be
maintained in a regular home freezer for up to five months
(Anonymous, 1984). Cultures distributed from Chris
Hansen's Laboratories are dried in the frozen state and
contain large numbers of viable organisms with optimum
activity. Cultures can be sent through the mail in
lightweight aluminum pouches and stored under refrigeration
with little activity loss.

Freeze dried cultures can be stored for 5 to 6 months

30

without activity loss at below freezing temperatures

of - 20 C ( - 4 F ), (Sellers, 1978). Morichi (1974)
agrees that freeze dried cultures should be stored at as
low temperatures as possible. Cell death occurs

logarithmically during storage.

Starter cultures preserved by freeze drying tend to have a
prolonged lag phase upon reconstitution and are mainly used
for propagation of mother cultures (Tamime,1981). Recent
developments have made it feasible to produce freeze dried
cultures for direct inoculation into the bulk starter tank

or alternatively into the processed milk (Robinson, 1981).

During preparation for freeze dehydration preparation and
during the process as few as 50% of the cultures may
survive. Those cultures which do survive normally are
acceptable acid producers in milk. High initial
concentrations of cells are favorable for an increased
survival rate. Concentrations of more than 1 x lollper ml.
is recommended by Morichi (1974) to insure adequate
survival. Gilliland (1981) suggests maintaining cultures
under vacuum after drying to increase survival since freeze
dried cultures are extremely sensitive to oxygen.

Naghmoush (1978) agrees that storage of freeze dried

cultures under vacuum and or at low temperatures will

prolong their activity.

Robinson (1981) describes freeze dried cultures as being in

a state of "suspended animation." Although viability is

31

excellent freeze dried cultures need a longer time to
attain optimum performance. It may take 14 to 16 hours to
develop proper acidity but once acidity has been reached
freeze dried cultures will be virtually undistinguishable
from other types of inoculants. Re-hydration and
reactivation of the cultures is a critical step in
obtaining maximum cell activity. A major concern is cell

sensitivity of L. bulgaricus to re-hydration temperatures

 

of 20 - 25 C (68 - 77 F).

Under unfavorable re-hydration conditions cells which have
survived the freeze dehydration process may become injured

or lose their viability.

deValdez et-al (1985) have studied the relationship
between residual moisture content and the viability of
freeze dried lactic acid bacteria. Their findings are
summarized as follows:

- overdrying is harmful to survival

- cell survival decreases as dehydration increases

- a minimal amount of water must remain for a
satisfactory survival rate.

- increased survival rates could be obtained by
maintaining optimal moisture content in the freeze
dried samples and by adding adonitol (a polyhydric
alcohol C H O ) as a cryoprotective agent.

5 12 5
The ability of a compound to preserve the viability of
cells has been associated to the presence of an amino group
or a secondary alcohol group (deValdez et-al, 1983).

Morichi (1974) found 1% to 3% residual moisture was

32

optimal to ensure the best survival of freeze dried
organisms during storage. Less than 3% moisture could be
obtained in freeze dried preparations stabilized with
glutamate, arginine or lactose. Porubcan and Sellers
(1975) developed a spray-drying process for yogurt and
related cultures. The resulting culture powder has a
concentration and activity equal to that produced by
lyophilization. Residual moisture was less than 5%,
product quality suffered little or no change after 6 months

at 21 C (70 F).

Various additives and suspending media have been examined
for their role in protecting lactic acid bacteria against
freeze drying. Skim milk has been used extensively as a
suspending medium for freeze drying of bacteria. Addition
of additives can help improve the protective activity of
skim milk. L. bulgaricus in particular responds poorly to

lyophilization, spray drying and conventional frozen

 

storage. Because freeze drying will kill bacterial cells,
protective agents must be added before freeze drying.
deValdez et-al (1983) explained the mechanisms by which
microorganisms are killed by freezing or dehydration and
the complexity of protecting organisms from injury. The
major factor causing cell death is osmotic shock. Morichi
(1974) felt that suspending medium is the most important
way to increase the survival rate of bacteria after freeze
drying and to minimize the death rate of dried organisms

during storage. Naghmoush et-al (1978) states that the

33

activity of freeze dried cultures following storage is the
most important point effecting culture behavior and
performance. Cabrini et-al (1982) examined yogurt cultures
freeze dried with various substrates and stored for 1 to 2
years at 4 C (39.2 F). They found yogurts freeze dried
with cryoprotective agents had high bacterial counts after
storage but were not given much protection during the
freeze dryingéprocess5 After one year most samples had

counts of 10 to 10 per ml. and were suitable as

starters.

Wright and Klaenhammer (1983) studied the use of calcium as

a protective agent for L. bulgaricus during freeze drying.

They found superior survival of L. bulgaricus although the

specific role of calcium was unknown. Adonitol was

 

 

strongly recommended by deValdez et-al (1983) as a
cryoprotective agent for production of freeze dried
cultures for use as starters. Both S. thermophilus and L.

bulgaricus were included in this study. In a separate

 

study performed by deValdez et-al (1983) cysteine was found

to be the best protectant for L. bulgaricus. B.

glycerophosphate in milk or sodium glutamate offered the

 

best protection for S. thermophilus. An appropriate

selection of the suspending medium and cryoprotective agent

 

was essential in order to obtain a maximum number of viable
cells. Choice of protecting agents is limited based on

processes, types of bacteria and storage conditions.

34

Additional work was done to determine the effect of storage
on total cell count, viable count and cell activity of
treated lactic acid cultures by (Naghmoush et-al, 1978).
The likelihood of maintaining active freeze dried cultures

for extended time periods at room temperature was examined.

Lyophilized concentrates can be expected to perform well
under industrial conditions and should be able to replace
more conventional cultures and eliminate excessive handling
(Speckman et-al, 1973). Problems include the expense of
lyophilization as well as the cryoprotective ingredients
and re-hydration media needed. Robinson (1981) and others
agree that freeze dried cultures provide convenience and

reliability as the ideal inoculum for a bulk starter.

2.6 Packaging Dried Products
The package affects the quality of foods by controlling the
degree to which processing, storage and handling can
effect components of foods (Karel, 1974). Storage factors
which can be controlled by packaging are light, oxygen,
moisture, contamination and attack by biological agents.
Campbell (1975) describes a desirable package as being
impervious to moisture, light, gasses and easily filled,

sealed, handled and emptied with reclosing features.

Packages for dehydrated foods must take into account the
products sensitivity to moisture, oxygen and bacteria. In
dried products where the moisture content is below 3%

growth of bacteria is eliminated. Thus, packaging should

35

focus on the two prime causes of spoilage, moisture and

oxygen.

The protection of foods from moisture exchange is an
ancient requirement (Labuza, 1981). Dried foods in the
range of 1% to 3% moisture have equilibrium relative
humidity values below 20%. The relative humidity of
ambient air is rarely this low, which allows low moisture
foods to absorb water rapidly (Paine, 1983). Absorption of
atmospheric moisture is critical for freeze dried products
because of their open structure. The slightest degree of
moisture absorption in a freeze dehydrated product can
cause discoloration, flavor loss and lumpiness which
results in poor reconstitution. Although these factors may
not cause complete spoilage in dehydrated foods, they are

unacceptable for the consumer (Sacharow, 1980).

Low moisture foods can also be very susceptible to oxygen.
This is especially true of freeze dried products because of
large internal surface areas. .Many dehydrated foods are
sensitive to light. Light accelerates the development of
rancidity by means of oxidative spoilage. To protect dried
products from oxygen, gas packaging or packaging under

vacuum is suggested.

2.6.1 Vacuum Packaging:
Vacuum packaging is used to remove most of the oxygen from
a package. The package is evacuated and sealed under

vacuum to increase product shelf-life. Packaging

36

dehydrated foods in flexible packages under vacuum may
cause problems. Materials must withstand the vacuum
process and not rupture at corners and seals. The shape of
vacuum packages may also be considered a negative attribute

due to their bulky appearance.

2.6.2 Gas Packaging:

Gas packaging is another method used to reduce the oxygen
level to 1% to 2% in the package. Air is replaced by
flushing the package with an inert gas. Nitrogen is the

most common gas used.

Another option when packaging dehydrated products which are
prone to deterioration from oxygen, is the use of
antioxidants. Antioxidants are defined by the United
States (U.S.) Food and Drug Administration (FDA) as
substances used to preserve food by retarding
deterioration, rancidity or discoloration due to oxidation
(Anonymous, 1986). Antioxidants are those compounds that
terminate the free-radical chain in lipid oxidation;
Chelators, which bind metal ions such as iron and copper
that catalyze lipid oxidation; oxygen scavengers, or those
compounds that react with oxygen in closed systems; and
"secondary" antioxidants, which function by breaking down
the hydroperoxides (Anonymous, 1986). The most common food
antioxidants are phenolic substances. Many natural
occuring antioxidants tend to be less effective and are
often inactivated by mild heat treatments. Antioxidant

effectiveness may also be dependent on the water present in

37
the system. Selection of an appropriate antioxidant is
determined by its effectiveness in certain fats, animal
fats or vegetable oils, its carry through properties and

possible synergistic effect (negative or positive).

Bachelors Foods, Ltd., (United Kingdom) produce dehydrated
foods. They use flexible pouches without either gas or
vacuum packaging, having found the amount of oxygen inside
the package is seldom a problem. Instead barrier films
are used to prevent external oxygen from reaching the

product (Anonymous, 1980).

2.6.3 Polyethylene:

Polyethylene is a very versatile film and an important
component of laminates. It is a good moisture barrier,

has excellent heat sealing properties and is resistant to
acid or alkali solvents. Low density polyethylene (LDPE)
is a poor oxygen barrier. Polyethylene is the major
plastic material used for flexible packaging. Polyethylene
finds wide use in laminations or co-extrusions because of
its toughness, heat sealability and low cost (Anonymous,

1986).

2.6.4 Aluminum Foil:

For packaging dried products, aluminum foil is used mainly
in laminate systems. Aluminum foil is available in many
thicknesses and is impermeable to water vapor, gases and
light. It is tasteless, colorless, non-toxic and will not

support bacterial or mold growth.

Aluminum foil is not heat sealable and thin gauges (which

38

are used for economic reasons) do not provide the strength
or abuse resistance needed (Martin, E.L., 1986). These
problems are overcome by laminating aluminum foil to paper
and/or plastic films. Thus barrier properties can be
obtained with the thinnest amount of foil. In addition,
strength, scuff resistance, durability, puncture resistance

and heat sealability are offered from the laminate system.

2.6.5 Polyester:

Polyester has excellent tensile strength, is tear, heat,
abrasion and chemical resistant. Polyester is a good gas
and water vapor barrier. It has good resistance to grease
and oils. Polyester has good thermal properties in
applications over a wide temperature range. It is a
primary material for boil in bags and retort pouches

(Rounsville, 1986).

2.6.6 Polypropylene:

Polypropylene is more rigid, stronger and lighter than
polyethylene. It is resistant to grease, abrasion and
chemicals. Polypropylene is an excellent water barrier and
with PVDC coating, an excellent gas barrier. Polypropylene
is stable to high temperatures and has good tensile
strength (Rice, 1982). A special advantage of
polypropylene is its lightness. It is the lightest in
weight of the plastics used in packaging. Biaxially
oriented polypropylene has become the most widely used of
the transparent plastic films due to the versatility

provided by coatings and co-extrusions (Martin, 1986).

39

A three-ply laminate structure of polyester/aluminum
foil/polypropylene for use with dried yogurt has the
following properties. Polyester on the outside provides
toughness, abuse resistance and has a good printing
surface. Aluminum foil as the middle layer provides a
barrier to light, moisture and oxygen (Kahn, 1982). On the
inside, polypropylene is compatible with the product,
provides additional protection against moisture and can be

sealed.

Rasic (1978) suggests packaging yogurt powder into
composite containers for storage at room temperature. He
recommends gas flushing to extend the shelf life. Ardito
et-al (1980) examined dried skim milk packaged in

(i) Nitrogen flushed cans (ii) Pigmented LDPE and

(iii) a Paper LDPE/Al/LDPE laminate. Product quality was
acceptable in all three packages stored at 23 C (73.4 F),
65% RH and at 30 C (86 F), 80% RH for six months. After
120 days at a higher temperature of 38 C (100.4 F), 90%
RH, product in the pigmented LDPE was unacceptable.
Product was good in the other packages. The package
recommended for this product was the laminate versus the
cheaper LDPE or more expensive cans. Sacharow (1980)
suggested that non-fat dry milk (NFD) is not as sensitive
to oxygen and does not require vacuum or gas packaging.
Most consumer portion packages are made from aluminum foil
laminates comprised of Paper/PE/Al/PE. Carnation Company,

found success for their small pouches of powder products

40

with metallized polyethylene laminated to paper. This
package enabled them to maintain the 12 month shelf life

(Anonymous, 1985).

For packaging dehydrated powder products flexible pouches
were emphasized. Flexible pouches are lightweight which
leads to tremendous space and shipping advantages. They
are economical, requiring no energy usage during storage.
Flexible pouches also lesson disposal problems often
encountered with other containers (Kahn, 1982). Further
development will insure product package compatibility with
better barrier characteristics to keep the product dry,
retain flavors-within the package and prevent the entry of

oxygen and light.

The final package choice must fulfill the requirements
imposed by the product. Packaging development has an
essential contribution to make, both in packaging new
products and making existing ones more efficient either in

production, storage or convenience (Anonymous, 1980).

2.7 Yogurt Products and Uses
Yogurt is sold in various forms in the United States.
Yogurt is available for commercial use as a liquid, spray
dried and freeze dried powder. Dried yogurt is sold as an
ingredient for bakery mixes, soups, salad dressings and
confectionary products (Vedamuthu, 1982). Advantages of
dried yogurt versus fresh include functionality,

availability, cost, shelflife and product standardization

41

(Anonymous, 1978). Like most fresh products yogurt is
subject to degradative changes typical of perishable
products. Freeze dried yogurt contains more viable
bacteria than spray dried yogurt due to shorter drying
times. Freeze dried yogurt is generally priced higher as a

result of higher processing costs.

The drying of yogurt is an old method of preservation.
Dried yogurt can be produced by sun drying, spray drying or
freeze drying. Sun drying is used mostly in rural areas of
the middle east while the other methods are used for

commercial purposes (Tamime and Deeth, 1980).

2.7.1 Freeze Drying:

Freeze drying is accomplished by removing water from the
frozen product under high vacuum. The final product is a
fine powder which can be stored at room temperature, if
packaged under vacuum or inert gas. To reconstitute the
product the original amount of water is added to the powder
with stirring. Reconstituted yogurt has a weaker
consistency than freshly made product which is overcome by
the use of stabilizers. Dried yogurt will also have
reduced flavor and fewer numbers of lactobacilli than the

initial yogurt (Rasic and Kurmann, 1978).

Several dried yogurts are manufactured in France, Russia,
Switzerland and West Germany. The powder has a moisture
content between 1% to 3%. Manufacturers claim a product

virtually undistinguishable from original yogurt

42

(Helferich, 1980). Little product of this nature has been
made thus far in the United States. Yogurt-based foods are
an untapped market which could help boost sales and

consumption of yogurt (Steinberg, 1983).

DMV Campina one of Europe's largest dairy cooperatives has
found great success with yogurt based products. They first
introduced a yogurt based diet drink. Since then they
have developed a high protein yogurt spread, yogurt bars
and yogurnaise, a yogurt mayonnaise containing 25% oil
which is 55% less than normal mayonnaise. They have also
been able to stabilize yogurt in various liquors (Krier,

1984).

Nolan (1982) described how yogurt is used in baked goods to
increase sales appeal, improve moisture, shelflife and the
flavor of bread, cookies, donuts and a variety of other
yeast raised products. Donuts with 30% yogurt were
preferred to regular donuts. Yogurt donuts were able to
mask the fried fat flavor and enhance glaze, fillings and
icings. The following reasons were given for adding yogurt
to breads.
- Improve moisture in the dough and in the
finished loaf. The yogurt acts as a humectant
which holds moisture in the loaf. Bread can be

held on the shelf an additional 12 to 24 hours.

- Yogurt improves the product flavor making it
"cleaner" tasting.

- Yogurt improves breads protein level

- Yogurt adds sales appeal as a "health food"

43

- Mold growth is slightly deterred as a result of the
lower pH.
Salsburg (1983) described a spray dried yogurt that is
"entirely nutritious with a mild lactic flavor." The powder
is semi-instant and can be dispersed easily with agitation
in cold or preferably warm water. It is packaged in a

multiwall paper sack with a sealed polyethylene liner.

Stauffer Chemical Company in the U.S. has a variety of
spray dried cultured dairy products which duplicate the
characteristics of fresh yogurt (Anonymous, 1978). The
spray dried yogurt powder is suitable for use as a flavor
base. Suggested applications for the dried yogurt powder
are: chocolate confections, cream fillings for candy,
fruit toppings, baked potato toppings, cheese spreads and
process cheeses, snack products (dusting powder), cookies
and cake fillings, whipped toppings, meat sauces, gravy

bases, salad dressings, breads and other baked goods.

Carnation Company introduced yogurt bars and Slender Yogurt
diet drink (Anonymous, 1982). Beam Import Company combines
yogurt with French Cognac. Yogurt is thought of as being
nutritious and having fewer calories than cream, while
still having the smooth, silky mouthfeel that is expected
in a liqueur. Yogurt also helps extend product shelf life
with no refrigeration required. The technology that has
been developed will be used to develop a yogurt powder to
be used in food applications similar to powdered milk

(Anonymous, 1984).

44

More recently, Pet Inc., introduced a calcium fortified
cereal called "Dairy Crisp." The cereal is fortified with
nonfat dried yogurt (Anonymous, 1985). To-Fitness, Inc., a
Florida based manufacturer has the first instant yogurt
with active cultures to be manufactured and marketed in the
United States (Anonymous, 1986). The products containing
active cultures, are available in powder form and can be
stored without refrigeration indefinitely. "Instant
Culture" the name given to the product comes in three forms
and fourteen flavors. Jet-Set is the name given to a
yogurt powder the consumer can reconstitute to the
consistency of regular yogurt in five minutes.

Expectations are that these new products will revolutionize
the $1.5 billion U.S. yogurt market (Anonymous, 1985).
Steinberg (1983) suggests that yogurt is an ideal base for
dips and salad dressings. Suitable not only for salads, it
can be used with hot or cold vegetables, meats and fish.
The tangy lactic acid taste will intensify flavors and
aromas and enhance added ingredients. Miller Cheese
Company of New York has introduced a yogurt based cheese.
Described as tasting like muenster with a slightly tangy
aftertaste, it is available plain and with herbs and spices

(Anonymous, 1986).

Dried yogurt is suitable for export to foreign countries,
especially those with a protein deficiency. It has great
potential for use in warm regions and can be easily

reconstituted. Robinson (1978) describes a dried

45

concentrated yogurt and cereal flour mixture that has found
widespread acceptance in certain areas of the middle east.
The dried mixture is valuable during periods of the year
when fresh milk is unavailable. A.B. Milkfoods of Sweden
developed a yogurt powder which is said to look and taste
like natural yogurt when reconstituted with water
(Anonymous, 1983). They suggest it provide an alternative
for countries lacking in fresh milk. The powder is
packaged in 25 kilogram paper bags with an inner liner and
has a one year shelf life. Yogurt powder of various
consistencies is available for use in medical preparations,

desserts, sauces, spreads and other products.

With new styles, flavors and uses for yogurt being
introduced they should appeal to both the health and
quality conscious consumer. As a result yogurt sales
should experience a period of renewed growth (Anonymous,

1986).

3.0 MATERIALS AND METHODS

3.1 Yogurt Preparation
The lowfat yogurt used in this study was manufactured in
the M.S.U. dairy plant (for formulation see Appendix A).
Yogurt preparation followed the sequence of steps shown in

Figure 3.

3.1.1 Bulk Starter Culture:
Bulk starter culture obtained from the Michigan State
University (M.S.U.) Dairy Plant was prepared as follows:

1. Five gallons of whole milk was sterilized by
heating in boiling water (212 F, 100 C) for over
one hour.

2. The sterilized milk was cooled to 115 F (46 C).

3. One 70 ml can of Chris Hansen's R-l Redi-Set
concentrated deep-frozen yogurt culture was
thawed and aseptically added to the mix. The
ratio of Streptococcus thermophilus to
Lactobacillus bulgarIcus was approximately 1:1.

4. The culture was incubated for approximately three
hours at 112 - 114 F (44.4 - 45.6 C).

3.1.2 Uncultured Mix:

Milk received from the M.S.U. Dairy Plant was standardized
into the composition desired (see Appendix B).

The uncultured mix was blended with a Lightin Mixer (Model

10 X) and vat pasteurized at 190 F (87.8 C) for 30 minutes.
The mix was then cooled to 150 F (65.6 C) prior to

homogenizing. All utensils and equipment were sanitized

before use.

46

47

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NFDM Milk } % Water
Mix I
Pasteurization
at 190 F (88.8 C) for 30 min.
Cooling
at 140 - 160 F (60 - 71.7 C)
Homogenization
First Stage . ..... . ..... 1000 psi
Second Stage ........... 500 psi
Cooling
at 112 F (44.4 C)
Inoculation Bulk Starter
2-4% W/W Culture
Incubation
at 110 - 112 F (43.3 - 44.4 C)
3 - 4 hours, pH = 4.0 - 4.5
Refrigeration
39.2 F (4 C), 24 hrs
FIGURE 3: Flow Diagram for Yogurt Preparation.

48

3.1.3 Homogenization:

The mix was homogenized at 1500 pounds per square inch
(p.s.i.) in two stages using a Manton Gaulin 75 K pilot
plant homogenizer. The first stage was operated at 1000
p.s.i. (70.3 Kg/sq.cm.) and the second stage at 500 p.s.i.
(35.1 Kg/sq.cm.).

3.1.4 Yogurt Mix:

Uncultured mix was cooled after homogenization to 112 F
(44.4 C). Bulk yogurt culture was then added to the mix.
The formulation used is described in Appendix A.
Incubation was carried out at 110 - 112 F (43.3 - 44.4 C)
for a period of 3 to 4 hours or until a pH of 4.3 to 4.5
was obtained. Yogurt was refrigerated at 39.2 F (4 C) for

at least 18 hours prior to evaluation.

3.1.5 Freeze Dehydration:

A Virtis pilot plant Freeze Dryer (Model FFD42 WS) with a
50 1b condensing capacity was used to freeze-dry the fresh
yogurt. The freeze dryer was operated according to
procedures in the instruction manual. Prior to freeze
drying, yogurt was frozen on stainless steel trays at 0 F i
4 F (-17.8 C i 2.2 C). Yogurt was spread evenly over each
tray approximately one half inch thick. Four to five trays
were dried per run. Each drying cycle lasted at least 48
hours or until the yogurt was dry and crumbly. Heat was
applied to the shelves at 112 F i 5 F (44.4 C i 2.8 C) to
aid in the drying process. Vacuum was maintained at 50

microns or less throughout the drying cycle and was

49

measured using a McCloud Gage. Five runs were needed to
dry 100 pounds of fresh yogurt. Figure 4 shows the flow

diagram describing the dehydration and packaging of yogurt.

3.2 Packaging

3.2.1 Dried Yogurt:

Approximately 1120 grams of yogurt was packaged under
atmospheric conditions and stored under ambient conditions
of 72 F (22.2 C), 50% Relative Humidity (RH) and accelerated
conditions of 100 F (37.7 C), 85% RH. Two of each of the
following package types were stored at ambient temperature

for 10 weeks and at accelerated conditions for 4 weeks.

1. The first package was a laminated Pouch (5.75 in. by
7.87 in.) of polyester/foil/polypropylene with a thickness
of 5.5 mils. Pouches were sealed on a Sentinel Thermo
Heatsealer under atmospheric conditions. Bar temperature was
set at 375 F (190.6 C) with 40 pounds of pressure for 1.5

seconds. A half inch top seal was made.

2. Dow Ziploc Freezer Bags (2.7 mil Low Density
Polyethylene (LDPE)) were also used. LDPE bags, 7 in. by
8 in. were made into pouches by reducing the size of the
Ziploc bags to that of the laminated pouch. Edges were
reinforced by a Sentinel Impulse Heatsealer. Bar pressure
was maintained at 30 p.s.i. (5.3 Kg/sq.cm.), with 0.5

seconds heating and 4 seconds cooling time.

50

 

Prefreeze
at -4 F (-20 C), 24 hrs.

I

Freeze Dehydration
approximately 48 hours

I

1——~ Packaging -—|

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Polyethylene Pouch Laminate Pouch
-—————1 Storage
Accelerated I Room
100 F(37.7 C), 85% RH 1 72 F (22 C), 50% RH

 

 

 

 

 

 

 

 

Testing and Evaluation I

 

FIGURE 4: Flow Diagram for Yogurt Dehydration
and Packaging.

51

3. Preliminary tests were also performed using Kraft
paper bags. The dimensions of these bags were 10.5 in. x 9
in. x 5 in. and made from virgin kraft paper. Due to high
rates of oxygen transmission and moisture gain, product
deteriorated within one week. Therefore no further tests

were performed using this construction.

3.3 Sampling
3.3.1 Fresh Yogurt:
Prior to freeze drying, portions of the fresh yogurt were
removed. The following tests and evaluations were made to
monitor the quality of fresh yogurt.
1. Color Analysis - using a Hunterlab D25D2 L Digital color
and color difference solid state digital colorimeter

(see section 2.3 for details).

2. pH Measurement- using a Orion Research Analog Model 301,
electronic pH meter.

3. Sensory Analysis - flavor only based on the scorecard
for the Collegiate Dairy Products Evaluation Contest
(see Appendix C).

4. Culture Viability - plating on Lee Agar (see Appendix D)
for the simultaneous enumeration of Lactobacillus
bulgaricus and Streptococcus thermophilus.

 

 

3.3.2 Dried Yogurt:
After the product was freeze dried, moisture content, color,
sensory properties and culture viability were initially

evaluated.

Moisture content was measured on triplicate samples dried
in a vacuum oven at 155 - 158 F (68.3 - 70 C) for 15 to 18

hours under a vacuum of 735 mm Hg. The procedure initially

52

followed was that described in the AOAC (1975) for dry milk.
However due to fluctuations in moisture readings and a
great degree of caramelization, yogurt was dried at the

lower temperature for a longer time period.

3.3.3 Stored Dried Yogurt:

Two pouches each of the laminate and LDPE pouches were
removed from ambient and accelerated storage conditions for
testing and evaluation. Product was examined after one,
two, three and four weeks. In addition, product from both
pouch types stored at room conditions was examined after
two months. Pouches of product from the same conditions

were combined and evaluated by the following methods:

1. Headspace Analysis - Duplicate readings from
each pouch were made to determine percentage of oxygen,
nitrogen and carbon dioxide present in the package
headspace. A 5 cc sample was removed and injected into a
sampling port of a Model 2153 Carle Gas Chromatograph.
Percent of each gas was determined using a Spectra Physics
400 Dual Channel Computing Integrator. Series - Bypass
technique was employed to allow carbon dioxide to exit the
column first.

The following conditions were maintained:

Column Temperature - 95 C

Carrier Gas - Helium at 40 psi with a flow
rate of ml/min

Chart Speed - 5 in/min

Bridge Setting - Thermistor

Output - 64

53

Samples were run on the left column of the gas chromatograph
which consisted of an 8 inch 80% Porapak N + 20% Porapak Q

and a 6 inch Molecular Sieve - 5 A.

2. Moisture Content - was determined using the
vacuum oven technique described earlier. Triplicate
samples from each package type and storage condition were

evaluated.

3. Color Analysis - Triplicate readings of dried
yogurt from each package type and storage condition were

made using the Hunterlab Color Cell as described before.

4. Culture Viability - After 2 and 4 weeks, samples
of dried yogurt were taken and prepared according to Standard
Methods for the Examination of Dairy Products (1985). Each
sample was plated in triplicates to determine viability of

Lactobacillus bulgaricus and Streptococcus thermophilus

 

 

 

during storage. Samples from both pouch types stored at
room conditions were also evaluated after more than two

months storage.

5. Reconstitution - The dried yogurt was
reconstituted using tap water with known pH as described
in Appendix E. Hardness content of this water was
measured at 340 ppm. Tap water was used for the
reconstitution because it represents a readily available

source for the consumer.

6. Sensory Analysis - Qualified judges rated samples

of reconstituted yogurt which had been refrigerated for 24

54

hours. Product from each package and storage condition was
evaluated for flavor change. The scoring was done using a
procedure similar to that described in the ADSA guide used
at Collegiate Dairy Products Evaluation Contests (Appendix

C and F).

7. pH - The pH of each reconstituted yogurt sample

was determined. pH of the tap water used for reconstitution

was also determined.

:.r-m- mm;-

‘
h

3.4 Material Permeability Tests
3.4.1 Water Vapor Transmission Rate (WVTR):
The WVTR of the LDPE material was measured gravimetrically
at 100 F (37.8 C) and 85% RH. The dish method described
in ASTM Standard E 96 (1980) was employed. Triplicate
samples were stored for 20 days at the above conditions.
WVTR values (weight gain per day) were determined using a
Spectra Physics 400 Dual Channel Computing Integrator. The
computations used to obtain the relative constants are

described in Appendix G.

3.4.2 Oxygen Permeability:

Oxygen permeability of the LDPE film was measured on a
Modern Controls Inc. (Minneapolis, MN) Ox-Tran 100 Oxygen
Permeability Tester. The procedure followed was in
accordance with the manufacturer's operation manual and ASTM
Standard D3985-81 (Oxygen Gas Transmission Rate Through

Plastic Film and Sheeting Using a Caulometric Sensor,

55

Annonymous,l984). Sample calculations are described in

Appendix H.

3.4.3 Carbon Dioxide Permeability:

Modern Controls Inc., Permeatran C (Minneapolis, MN) Carbon
Dioxide Permeability Tester was used to measure the carbon
dioxide transmission rate through the LDPE film sample.
(See Appendix I). Test samples were mounted with a masking
element to reduce the effective surface area to 5 sq.cm.
The procedure used followed the instructions found in the

operators manual.

4.0 RESULTS

4.1.1 Culture Viability:

Culture viability of S. thermophilus and L. bulgaricus

was monitored throughout the study. Initially in the fresh

 

yogurt there was 2-3 times the number of L. bulgaricus

organisms in comparison to S. thermophilus. Freeze drying

resulted in a 1800 times reduction of S. thermophilus and a

39,000 times reduction of L. bulgaricus (Table 5). It is,

 

 

 

therefore, important to use procedures that insure high

 

 

initial counts of organisms to compensate for this sharp

reduction during the freeze drying process.

During storage at room conditions (72 F, 50% RH) S.

thermophilus and L. bulgaricus decreased slightly after

the initial freeze drying process (Table 5). Storage at

 

 

accelerated conditions (100 F, 85% RH) resulted in a

larger reduction of both organisms. Yogurt was packaged in
a LDPE pouch and a laminated foil pouch. A very slight
difference in survival rates of the organisms was observed
in the different package types.

Initial counts of S. thermophilus and L. bulgaricus were

5 - ——— 4 -—————— —
4.10 x 10 and 5.30 x 10 respectively, after freeze

 

drying and storage for ten weeks at room conditions (72 F,
3
50% RH) in the LDPE pouch, a reduction to 3.7 x 10 for
3
S. thermophilus and 3.0 x 10 for L. bulgaricus was

found. This indicates that organism survival rate is only

 

 

56

TABLE 5:

Average Values of Streptococcus thermophilus and

Lactobacillus
Initia

bulgaricus Present in Fresh Y6gurt,

Freeze Drie
Yogurt plated on Lee Agar.

an Packaged Freeze Dried

 

 

Samples Storage S. thermophilus L. bulgaricus
Time (weeks)
9
Fresh 0 7.7 10 2.1 x 10
Yogurt
4
Initial 0 4.1 10 5.3 x 10
Freeze Dried
3
LDPE 2 2.9 10 1.9 x 10
3
4 3.2 10 2.3 x 10
r 4
LDPE 2 5.2 10 1.2 x 10
4
4 4.6 10 2.4 x 10
3
10 3.7 10 3.0 x 10
3
Laminate 2 3.3 10 3 9 x 10
3
4 2.7 10 3 2 x 10
3
Laminate 2 6.6 10 7.3 x 10
3
4 4.7 10 6.5 x 10
3
10 3.8 10 4.0 x 10

 

NOTE: All average values determined from triplicate samples.

a. Accelerated Conditions 100 F, 85% RH.
r. Room Conditions 72 F, 50% RH.

57

58

moderately affected during storage in both packages.

4.1.2 Headspace Gas Composition:

Head space analysis was done to monitor the change in the
concentrations of oxygen, carbon dioxide and nitrogen,
which can affect the shelf life of the product during
storage. Storage of freeze dried yogurt at room conditions
in both types of pouches showed a decrease of oxygen in the
headspace to about 14%. during the first four weeks. The
oxygen content increased back to the initial level of 17%
after ten weeks (Table 6). The decrease in oxygen content
in both package types is probably a result of oxidative
rancidity or protein oxidation. Extended storage of the
product in the LDPE pouch allowed oxygen to permeate back
into the headspace to reach equilibrium. In the laminate
pouch subjected to accelerated conditions, carbon dioxide
was detected in the headspace. This is probably due to
nonenzymatic browning reactions which produce carbon
dioxide. The presence of carbon dioxide detected during the
first week of storage in the laminate pouch at room
conditions may have been due to a sampling error resulting
from the presence of residual carbon dioxide from the
previous sample. Inadequate flushing of the syringe may

account for the carbon dioxide detected.

4.1.3 Color Analysis:

The Hunter Spectrophotometer was used to compare color

TABLE 6: Average Values of Gas Present in the Package
Head Space of Freeze Dried Yogurt Stored at
Room and Accelerated Conditions.

 

 

Percentages
Samples Storage CO 0 N
Time (weeks) 2 2 2
a b
LDPE 1 N 20.99 77.57
2 N 14.55 85.46
3 N 14.46 85.54
4 N 14.25 85.75
r
LDPE l N 17.66 82.33
2 N 14.73 85.21
3 N 14.54 85.46
4 N 14.38 85.67
10 N 17.71 83.04
a
Laminate 1 0.36 15.83 83.81
2 0.17 14.90 84.94
3 0.48 14.44 85.04
4 0.47 14.47 85.07
r

Laminate 1 0.23 16.96 82.78
2 N 14.60 85.40
3 N 14.52 85.48
4 N 14.23 85.77
10 N 17.30 82.43

 

NOTE: All average values determined from duplicate

readings from each of the two pouches.

a. Accelerated Conditions 100 F, 85% RH.
b. Gas concentration insufficient to detect.

r. Room Conditions 72 F, 50% RH.

59

60

changes in freeze dried yogurt during storage in different
packaging materials. The Hunter L,a,b scale was designed
to give measurements of color units of approximate visual
uniformity throughout the color solid. Thus, "L" measures
lightness and varies from 100 for perfect white to zero for
black, approximately as the eye would evaluate it. The
chromaticity dimensions ("a" and "b") give understandable
designations of color as follows:
- "a" measures redness when plus, gray when zero and
greenness when minus
- "b" measures yellowness when plus, gray when zero and
blueness when minus
Measured L,a,b, values (Table 7) do not indicate an
absolute measure of product whiteness. Whiteness is the
attribute by which an object is judged to approach the
preferred white. Ideal white is bluish, and departures
from this ideal white toward yellow reduce whiteness
ratings about four times as much as departures toward gray.
Whiteness indices can be used to provide correlation with
visual whiteness rankings of "product whiteness" for a
number of product applications whereby, blue whites will be
evaluated higher than neutral or yellow whites of the same

lightness (ASTM E 313-73).

Whiteness indices were calculated using the experimentally
determined L,a,b values. The relationship between Hunter

L,a,b values and the International Commission on

TABLE 7: Average Hunter Color Cell Values of Fresh Yogurt,
Initial Freeze Dried and Dried Yogurt Stored in
Polyethylene and Laminate Pouches.

 

Hunter Cell Values

 

Whiteness
Samples Storage L a b Index (WI)
Time (Weeks)

Fresh 0 80.4 - 2.2 9.6 20.54

Yogurt

Dried O 85.6 - 1.4 15.7 - 3.52

Initial

a

LDPE 1 82.4 - 1.8 15.8 - 6.50
2 81.3 - 2.0 16.2 - 9.56
3 80.6 - 2.4 17.0 -13.33
4 78.4 - 2.8 17.6 -17.38

LDPE 1 84.2 - 1.6 15.6 - 4.16
2 82.2 - 1.8 15.8 - 6.65
3 82.6 - 2.4 16.4 - 9.18
4 82.8 - 2.2 16.2 - 8.09
0 78.5 - 2.8 15.8 - 9.25

Laminate 1 82.2 - 2.0 15.6 - 5.71
2 80.2 - 2.4 15.8 - 8.09
3 80.2 - 2.6 16.2 - 9.92
4 80.4 - 2.6 16.3 -10.25

Laminate 1 80.2 - 2.4 15.2 - 5.34
2 80.3 - 2.4 15.2 - 5.27
3 80.1 - 2.4 15.2 - 5.41
4 80.4 - 2.4 15.4 - 6.11
0 82.7 - 2.5 15.8 - 6.27

 

NOTE: All average values determined from triplicate
readings.

a. Accelerated Conditions, 100 F, 85% RH
r. Room Conditions, 72 F, 50% RH.

61

62

Illumination (CIE) X,Y,Z values is as follows:

2 aL
X = 0.98041 0.01L +
(175)
2
Y = 0.01 L
2 bL

1.18103 0.01L - -———
(70)

N
ll

Whiteness Index (WI) per ASTM Method E 313-73 is:

Z
WI = 4 ——-————- - 3Y
1.18103
The calibrated standard white tile has a maximum WI of
72.0. Freeze drying resulted in a sharp reduction of WI
from 20.5 for fresh yogurt to -3.5 (Table 7). During
storage WI values decreased indicating a change in the
products color from white to yellow. Throughout storage
under accelerated conditions whiteness decreased in both
package types. Four weeks of storage at accelerated
conditions lowered the WI to -10.3 in the laminated pouch
as compared to -17.4 in the LDPE. A similar trend was
observed for storage at room conditions in both packages.
Ten weeks of storage at room conditions resulted in a WI of
-6.3 in the laminated pouch as compared to -9.3 in the
LDPE. In general color changes were less prominent in the
laminate pouch versus the LDPE pouch, due to its

impermeable structure (Figure 5).

Whiteness Index

 

63

 

13
A an (autumn: A III (one)
IR [IL-dun. Illfllfll(lld)
isT
1|.
:.

 

 

 

 

4

t_‘

l I i l 3
Storage Time (weeks) ———-'

FIGURE 5: Whiteness Index for Yogurt

 

64
4.1.4 pH:
Rasic and Kurmann (1978) reported that the optimum flavor
of yogurt is usually obtained at a pH value between 4.40
and 4.00. Low acidity of yogurt (pH above 4.6) induces
whey separation due to a weak structure. High acidity (pH
below 4.0) can cause a contraction of the coagulum and
whey separation due to a decrease in protein hydration.
pH values (Table 8) indicate that freeze drying did not
affect the acidity. However, extended storage showed a
decrease in pH from 4.0 to 3.9 indicating a slight

increase in acidity.

4.1.5 Moisture Content:

Initial moisture content of freeze dried yogurt was found
to be 2.71 (Table 9). Storage in LDPE pouches resulted in
an increase in moisture content over time. An average
moisture content of 3.80 was reached when stored at room
conditions for ten weeks and 5.78 at four weeks under
accelerated storage conditions (Figure 6). However, in the
laminate pouch, the moisture content decreased to 1.50 and
then stabilized at 2.00 after extended storage. This may be
due to crystallization of lactose which could affect the
amount of moisture in the freeze dried yogurt in the

laminate pouch (Iglesias and Chirife, 1982).

Permeability constants for LDPE were determined at 70 F,
50% RH (Table 10). No moisture gain was observed for the
laminate pouch subjected to storage for two weeks at 100 F

(37.7 C) and 70% RH.

65

TABLE 8: pH of Fresh Yogurt, Initial Freeze Dried
and Reconstituted Yogurt.

 

Samples Storage pH
Time (Weeks)

 

Fresh 0 4.0
Yogurt
Initial 0 4.0
Freeze dried
a
LDPE 1 4.0
2 4.0
3 4.0
4 3.9
r
LDPE 1 4.0
2 4.0
3 4.0
4 3.9
10 3.9
a
Laminate 1 4.0
2 3.9
3 3.9
4 3.9
r
Laminate 1 4.0
2 4.0
3 4.0
4 3.9
10 3.9

 

NOTE: Average pH value of water used for reconstitution was
determined to be 7.1.

a. Accelerated Conditions, 100 F, 85% RH.
r. Room Conditions, 72 F, 50% RH.

TABLE 9: Average Values of Moisture Content for Freeze
Dried Yogurt.

 

Samples Storage Moisture Content
Time (Weeks)

 

Initial 0 2.71
Freeze dried

a
LDPE 2.74
3.92
4.84

5.78

ubWNH

LDPE 2.00
1.92
2.65
2.37

3.80

ObWNH

a
Laminate 1.77
1.55
1.81

1.83

ubUJNl-J

r
Laminate 1.88
1.59
1.53
1.57

2.00

ObWNH

 

NOTE: Average values based on triplicate readings.

a. Accelerated Conditions, 100 F, 85% RH.
r. Room Conditions, 72 F, 50% RH.

66

Moisture Content

67

 

 

 

£1UII(-nunmfl)' lllllunun
' Ill-lunetunnuHMHD [Ilium-an.»
&

t
l
L

“ t
I- \

.\\ 4
L

 

 

Storage Time (weeks) -————-’

FIGURE 6: Moisture Content for Freeze Dried Yogurt.

 

TABLE 10: Permeability Constants for WVTR, Oxygen and
Carbon-dioxide Transmission of LDPE.

 

 

 

Permeability LDPE
Constant
1 gm - mil
Water Vapor 0.4333

sq.m. - mmHg - days

2 cc - mil
Carbon Dioxide 32741

 

sq.m. - atm - days

3 cc - mil
Oxygen 3780

 

sq.m. - atm - days

 

1,2,3 Test Conditions 72 F (22.2 C) and 50% RH.

68

69

4.1.6 Flavor:

The flavor of reconstituted freeze dried yogurt during
storage was monitored and compared to fresh yogurt. The
sensory panel consisted of three expert taste panelists.
Sensory evaluation was based on the American Dairy Science
Association Flavor Scoring Guide for Swiss Style Yogurt
(see Appendix C). Scoring ranged from a ten for yogurt
with no defects to a one with several pronounced defects.
A score below five indicates one or more serious flavor

defects.

Freeze dried yogurt was packaged in pouches made from three
different materials. These were:

a. Kraft Paper

b. LDPE Film

c. Foil Laminated Film

Freshly made yogurt was rated a nine by the judges (Table
11). Flavor tests of reconstituted yogurt stored in Kraft
paper pouches was discontinued after two weeks because it
was rated unacceptable by the judges. Initially,

reconstituted freeze dried yogurt prior to storage scored

an eight by two judges and a seven by the third (Table 11).

After ten weeks of storage at 70 F and 50% RH, the
reconstituted freeze dried (LDPE) yogurt was evaluated a
seven by two judges and a six by the third. Reconstituted
yogurt (laminated pouch) after similar storage was
evaluated to be a five by two panelists and a six by the

third. Both of these were considered to be acceptable by

TABLE 11: Flavor Ratings of Reconstituted Freeze Dried
Yogurt Based on the Collegiate Dairy Products
Scorecard for Swiss Style Yogurt.

 

Flavor Panelists

Samples Storage A B C
Time (Weeks)

 

Fresh 0 9 9 9

Yogurt

Initial
Freeze Dried 0 8 7 8

a
LDPE 1 7 7 7
2 7 5 7
3 5 5 6
4 3 6 3
r
LDPE 1 8 6 8
2 7 6 7
3 6 7 7
4 6 7 7
10 6 7 7
a
Laminate l 7 6 6
2 6 7 7
3 7 6 7
4 6 6 6
r

Laminate l 8 5 9
2 8 6 7
3 7 6 7
4 6 7 7
10 5 6 6

 

a. Accelerated Conditions, 100 F, 85% RH.
r. Room Conditions, 72 F, 50% RH.

70

71

the panel.

Storage for four weeks under accelerated conditions of 100 F
and 85% RH (LDPE) pouch reSulted in an unacceptable

flavor. However, storage of the dried powder in the
laminated pouch under the same conditions resulted in a
reconstituted yogurt which was evaluated to be acceptable

(Table 11).

Defects that were noted in the sensory analysis of

reconstituted yogurt were:

Bitter Taste

- High Acid

- Lacks Freshness
- Weak Consistency
- Old / Stale

- Lacks Fine Flavor

Results indicate that the laminate pouch showed better
flavor retention during extended storage (accelerated
conditions) as compared to the LDPE pouch because of its

better barrier properties (Table 11).

5.0 CONCLUSIONS

Reconstituted freeze dried yogurt is favorably accepted
after being subjected to extended storage. Longer

shelf life was obtained using a laminate (Polyester / Foil
/ Polypropylene) pouch versus a LDPE pouch or paper bag.
The product had acceptable flavor ratings at the end of ten
weeks of storage under room conditions for both the
laminate and the LDPE pouch. Freeze dried yogurt packaged
in LDPE pouches had similar properties under room
conditions as yogurt in the foil laminated pouch but
showed more deterioration under accelerated conditions as
compared to the laminate pouch. High moisture and oxygen
transmission rendered the third package, the paper bag,

unacceptable within one week of storage.

Color degradation was more prominent in the freeze dried
yogurt packaged in the LDPE pouch versus the laminate

pouch. The pH remained fairly constant during storage in
both package systems. The laminate pouch being a better
moisture barrier maintained a lower moisture content as
compared to the LDPE pouch. Viable counts of both yogurt
organisms decreased during storage in the two packages.
Levels of both organisms decreased by an order of 1 x 106

to 1 x 107 due to freeze drying and storage. Higher initial

counts are therefore recommended prior to freeze drying and

storage to ensure a quality product on reconstitution.

72

73

Some future areas of investigation are:

Use of stabilizers for improved shelf life
Packaging under inert gas or vacuum

Using selected strains of yogurt organisms to
better withstand freeze drying and storage or the
use of cryoprotective agents to protect yogurt
cultures during freeze drying and storage

Flash freeze fresh yogurt to minimize destruction
prior to freeze drying

Storage of freeze dried yogurt at lower
temperatures

Effect of flavor changes in products using
reconstituted yogurt

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Anonymous. 1986. Yogurt Cheese. Dairy Foods, 87(3):46.

Anonymous. 1986. Preservatives: Antioxidents. Food
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Anonymous. 1985. Metallized Materials Move On Foods.
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Anonymous. 1985. Yogurt Provides Cereal's Calcium. Dairy
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Anonymous. 1984. Cultured and Culture - Containing Dairy
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Anonymous. 1984. Yogurt Maintains Market Niche. Dairy
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Anonymous. 1984. Freeze Dried Culture Shipping and
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APPENDICES

r~.----:-~-n;

APPENDIX A

LOW FAT YOGURT FORMULATION

 

 

 

 

Ingredients Amount for 100 lb batch (lbs)
Small Can Large Can
1
Raw Whole Milk 15.0 27.0
Water 17.6 31.8
Non Fat Dry Milk 3.2 5.7
TOTAL 35.8 64.5
2

Yogurt Culture added later 2.4 lbs

 

1. Milk fat was determined to be 3.6% by the M.S.U. Dairy
Plant.

2. Bulk starter culture using Chris Hansen's deep frozen
concentrated R-l yogurt culture

81

”4

MM- *-

APPENDIX B

MILK STANDARDIZATION FOR LOWFAT YOGURT

Raw whole milk contained 3.6% fat as determined by the
M.S.U. Dairy Plant.

Percent Fat Required = 1.5% , 12.6% Solids Non Fat (SNF)
Assumed SNF in skim milk = 8.8%
Calculations: Based on a 100 lb batch.

1. For each 1.5 parts at 3.6% fat use 2.1 parts Non Fat Dry
Milk (NFDM) plus water

2. Whole Milk Required = 100 (lbs) x (1.5 / 3.6) = 41.7 lbs.

3. Reconstituted NFDM Required

100 (lbs) x (2.1 / 3.6)

58.3 lbs.
42.0 - 1.5 = 41.5 lbs

4. Serum Required
5. SNF Present = 100 (lbs) x 12.6% = 12.6 lbs.

6. SNF from Water and NFDM 12.6 - (41.5 x 8.8%)

8.95 lbs.

7. Amount of Water Required = 58.30 - 8.95 = 49.35 lbs.

Therefore a 100 lb batch consists of 41.7 lbs of milk, 49.4
lbs of water and 8.9 lbs of NFDM.

82

APPENDIX C

FLAVOR SCORING GUIDE FOR YOGURT

 

 

Flavor Criticism Intensity of Defect
Slight Definite Pronounced
Bitter 9 7 5
Cooked 9 8 6
Coarse 9 7 5
Foreign 8 7 6
High Acid 9 7 5
Lacks Fine Flavor 9 8 7
Lacks Flavoring 9 8 7
Lacks Freshness 8 7 6
Lacks Sweetness 9 8 7
Low Acid 9 8 6
Old Ingredient 7 5 3
Oxidized 6 4 1
Rancid 4 2 *
Too High Flavoring 9 8 7
Too Sweet 9 8 7
Unnatural Flavoring 8 6 4
Unclean 6 4 1

 

Source: ASDA guide for scoring flavor of swiss style yogurt

83

APPENDIX D

COMPOSITION OF LEE AGAR AND PLATING TECHNIQUES

Preparation for 1 liter (1000 ml) solution.

 

 

Ingredient Amount

Tryptone 10.0 grams

Yeast Extract 10.0 grams

Lactose 10.0 grams

Sucrose 5.0 grams

Calcium Carbonate 3.0 grams

Potassium Hydrophosphate 0.5 grams

Agar 18.0 grams
Bromcresol Purple (BCP) 0.2 grams in 100 ml

distilled water

 

Ingredients were weighed and distilled water added to make 1
liter. Components were thoroughly mixed on a magnetic hot
plate. Prior to sterilization the pH of the medium was
adjusted to 7.0 i 0.1. The medium was sterilized by
autoclaving at 249.8 F (121 C) for 20 minutes. The BCP
solution was sterilized at the same temperature for 15
minutes.

Prior to pouring again, sterile petri dishes should be
chilled to insure uniform distribution of the Calcium
Carbonate. Per one liter of Agar, 10 ml of BCP is used. Agar
must be thoroughly mixed to evenly suspend Calcium
Carbonate. Plates are poured 4 to 5 mm thick and allowed to
solidify. Plates may be refrigerated until used. Incubation
of plates is at 98.6 F (37 C) for 48 hours. Time and
temperature must be exact to assure culture identification.

84

 

APPENDIX E

RECONSTITUTION OF DRIED YOGURT

Based on a 14% solids and 86% water composition in yogurt.

A 100 lb yogurt batch contains 1.5 lbs. fat (1.5%) and 12.6
lbs. SNF (12.6%).

Hence amount of water in
a 100 lb batch of yogurt = 100 - 12.6 - 1.5 = 85.9 lbs

Therefore for each 14 parts of dried yogurt, 86 parts of
water was used for reconstitution.

85

.—n--. _——.._,—
' 1. r A".

 

APPENDIX F

YOGURT SCORE CARD

Flavor

score 10 0.0.0.0000...OOOOOOOOOOOOOOOO No critiCism

Score 9 or less..Bitter Lacks sweetness
Cooked Low Acid
Coarse Old Ingredient
Foreign Oxidized
High Acid Rancid
Lacks Fine Flavor Too High Flavor
Lacks Flavoring Too Sweet
Lacks Freshness Unnatural Flavoring

Unclean

Source: ASDA score card for swiss style yogurt

86

APPENDIX G

WATER VAPOR PERMEABILITY CONSTANT

Water Vapor Transmission Rate:

Weight Gain
WVTR = = 0.0388 g/day
Time in Days

 

 

_ WVTR
P ::
H O AREA (sq.m.) x VAPOR PRESSURE (mm.Hg.)
2
Vapor Pressure
9 100 F, 90% RH = 46.92 x 0.90 = 44.72 mm.Hg.

- 0.0388 g/day x 2.7 mil
P =

 

H 0 0.00541 (sq.m.) x 44.72 (mm.Hg.)
2
P = 0.4333 g - mil / sq.m. - day - mm.Hg.
H O
2
P = 0.0281 g - mil / 100 sq.in. - day - mm.Hg.
H O
2

87

APPENDIX H

PERMEABILITY CONSTANT FOR OXYGEN

- VOLUME OF GAS PERMEATED (CC) x THICKNESS (milS)

 

P =
O AREA (sq.m.) x TIME (days) x PRESSURE (atm.)
2

Film Thickness = 2.7 mils. F
1

Reading from Ox-Tran = 14 mV 1

Ox-Tran Calibration : 1 mV = 100 cc / sq.m. - days a

Gas Permeated from = 1400 cc / sq.m. - days

calibration equation

P = 1400 x 2.7 = 3780 cc - mils / (sq.m. - days
0 - atm)
2
P = 244 cc - mils / 100 sq.in. - days - atm
O
2

88

CO

V I

m 1

CO

CO

CO

APPENDIX I

PERMEABILITY CONSTANT FOR CARBON DIOXIDE

VOLUME OF GAS PERMEATED (CC) x THICKNESS (mils)

 

AREA (sq.m.) x TIME (days) x PRESSURE (atm.)

0.02 (cc) x 2.7 (mils)

 

0.0005 (sq.m.) x 0.0033 (days) x 1.0 (atm)

32741 cc - mils / sq.m. - days - atm.

2112 cc - mils / 100 sq.in. - days - atm.

89

MICHIGAN STATE UNIV. LIBRARIES
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