Potato taste defect (PTD) is described as potato-like or peasy flavor that is perceived in coffee beverages. Consequently, it has been an obstacle in the coffee business affecting the value chain from producers to consumers. The quality of coffee is evaluated by professional cuppers who take decisions, that in turn determine the price of coffee. Hence, a good price is offered to a superior quality and defect-free coffee such as specialty coffee. Since PTD is detrimental to coffee quality, various research studies have been carried out to understand its causes. Pyrazines, particularly 2-isopropyl-3-methoxypyrazine (IPMP) and 2-isobutyl-3-methothoxypyrazine (IBMP), were identified as the main compounds associated with PTD in coffee. However, existing information on development of these compounds and their extent to produce detectable PTD are limited. Cupping is the only method that has been applied by the industry to detect PTD in coffee. However, its efficiency has not been documented. This project was conducted in Rwanda with the aim of investigating, predicting and managing the occurrence of PTD in coffee. Gas Chromatography - Mass Spectrometry (GCMS) was applied to identify and quantify IPMP and IBMP in green and roasted coffee beans collected from coffee washing stations in Rwanda. The occurrence of PTD in coffee was assessed by trained professional cuppers using the commercial standard cupping method. The same cuppers were assessed for their efficiency to detect PTD and determine sensory qualities of coffee using a method of generalizability theory. Furthermore, the best estimate thresholds (BET) of cuppers to detect IPMP, IBMP and a blend of IPMP-IBMP dissolved in water and coffee beverage were determined. Based on GCMS analysis of 32 coffee samples, the mean concentrations of IPMP and IBMP were, respectively, 20.7 ℗ł 1 ng/g and 85.8 ℗ł 0.9 ng/g in green beans and 114.8 ℗ł 0.7 ng/g and 158.1 ℗ł 2.7 ng/g in roasted beans. Logistic regression analysis identified a relationship between PTD occurrence and two potential predictors, IPMP concentration in green beans and the ratio of IBMP to EDMP (2-ethyl-3,5-dimethylpyrazine) in roasted coffee beans. Coffee roasting impacted the contents of IPMP and IBMP in coffee a non-linear manner. Two main phases were observed, with loss of the two compounds at temperatures below 100℗ʻC, described by an exponential decay regression model; and formation of the compounds at roasting temperatures above 120℗ʻC, described by logistic dose response model. A panel of cuppers who were assessed demonstrated mean sensory detection thresholds of 0.7 ng/L, 1.3 ng/L, 1.4 ng/L for IPMP, IBMP and a blend of IPMP-IBMP in water, respectively. When the thresholds were measured in coffee, higher values were obtained with IPMP, IBMP and their blends detected at 110 ng/L, 384 ng/L, and 66.7 ng/L; respectively. The assessment of cuppers' efficiency has demonstrated a disagreement among cuppers to identify samples with detectable PTD, indicating a poor performance of cuppers. Generally, this project demonstrated that the prediction of PTD occurrence in coffee was influenced by the random distribution of PTD-associated pyrazines in coffee beans, the roasting profile and the variations in sensory sensitivity of cuppers to detect PTD. Consequently, regular refresher trainings of cuppers are recommended to improve PTD cupping efficiency. This research generated new knowledge with respect to the impact of roasting profiles on concentrations of PTD-associated compounds in coffee. The identified concerns that affect the efficient detection of PTD will open up opportunities for further research to enhanced understanding of the causes and origin of PTD in coffee.
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