The effect of processing method, storage temperature and length of storage on chemical and sensory attributes of low-fat milk

Ultrapasteurized milk (UP) stored at 4, 7 or 10°C for up to 36 d and high temperature-short time (HTST) pasteurized milk stored at 4 or 7°C for up to 15 d were analyzed to determine the effect of processing method and storage time and temperature on intensities of sensory characteristics and concentrations of free fatty acids and volatile compounds, including sulfur. Eight characteristics, bitter, buttery, cooked, feed, painty, rancid, sour and sweet were selected by a trained panel to describe UP and HTST milk. Intensities of bitter, painty and rancid increased between 8 and 15 d in UP and HTST milk stored at 4 or 7°C. Intensity of rancid flavor was greater in milk stored at 7°C compared to 4°C. UP milk had higher intensities of painty, rancid and buttery flavor than HTST milk. Fourteen fatty acids ranging from 4 to 18 carbons in length were identified in the ether-hexane extracts of UP and HTST milk. Hexanoic, octanoic, decanoic and dodecanoic acids increased in content during storage of UP and HTST milk, however, only decanoic and dodecanoic acids were significantly influenced. Decanoic and dodecanoic acids were positively correlated with intensities of painty and rancid flavors. Seventy volatile compounds, including aldehydes, ketones, a lactone, furans, an alcohol, an acid, straight and branched chain hydrocarbons, aromatic and sulfur compounds, were isolated using simultaneous distillation extraction and analyzed using gas chromatography. Concentrations of hexanal and unknown 18 decreased in concentration during storage whereas concentration of 2-nonanone increased in UP and HTST milk. Hexanal was negatively correlated with painty flavor. Three sulfur compounds (dimethyl sulfide, methyl isothiocyanate and benzothiazole) were identified in milk extracts. Benzothiazole concentration was influenced by the interaction of processing method and storage time and temperature. Dimethyl sulfide concentration increased in UP milk with increased temperature. In HTST milk, content decreased with increased temperature. The data indicate that short to medium chain fatty acids contribute to painty and rancid flavors. The characteristic identified as painty appears to be the same as the stale characteristic identified by other researchers.