HEAT-INDUCED CHANGES IN VOLATILES OF MILK AND EFFECTS OF THERMAL PROCESSING ON MICROBIAL METABOLISM OF YOGURT

Abstract

Flavor and texture of yogurt are highly associated with the consumer acceptability. The volatiles in the headspace of heated milk and yogurt were analyzed to assess the effects of preheat treatment on microbial metabolism. Seventeen and 25 volatiles were found in fresh raw milk and reconstituted milk using Solid Phase Microextraction-Gas Chromatography-Mass Spectrometry, while 23 and 31 volatiles were investigated in their yogurt products, respectively. The levels of aldehydes, methyl ketones, alkanes, olefinic aldehydes and ketenes increased in liquid milk with increasing the intensity of heat treatments. Meanwhile, their levels in respective yogurt showed similar changes. 2-Pentanone, 2,3-butanedione and 2,3-pentanedione that were metabolized by starter culture organisms decreased due to increasing the intensity of heat treatments, suggesting that these heat treatments can inhibit the flavor metabolism of organisms. By analyzing the texture profile of yogurt, hardness, adhesiveness and gumminess increased with increasing the intensity of heat treatments; however, little difference was observed between the raw milk samples and the reconstituted milk samples. PRACTICAL APPLICATIONS This study demonstrated the differences in flavor impact compounds and textural properties of yogurt due to the preheat treatment and microbial metabolism. Results explain the source of volatile compounds in yogurt processing. Information on the changes in flavor as affected by heating processes would help in understanding their suitability in yogurt production. The research on flavor metabolism would shed light on the final application of some approaches to improve yogurt such as modification of milk medium, screening of the flavor-producing bacteria and the processing control of flavor production. Also, the finding could be used in the production of flavoring agent.