Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


The main objective of this research was to determine the effect of psychrotrophic bacterial contamination and high moisture curd on the major chemical components and flavor of Cheddar cheese. Also, it was of interest to establish chemical profiles of "good" and "bad" Cheddar cheeses and to gain information which would aid in the prediction of Cheddar cheese flavor. The research was divided into two parts. Part I concentrated on the effects of psychrotrophs in raw milk on Cheddar cheese quality. Part II examined the effects of psychrotrophic and coliform post-pasteurization contamination, high moisture curd and low activity starter on Cheddar cheese quality. After processing, Cheddar cheese was stored at 7(DEGREES)C and sampled at 0, 5, 30, 60 and 180 d. Analyses performed included: sensory evaluation, yield, microbiological profile, proteolytic activity, gas chromatography with headspace sampling (GCHS) and high performance liquid chromatography (HPLC). Data revealed that the control cheese had significantly superior flavor and body/texture scores than all treated cheeses. Yield of the control cheese was significantly higher than cheese made from psychrotrophic-treated raw milk. Statistically higher levels of citrate, acetone, 2-butanone, proteolytic activity, lactate, ethanol, propanol, formate and propionate and lower levels of acetate, 2-pentanone, pyruvate, pH and salt were noted in "bad" Cheddar cheese as opposed to "good" Cheddar cheese. All microbial counts tended to decrease while the proteolytic activity tended to increase with storage time. HPLC analysis indicated significant increases of citric, pyruvic, lactic, formic, acetic and propionic acids with aging of the cheese. Only orotic acid decreased with aging. GCHS values indicated increases of acetone, 2-butanone and ethanol with aging; 2-pentanone decreased with aging. Excellent Cheddar cheese had low concentration of short chain free fatty acids (FFA). Young Cheddar cheese contained low concentration of FFA while aged, desirably-flavored Cheddar cheese had intermediate concentrations of individual FFA. Aged cheeses treated with psychrotrophs were signaled by measurement of high concentrations of individual short-chain and long-chain FFA. Finally, regression equations were established which laid a foundation for further study in the area of flavor prediction.