Preparation And Ripening Characterization Of Fuctional Low-fat Cheese With Lactobacillus Acidophilus

The processing techniques of functional low-fat cheese made with Lactobacillus acidophilus were studied in this dissertation, including degreasing process, starter culture, adding rennet , cutting, mixing, salting and other approaches associated with the functional low-fat cheese quality (especially the viable count ). With the optimal techniques, fresh functional low-fat cheese was prepared, and then incubated at 4℃until maturation. In the phase of cheese maturation, the microbiological quality, physicochemical and sensory properties were monitored.In the first step, the optimal temperature of milk preliminary heating and rotation rate of separtor were confirmed to keep low-fat content of milk. The results showed that the fat content of skimmed milk was decreasing with the increasing preliminary temperature and separator rate. The optimized conditions were set as 50℃of preliminary tempertaure and 7000r/min of separate rate.Subsequently, L. acidophilus, S.thermophilus, and Bifidobacterium were screened for functional agents of cheese by analyzing their growth and acid production characterizations in the starter culture. The selective harvest times were determined according to the growth curves of three lactic acid bacteria. The effects of types and amounts of starter cultures on pre-acidification time and viable count were studied.The results showed that the starter culture mixed with L. acidophilus and S. hermophilus (Ratio of 1:1) work well, and the optimal addition was 6%.The selective rennet conditions in low-fat cheese were determined by the curd procedure orthogonal experiment. The optimized parameters were: rennet additions 0.01% (w/v), curd temperature 35℃, curd pH 6.0, and CaCl2 addition 0.02% (w/v). With the optimal techniques, the low-fat cheese had highest yield 25.5%(w/w), and milk whey had lowest non-fat-solid content 7.5%(w/v). The Viable count was above 107 CFU/mL, the low-fat cheese was good taste. The procedure after curd inculded curd stirring and burning brining, the final optimal parameters were 30 min of stirring and burning time, 40℃of temperature, and 4%(w/w) of salt concentration.After preparing the fresh low-fat cheese, the low-fat cheese was vacuum packed, and ripened at 4℃. In order to study the effect of fat content on cheese quality, the full-fat cheese acted as the control substance was made by the same techniques above and its ripened characterizations were monitored too. The low-fat cheese added with Lactobacillus acidophilus had higher viable count and lower living contaminant comparing to full-fat cheese after 90 day ripenning. The viable counts of low-fat cheese was 7.67 log(CFU/g)(full-fat cheese: 7.57), the counts value of L. acidophilus was 7.63 log(CFU/g)(full-fat cheese: 7.46),the counts value of mould was less than 10 CFU/g(full-fat cheese: less than 50),yeast count was less than 50 CFU/g(full-fat cheese was less than 50),the counts value of coliform group was less than 30 CFU/g(full-fat cheese: more than 50).The viable counts and L. acidophilus counts remained viable in low-fat cheese for up to six months having counts of over 107 CFU/g,but their counts in the full-fat cheese decreased blow 106 CFU/g after six months.After the ripen period, the low-fat cheese had higher pH value 6.34 (full-fat cheese 5.04) and moisture content 54.38%(w/w)(full-fat cheese 40.98%, w/w) when compared to the full-fat cheese by investigating the change of moisture, pH, ash content in cheese, and the ash content of low-fat cheese (4.43%, w/w) higher than full-fat cheese (3.74%, w/w).Meanwhile, the protein content, fat content, soluble nitrogen (SN) content at pH 4.6, and in 12% (w/v) trichloracetic acid (TCA-SN) of cheese were obtained, in order to determine the change tendency of proteolysis and lipolysis of low-fat cheese and full-fat cheese in ripen period. These results showed that protein content of low-fat cheese was much higher than full-fat cheese (the protein contents of low-fat cheese and full-fat cheese were 40.45%(w/w) and 28.68% (w/w) respectively after 90 d ripened), and the protein hydrolysate content of low-fat cheese was higher than that of full-fat cheese too. (the pH 4.6 SN value and 12% TCA-SN value were 4.97% and 5.17%, w/w respectively,correspond indexes in full-fat cheese were 3.88% and 3.74%, w/w). The same phenomenons were verificated in electrophoresis test and amino acids mensuration of both cheeses, meanwhile, flavor substances of both cheeses were detected by GC-MS. The result showed that full-fat cheese has more types and contents of flavor components than low-fat cheese(these were 28 types of main flavor components in full-fat cheese, and 20 types in low-fat cheese.)Finally, the textual properties of cheeses were tested during the 90 days of maturation, meanwhile the microstructure of cheese were observated by scanning electron microscope. These results showed that the low-fat cheese had higher hardnese, gumminess and chewiness than full-fat cheese, but values of springiness, cohesiveness and adhesiveness were lower than full-fat cheese. The optimal ripen period of low-fat cheese was 90 day according to sensory evaluation, and the ripen period of full-fat cheese was longer than low-fat cheese.The new ideas: the functional low-fat cheese with L. acidophilus has good quality of remaining high vaible counts and having trophic function, and the optimal techniques were obtained in this experiment by using a English cheese machine, so the optimal techniques can be using in industrialized production. For above reasons, the the functional low-fat cheese with L. acidophilus will has a good development perpective in dairy product market.