The optimisation of accelerated yogurt production

[摘要] English: Yogurt is one of the most popular of the dairy product range and a reduction in price would increase its popularity. If ways to accelerate the fermentation process could be found, making it possible to produce larger quantities with existing infrastructure, the affect would be reduced unit costs. This in turn should lead to a reduced shelf price. It was therefore decided to undertake a study into identifying a way or ways to accelerate the process without affecting the quality of the final product. Initially two main problems with the current yogurt fermentation process were identififJd. Firstly, the variation in the composition of fresh milk influences the repeatability of independent yogurt fermentations. This problem was eliminated by substituting fresh milk with milk powder. : Secondly, the need arose to standardise the starter culture's inoculation load and grpwth stage. In order to do this, a suitable growth medium which enables spectrophotometrical growth rtJonitoring was developed. This medium is referred to as milk serum in this study. The standardisatipn of the preinoculum load enabled repeatability and resulted in a 17 % decrease in fermentation tire. Thus, any variation observed in fermentation time can be attributed to the specific suppleme~t or condition evaluated. The development of the milk serum was an important breakthrough because it enabled 'the evaluation of the impact of several supplements and physical conditions on the yogurt fermentation! process. Various supplements were evaluated. They included a wide array of vitamins, minerals, sugars and amino acids. It was found that only a small number were capable of decreasing the fermentation time, and in those instances the final product was unsuitable for human consumption. This is an indication that the starter bacteria do not have a deficiency for any of those supplements when milk was used as growth medium. It could therefore be concluded that milk is an ideal and complete growth medium for the starter organisms. After the evaluation of supplements had been completed, the effect of different conditions on the fermentation process was investigated. This included proteolytic treatment, eleatrical current application, pressure applications, de-aeration and different incubation temperatures. The treatment of milk with a commercial proteolytic enzyme prior to fermentation increased the pH at which the yogurt coagulated marginally and also resulted in a sweeter product. Milk pre-treated with an electrical current did not significantly affect the free calcium content, which indicates that the structure of the casein micelle was still intact. Various pressure treatments of yogurt milk did not significantly influence the yogurt fermentation time. It did however influence the yogurt setting pH. As the pressure treatments increased, the setting pH subsequently also increased, which resulted in a sweeter yogurt. I The de-aeration of milk did not significantly influence the yogurt fermentation time or the setting pH. I Pasteurisation of milk resulted in the decrease of yogurt fermentation time in ~omparison to unpasteurised milk. During studies done on various incubation temperatures, the optimal growth temperatures of the respective starter bacteria was clearly noticeable/visible. The individual yogurt batches obtained by incubations at 39 °C, 42 °C and 45 •c exhibited properties unique to each of the two starter bacteria. At the optimal growth temperature of L. bulgaricus, 45 °C, the yogurt exhibited hig,her lactic acid content (pH reduction), whereas the yogurt produced at 39 °C, the optimum growth terpperature of S. thermophilus, had increased slime production. It was evident during incubation temperature studies that the incubation temperature during yogurt manufacture influences the rate lof lactic acid production, as well as the rheological properties of the final product. The results obtained in this study indicate that further research, whether it be to decrease yogurt fermentation time or to improve the product, would be of great value.

[发布机构] University of the Free State