Yoghurt is a cultured dairy product which is made from the mixing of standardized milk with the activity of a symbiotic blend of Streptoccoccus thermophilus and Lactobacillus delbruekii subsp.Bulgaris (LB) cultures. Usually, the making of yoghurt involved the pretreatment of milk follow by the homogenization, heat treatment, cooling to incubation temperature, inoculation with starter, fermentation , cooling, post fermentation treatment, chilling and lastly packaging for commercial yogurt production. In this experiment, the making of yoghurt followed the batch process. The 1000ml of UHT milk is mixed with some milk powder to increase the content of milk solid non fat in the milk in order to improve the texture with the help of protein functionality. The equipment used for mixing is sterilized so that the contamination of products with phage and spoilage microbes is reduced to minimum. The mixture of milk is then double boil in boiling water to provide a sterile environment for the starter culture to grow while denature whey protein to from a stable gel that disallow the occurrence of syneresis. The heating of the milk is also done to break down the milk constituents to provide nutrients and reducing condition for the starter culture. One of the constituent is Sulphydral compounds which are the nutrients and reducing condition for this starter culture. In addition, heating process also remove air from the medium to aid the anaerobic process that is going to occur during fermentation. When the heated milk reach the desire temperature, it is cooled down in ice water bath. Then two spoon of fresh yoghurt which contained both Streptoccoccus thermophilus and Lactobacillus delbruekii subsp.Bulgaris (LB) cultures are added to the cooled milk. The mixture undergoes fermentation when placed in the incubator.
Based on the result obtained from this experiment, the pH value of the milk at 0 hour before incubation is pH 6.08 which near neutral while the result for the mixture of milk being incubated for 5 hours is pH 4.45. The result show that fermentation have occurred where the strain culture have started to utilized the lactose as an energy source and produced lactic acid which lower the pH value of the milk mixture. Generally, Streptoccoccus thermophilus strain tend to grow first before Lactobacillus delbruekii subsp.Bulgaris (LB) strain generating formic acid that stimulate and aid the grow of Lactobacillus delbruekii subsp.Bulgaris (LB) cultures. As Streptoccoccus thermophilus grow, they respired and removed the oxygen content in the milk to create an anaerobic condition. They responsible for the initial pH drop of the milk mixture. The pH value for the mixture after 24 hours of incubation is pH 4.44 which is much lower than the pH value after 5 hours of incubation. This is because, the longer the incubation process, the longer the fermentation process and the more the production of lactic acid. During this moment also, both Streptoccoccus thermophilus and Lactobacillus delbruekii subsp.Bulgaris (LB) cultures should be growing and worked together to produce lactic acid. These cultures grow rapidly when together than separately. When the acid accumulated, Streptoccoccus thermophilus strain are suppressed as it is less acid tolerant compared to Lactobacillus delbruekii subsp.Bulgaris (LB). Thus, Lactobacillus delbruekii subsp.Bulgaris (LB) will ferment the remaining lactose and decrease the pH value to near pH 4.0. As the pH value go lower and lower, the acid condition cause the casein the milk to curdle and form into yoghurt while giving yoghurt it sour flavour.
The pH value of the yoghurt after 1 week of storage continues to decreased to 4.38. This is because pH of yoghurt will continues to drop during refrigerated storage but with minimum acid production by the culture. Generally, the coagulation of milk starts at pH value below 5.0 and stops at pH 4.6, however the pH value of the yoghurt make in this experiment after the refrigerated storage is lower than pH 4.6. This may due to the high concentration of starter culture added. Thus, the texture of the yoghurt produced is more watery and much sourer when compared with the commercial yoghurt.
As shown in Table 3: Percentage of lactic acid at different time, the amount of lactic acid in the yoghurt at 0 hour before incubation, 24 hours after incubation and 1 week after refrigeration is 0.31%, 1.07% and 1.16% respectively. The results show that the concentrations of lactic acid do increase due to activity of Streptoccoccus thermophilus and Lactobacillus delbruekii subsp.Bulgaris (LB) cultures. The concentration of lactic acid increase dramatically from 0.31% to 1.07% as the activity of both starter culture grow rapidly during fermentation due to the fermentation of lactose to lactic acid. Then, the lactic acid in yoghurt after 1 week of storage is only slightly higher than the concentration of lactic acid in yoghurt at 24 hours of incubation because of the minimum activity of the starter culture.