STUDIES ON AGE GELATIONAND SEDIMENTATION OF UHT PROCESSED MILK DURING STORAGE

Abstract

Milk sterilized by the use of UHT temperature tends to be unstable during storage. This instability is a critical aspect of quality of UHT milk which limits its use. Age gelation and sedimentation of UHT milk are serious problem in milk processing plants in Pakistan, which lead to decreased profit margin and hinder the commercial exploitation of UHT milk. The research work was carried out to evaluate the effect of milk quality (commercial and improved quality milk), UHT processing temperature (135 o C, 138 o C, 141 o C and 144 o C), storage temperature (25 o C, 40 o C) on physicochemical characteristics, protein and protein fractions, proteolysis, age gelation (viscosity), sedimentation and sensory evaluation during storage of 90 days. Raw milk quality of was assessed through sensory parameters, adulteration and microbiological examination showed that commercial milk was found positive for some adulterants and possessed higher number of TPC and SCC than IQ milk. The pH of commercial and IQ milk samples were in the range 6.61-6.63. Commercial milk possessed lower fat, protein, solids not fat, lactose and total solids than the IQ milk, except ash content which was found higher in commercial milk. The commercial milk also contained lower concentration of calcium and phosphorus as compared to the IQ milk. IQ milk possessed more content of casein than commercial milk which was found significantly higher in non casein nitrogen and non protein nitrogen, respectively. The viscosity was higher in case of commercial milk than the IQ milk. It was found from the physicochemical analysis that the IQ milk had significantly the highest pH value and lower acidity than the commercial milk after UHT processing. However the acidity was higher and pH was lower in the UHT milk when stored at 40 o C than at 25 o C (6.59). Significantly the lowest (6.55) pH at144 o C and highest (0.132%) acidity at 135 o C was observed. Lactose content of different UHT processed milk indicated the commercial milk contained significantly the lowest content of lactose (3.79%) as compared to IQ milk (4.04%). Lower lactose content was observed in milk stored at 40 o C the lactose content decline in UHT milk, processed at a temperature 135 o C was lower than milk processed at 144 o C temperature. Commercial milk contained significantly lower (2.64%) casein content and high NCN and NPN content 0.63% and 0.25%, respectively than of IQ milk which contained higher casein and lower NCN and NPN content. The influence of storage temperature indicated highest casein content and the lowest NCN and NPN in milk stored at 25 o C and the lowest casein and significantly the highest NCN and NPN at 40 o C temperature. The processing of milk at 144 o C contained higher casein content and the lowest NCN and NPN content at 144 o C temperature. Less proteolysis was observed in IQ milk processed at 144 o C and stored at25 o C temperature which may not be directly related to age gelation but may directly or indirectly related to sedimentation. The viscosity of milk processed at 144 o C showed the lowest viscosity the highest viscosity was found at 135 o C temperature. The occurrence of sedimentation was significantly higher in commercial milk while lower sedimentation (4.15g) was exhibited by the IQ milk. Higher storage temperature (40 o C) yielded higher sedimentation and increasing trend was also found with the increasing UHT temperature. Sensory parameters based on scores assigned by the panelists are higher in IQ milk than commercial milk. The study further concluded that milk subjected to 141 o C UHT temperature was ranked at top based on the sensory scores.