Impact of the Supplementation with Partially Chymosin-Proteolyzed Buffalo's Total Milk Proteinate on the Properties of Cow's Set Yoghurt

The aim of this study was to experiment the supplementation of cow's yoghurt milk with partially chymosin-proteolyzed buffalo's TMP the relation to compositional, bacterial, biochemical, rheological and organoleptic attributes of the resultant product. Four treatments including the control were designed, where cow's full cream milk powder (FCMP) was reconstituted whether at the level of 14% FCMP with tap water (the control) or at the level of 13% FCMP with solution containing 1% TMP in the native form (i.e. non enzymatic hydrolyzed) or solution of TMP proteolyzed by chymosin for 4 or 8 h. Yoghurt milks were heat treated at 85 degrees C for 5 min. followed by temperature adjustment to 42 degrees C at which they were inoculated with 2% of activated yoghurt bacterial starter culture, filled into 100 ml polyvinyl chloride containers, covered, and incubated until complete coagulation (through about 3 h.). Thereafter, the container were transferred to the refrigerator (5 +/- 1 degrees C), where they were kept for the periodical analyses. The results indicated that, the viscosity value as well as the capacity and stability of the emulsion property of solution containing 5% TMP proteolyzed by 1% chymosin enzyme decreased as the proteolysis time prolonged. On the other hand both of the water soluble nitrogen (WSN) and non-protein nitrogen (NPN) of TMP solution increased as the proteolysis time increased. No significant differences in both of moisture, fat, titratable acidity contents as well as the Log count of lactic acid bacteria between yoghurt treatments but the supplementation with 1% TMP, whether in the native or rather in the proteolyzed form, was associated with significantly gradual increases in the protein, WSN/TN, NPN/TN contents as well as pH value and decrease in the ash content of yoghurt. The prolonging of cold storage period (CSP) was associated with significant increase in yoghurt acidity. Neither yeasts & molds nor total coliform exhibited any considerable count in yoghurt along CSP. Although the cohesiveness criterion was not influenced, the hardness of yoghurt strengthened either due the presence of TMP or/and by the progressing of the cold storage period. Moreover, the degree of TMP proteolysis was associated with increase of yoghurt hardness. The adhesiveness appeared opposite trending to those of the hardness in all cases. The control sample possessed a value higher than that of non proteolyzed TMP. The TMP proteolysis whether for 4 h or rather for 8 h heightened the springiness values of yoghurt supplemented therewith. The gumminess value of yoghurt heightened when TMP added and as TMP was proteolyzed or cold stored. Chewiness behaved similar trending to gumminess in yoghurt when fresh. At the end of CSP, chewiness value of the control and yoghurt containing native TMP decreased, while those of proeolyzed TMP whether for 4 or 8 h increased. All TMP containing yoghurt gained the highest judging score and the sample of native or 4h-proteolyzed TMP kept the best sensory preference until the end of CSP. It can be concluded that: Although the TMP derivative is a protein compound, its proteolysis with chymosin, which kappa-casein is its preferred substrate, resulting in the liberation of peptide, para kappa-casein that helped more in strengthening the weak curd, from which cow's yoghurt is suffered and homogenizing its consistency, provided that the period of proteolysis does not exceed 4 h by 1% enzyme.