Partitioning of individual caseins between rennet curd and whey: Impact of pre-acidification and pre-salting of reconstituted skim milk

Abstract

Acidification is one of the essential pre-treatments applied to milk for the manufacture of many cheese varieties while pre-salting is most common in Domiati-style cheeses. Both treatments play several safety and qualitative roles. This research examined the impact of pre-acidification (pH 5–7), pre-salting (0–3 M, NaCl) and combined pre-acidification and pre-salting (pH 5–7 at 3 M, NaCl) of reconstituted skim milk on partitioning of individual caseins between rennet curd and whey. No αs1- and β-caseins were detected in rennet whey from unsalted or pre-salted milk with ionic strength ≤ 0.9 M, NaCl whereas κ- and αs2- caseins were found at all pH and NaCl levels. The greatest loss of caseins (72.9 % of total-casein (T-casein)) into whey occurred when milk with 3 M, NaCl was adjusted to pH 7 whereas pH 6 (10 % T-casein loss) appeared optimal for this milk. Findings for unsalted samples are attributed to soluble κ-/αs2-caseins-whey protein complexes as evidenced by gel electrophoresis. Notably, varying levels of para-κ-casein lost in rennet whey from pre-salted milk reveal inhibition of aggregation phase of renneting which was dependent on ionic strength and pH. Partial inhibition of the enzymatic phase was also evident in highly pre-salted samples (3 M, NaCl) at pH values above or below 6.