Comparison of heat and pressure treatments of skim milk, fortified with whey protein concentrate, for set yogurt preparation: effects on milk proteins and gel structure

Abstract

Heat (85 degrees C for 20 min) and pressure (600 MPa for 15 min) treatments were applied to skim milk fortified by addition of whey protein concentrate. Both treatments caused > 90 % denaturation of beta-lactoglobulin. During heat treatment this denaturation took place in the presence of intact casein micelles; during pressure treatment it occurred while the micelles were in a highly dissociated state. As a result micelle structure and the distribution of beta-lactoglobulin were different in the two milks. Electron microscopy and immunolabelling techniques were used to examine the milks after processing and during their transition to yogurt gels. The disruption of micelles by high pressure caused a significant change in the appearance of the milk which was quantified by measurement of the colour values L*, a* and b*. Heat treatment also affected these characteristics. Casein micelles are dynamic structures, influenced by changes to their environment. This was clearly demonstrated by the transition from the clusters of small irregularly shaped micelle fragments present in cold pressure-treated milk to round, separate and compact micelles formed on warming the milk to 43 degrees C. The effect of this transition was observed as significant changes in the colour indicators. During yogurt gel formation, further changes in micelle structure, occurring in both pressure and heat-treated samples, resulted in a convergence of colour values. However, the microstructure of the gels and their rheological properties were very different. Pressure-treated milk yogurt had a much higher storage modulus but yielded more readily to large deformation than the heated milk yogurt. These changes in micelle structure during processing and yogurt preparation are discussed in terms of a recently published micelle model.

A rheological study of acid-set "simulated yogurt milk" gels prepared from heat- or pressure-treated milk proteins

“…Extensive denaturation (i.e., 91.4%) was evident after pressure treating WPI at 700 MPa which was comparable to, although slightly lower than, that caused by heating at 78 o C × 30 min (i.e., 93.7%). These results are comparable to those of Needs et al [26] who reported that both heat (80 o C × 20 min) and pressure (600 MPa × 15 min) treatments caused > 90% denaturation of β-lactoglobulin in skim milk. In an earlier study, Van Camp et al [40] found that the amount of native β-lactoglobulin in WPC was reduced significantly after pressurisation at 400 MPa for 30 min, in contrast to α-lactalbumin, for which no major changes were evident.…”

High hydrostatic pressure technology in dairy processing: a review

“…Acid gels obtained from high-pressure homogenized milk showed a linear decrease in whey holding capacity and retention of more than 20% whey after centrifugation for 25 min (Hernandez and Harte 2008). Similarly, Needs et al (2000) recorded lower values of fracture stress in set yoghurts made from pressure treated milk (60 MPa for 15 min) compared to heat treated milk. Yoghurt prepared from milk that was ultra high pressure homogenized at 200 and 300 MPa at 30 and 40°C considering modifications induced in the fat fraction that could delay the lipid oxidation and lower the degree of lipolysis, resulted in expected results (Serra et al 2008).…”

“…They observed that milk subjected to HHP treatment and thermal treatment followed by HHP, loses its white colour and turns yellowish. This might be due to reduction in size of casein micelles (Needs et al 2000). Whereas milk when first subjected to HHP followed by thermal treatment regained its whitish colour and this may be attributed to reversible nature of casein micelles (or reaggregation of disrupted micelles) towards HHP treatment when applied in the range of 300-676 MPa followed by thermal treatment.…”

Negative effect of heat sterilization on the free amino acid concentrations in infant formula

“…This advanced Maillard reaction may have additional undesirable consequences for nutrition (Kilshaw et al, 1982;Adrian, 1974;Rigo et al, 1994). The FAA composition or bioavailability in formulas may also be altered during processing and enzymatic hydrolysis (Donovan and Lonnerdal, 1989;Recio and Olieman, 1996;Needs et al, 2000).…”