Dissociation and coagulation of caseins and whey proteins in concentrated skim milk heated by direct steam injection

Abstract

Heat treatment of concentrated milk systems for preservation and long shelf life or at least a sufficient removal of food pathogens prior to spray drying is a crucial step due to the decreasing stability of these systems toward heat compared to unconcentrated milk. Heat-induced coagulation is observed when temperature-time combinations for the achievement of certain microbial inactivation effects are higher than the heat and colloidal stability of the concentrated milk system allows. In this work, the effects of direct steam injection on the stability of casein micelles in concentrated skim milk (CSM) of 18, 23, and 27% total non-fat solids, heat-treated by direct steam injection (DSI), were investigated. Quantitative differential centrifugation for the separation of aggregates, casein micelles, dissociated submicellar particles, and soluble proteins and subsequent analysis of caseins and whey proteins within these fractions by RP-HPLC were applied. Quantitative separation was monitored by particle size measurements. The dissociation of κ-casein as well as an increase in casein micelle hydrodynamic radius were observed to increase with increasing total solid content of CSM, heat treatment time, and temperature. Heat-induced dissociation of β-Lg-κcasein complexes at a critical level of 30-35% was found to induce severe coagulation of κ-casein-depleted calcium-sensitive casein micelles in CSM heated by DSI. Dissociated and aggregated proteins were found to be present as distinct colloidal particle classes differing in size from casein micelles.

Modelling of heat stability and heat‐induced aggregation of casein micelles in concentrated skim milk using a Weibullian model

“…The relevance of dissociated protein is, on the one hand, linked to the relative amount of protein present in the serum phase after complete coagulation, that is the lower asymptote of the Weibullian model, and on the other hand, heat-induced coagulation which is often related to dissociation of j-casein as one of the main detrimental effects on casein micelle colloidal stability. Using direct steam injection, dissociation of j-casein at the onset of coagulation was also found to be comparably low by Dumpler et al (2017). In the study of Dumpler et al (2017), j-casein was not included in protein aggregates together with calciumsensitive caseins to a large extent.…”

“…Using direct steam injection, dissociation of j-casein at the onset of coagulation was also found to be comparably low by Dumpler et al (2017). In the study of Dumpler et al (2017), j-casein was not included in protein aggregates together with calciumsensitive caseins to a large extent. The reason for the lower amount of coagulable protein in CSM using the tube heating system as compared to direct steam injection should be clarified.…”

“…Supernatants were analysed for soluble caseins by RP-HPLC and were also further fractionated by ultracentrifugation at 48 800 g/26 min. Centrifugal and ultracentrifugal supernatants were analysed for caseins as described by Dumpler et al (2017). Heat treatments of two individual samples at each temperature-time-total solids combination were performed at least in duplicate, and each sample was analysed (n = 4).…”

“…Centrifugal and ultracentrifugal supernatants were analysed for soluble caseins after dilution in guanidine buffer for complete solubilisation of proteins as described by Dumpler et al (2017) with modification of the elution gradient. It was observed that whey protein peaks became blurred and 'disappeared' from the chromatograms after about 200 s of heating.…”

“…In the previous paragraph, we referred to the dissociation of caseins from casein micelles that become nonsedimentable as they dissociate into the serum phase and form colloidal particles as shown by Dumpler et al (2017), either during heating or after cooling as assumed by Anema and Li (2000). The relevance of dissociated protein is, on the one hand, linked to the relative amount of protein present in the serum phase after complete coagulation, that is the lower asymptote of the Weibullian model, and on the other hand, heat-induced coagulation which is often related to dissociation of j-casein as one of the main detrimental effects on casein micelle colloidal stability.…”