Effects of pressurized thermal processing on native proteins of raw skim milk and its concentrate.

Heating, pressurization, and shearing can modify native milk proteins. The effects of pressurized heating (0.5 vs. 10 MPa at 75 or 95°C) with shearing (1,000 s-1) on proteins of raw bovine skim milk (SM, ∼9% total solids) and concentrated raw skim milk (CSM, ∼22% total solids) was investigated. The effects of evaporative concentration at 55°C and pressurized shearing (10 MPa, 1,000 s-1) at 20°C were also examined. Evaporative concentration of SM resulted in destabilization of casein micelles and dissociation of αS1- and ß-casein, rendering CSM prone to further reactions. Treatment at 10 MPa and 1,000 s-1 at 20°C caused substantial dissociation of αS1- and ß-casein in SM and CSM, with some dissociated caseins forming shear-induced soluble aggregates in CSM. The pressure applied at 10 MPa induced compression of the micelles and their dissociation in SM and CSM at 75 or 95°C, resulting in reduction of the micelle size. However, 10 MPa did not alter the mineral balance or whey proteins denaturation largely, except by reduction of some ß-sheets and α-helices, due to heat-induced conformational changes at 75 and 95°C.

Denaturation of selected bioactive whey proteins during pasteurization and their ability to modulate milk immunogenicity.

This research communication relates to the hypothesis that the consumption of raw or unprocessed cow's milk contributes to lowered prevalence of allergies. Thermal pasteurization of bovine milk can result in denaturation of minor whey proteins and loss of their bioactivity. Denaturation of bovine serum albumin (BSA), immunoglobulin G (IgG) and lactoferrin (LF) in skim milk was studied under different temperature (72, 75 or 78°C) and time (0-300 s) combinations. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) results revealed that denaturation of all 3 proteins occurred at 72°C and progressed with increase in temperature and holding time. About 59% of LF and 12% of IgG denatured under high-temperature short-time (72°C/ 15 s) pasteurization, while BSA was least impacted. To assess modulation of milk immunogenicity, secretion of selected T helper (Th)-type cytokines by human peripheral blood mononuclear cells (PBMCs) was studied in vitro in response to different concentrations of BSA (0.4-1.0 mg/ml) and IgG (0.8-1.6 mg/ml) in unheated skim milk. Addition of IgG at 1.6 mg/ml induced a prominent Th1-skewed cytokine profile that may not trigger a Th2-skewed allergic reaction. BSA did not appear to modulate milk immunogenicity to any significant extent.

Impact of controlled shearing on solubility and heat stability of pea protein isolate dispersed in solutions with adjusted ionic strength.

Pea protein isolates (PPI) have sustained an increasing demand in the food industry as a substitute for animal-origin proteins. Shearing is an integral part of food processing that can change properties of proteins and their functionality. PPI dispersions prepared at 4 or 8% concentration (w/w protein), pH 6.8 or 7.5 and under ionic strength (IS) 100, 200 mM or non-adjusted, were subjected to controlled shearing at two levels (100 or 1500 s-1) during heating at 90 °C for 5 min. All main factors had substantial effects on the tested dependent variables. Shearing at 1500 s-1 significantly improved the solubility and heat stability of 4% PPI at pH 6.8 or 7.5 and IS-100 or 200 mM by 27-43% in comparison to 100 s-1. Following 1500 s-1 treatment, all PPI dispersions showed >85% solubility and heat stability except 4% PPI at pH 6.8 and IS 100 mM (60%). Shearing appeared to alter structural and physicochemical properties of pea proteins as well nature of protein aggregation. Heating accompanied with 100 s-1 shearing mostly resulted in insoluble covalent aggregates while shearing at 1500 s-1 mainly contributed to formation of soluble hydrophobic aggregates.

Influence of lactic, citric and phosphoric acids on the properties of concentrated lactose solutions.

Physicochemical and thermal characteristics of concentrated lactose solutions containing 0.05, 1, or 4% (w/w) of three acids commonly used in the food industry, i.e., lactic, citric, and phosphoric acid, were studied. Properties of both lactose and water were affected by all acids studied. Thermographic analysis showed that interactions between water and acids hindered evaporation of water from most of lactose solutions. This effect was mostly related to the formation of a strong hydration layer around lactose molecules by hydrogen bonds. Acid-induced hydrolysis of lactose into glucose and galactose varied depending on the concentration, hydrolytic power of acids and molecular interactions in the system. The study concluded that the varying physical, chemical, structural and thermal characteristics of lactose as affected by the presence of different acids was mainly due to the manipulation of water-lactose interactions, whereas the hydrolysis of lactose by the acids plays a smaller role.

Structural changes of native milk proteins subjected to controlled shearing and heating.

Processing of milk results in structural modifications of proteins creating a foundation for various interactions. The present study aimed at identifying the effects of simulated processing conditions, the combination of temperature and shear, on native proteins in raw skim milk. The temperatures chosen (72 and 140 °C) were combined with selected shear rates (0, 500, or 1000 s-1) during processing. Impact of shear appeared temperature dependent, but it induced either reversible or irreversible changes in the secondary structure of milk proteins at all temperatures. Increase in shear may result in reversible structural modifications at 20 °C, while it could contribute to fragmentation of hydrophobically-linked protein aggregates at 500 s-1 and also reformation at 1000 s-1 during heating at 72 °C. The shearing at 140 °C appeared to enhance the formation of protein aggregates primarily by hydrophobic interactions, as well possibly thiol/disulphide interactions to a lesser extent.

In vitro immunogenicity of various native and thermally processed bovine milk proteins and their mixtures.

In vitro immunogenicity of various native and thermally processed (72°C/15 s and 100°C/30 s) bovine milk protein fractions, their mixtures, whey, and skim milk, was studied by analyzing the immune response of T helper (Th) cells in human peripheral blood mononuclear cells. The secretion of Th type cytokines induced by the protein stimulants was quantified while determining the heat-induced protein denaturation. Purified whey proteins, caseins and whey fraction, and skim milk provoked substantial immune responses at various degrees, indicating their potent immunogenicity. The protein mixtures prepared using the fractionated whey proteins with or without caseins appeared less immunogenic in both native and heat-treated forms, implying their potential of producing less immunogenic dairy products. The 100°C/30 s treatment significantly altered the immunogenicity of most of the potent protein stimulants, which mostly coincided with their levels of protein denaturation. The 72°C/15 s treatment caused the least protein denaturation but altered the immunogenicity of several protein stimulants notably, including heat-stable caseins and α-lactalbumin.