Impact of cryoconcentration on casein micelle size distribution,micelles inter-distance,and flow behavior of skim milk during refrigerated storage

Cryoconcentration combined with a cascade effect was used to concentrate skim milk up to 25.12% total dry matter. Size, shape, and inter-micellar distance of casein micelles were characterized by ZetasizerNano-ZS, transmission electron microscopy, and ImageJ analyses. Flow properties of the cryoconcentrated skim milk were evaluated during 5 weeks of storage under refrigerated condition at 4 °C. Milk color was also evaluated according to the L*, a*, and b* system. The cryoconcentrated skim milk obtained after three cryoconcentration cycles was characterized by a monomodal distribution of its micelles with a tendency to smaller casein micelles. Approximately 60% of the total micellar volume was occupied by the casein micelles with a size of 100–200 nm, less than 18% of the volume with a size of 50–100 nm and only less than 1% was occupied by micelles with a size > 350 nm. This result shows that cryoconcentration changed the distribution of the mean size of the casein micelles to smaller units. No significant difference was observed on the inter-micellar distance. Cryoconcentration significantly improved the color of skim milk by increasing the L* value up to 67 which was similar to that of whole milk. Transition from a Newtonian to a non-Newtonian behavior was observed from the fourth week storage with a slight increase of casein micelle size.Industrial relevanceA concentration procedure of skim milk based on a complete block cryoconcentration technique was proposed. Application of this sub-zero technology permitted the concentration of skim milk total dry matter up to 25%. The casein micelle size was positively affected by moving the major part of the micelles toward the smaller size, whereas the inter-micellar distance was not affected. This new knowledge can be exploited in milk-based products to enhance the product stability. The cryoconcentrated skim milk color was positively affected since its L* value, which represents the milk whiteness, was significantly improved. The flow behavior of the cryoconcentrated milk was of Newtonian type up to 4 weeks of storage at 4 °C. The generated knowledge in this study can be easily used by the milk processing industry in order to make stable milk product with high dry matter content without adding milk powder, which negatively affects the product sensory properties (floury consistency).     

High-pressure-induced changes in the rennet coagulation properties of bovine milk

The mechanism of high-pressure (HP)-induced changes in rennet coagulation properties of milk, particularly the role of whey protein-casein micelle associations, was studied. Treatment at 100 or 250 MPa reduced the rennet coagulation time (RCT) of raw skimmed bovine milk, compared with untreated milk. Treatment at 400 MPa had little effect, but at 600 MPa, RCT increased considerably. HP-induced increases in RCT did not occur in serum protein-free milk or milk treated with the sulphydryl-oxidising agent KIO₃, which prevents association of denatured β-lactoglobulin with casein micelles. Treatment at 5 or 10 °C at 250–600 MPa resulted in shorter RCT than treatment at 20 °C. In milk without KIO₃, coagulum strength was highest after treatment at 250 or 400 MPa, whereas in milk with KIO₃ it was highest after treatment at 400 MPa. These results indicate the significance of HP-induced association of whey proteins with casein micelles for rennet coagulation properties of milk.     

Effect of hydrolysis of casein by plasmin on the heat stability of milk

This study examined the effect of hydrolysis of casein by added plasmin (6 mg L⁻¹) on the heat stability of raw, pre-heated, serum protein-free or concentrated skim milk. Plasmin activity markedly affected the heat stability–pH profile of skim milk and serum protein-free milk, apparently by altering the properties of the casein micelles. It is probable that changes in the surface charge of the micelles, as a result of the hydrolysis of caseins, contributed to this effect. Hydrolysis by plasmin reduced the zeta-potential of the casein micelles from ∼−19 to ∼−16 mV. The effect of hydrolysis of casein by plasmin on the heat stability of pre-heated milk was less pronounced, shifting the heat stability–pH profile to more alkaline values; the heat stability of concentrated milk was unaffected by plasmin. A very high (50 mg L⁻¹) level of added plasmin resulted in clearing of the skim milk; the L^* value decreased from ∼75 to ∼35 after 24 h incubation at 37 °C. Clearing was correlated with a change in casein micelle diameter from an initial value of ∼175 to ∼250 nm. It is suggested that plasmin-induced changes in zeta-potential may promote micellar aggregation or changes in micelle stucture.     

Skim milk protein distribution as a result of very high hydrostatic pressure

This work studies the micellar size and the distribution of caseins, major and minor whey proteins in different fractions of skim milk treated up to 900 MPa for 5 min. Transmission electron microscopy showed that the smallest casein micelles were formed around 450 MPa with no variations at higher pressures. The changes found in micellar size correlated with the concentration of soluble casein, because treatments at 250 MPa significantly enhanced the level of non-sedimentable casein while, between 700 and 900 MPa, there were no further increases with respect to lower pressures. There was a severe β-lactoglobulin (β-Lg) denaturation at pressures ≥ 700 MPa, which reached 77–87%. α-Lactalbumin (α-La) was stable up to 550 MPa, but it denatured at higher pressures. The content of soluble lactoferrin (Lf) decreased with pressure, particularly from 550 to 800 MPa, while that of secretory IgA (sIgA) progressively decreased from 250 up to 700 MPa. Our results indicated that treatment of milk at very high pressures, from 700 to 900 MPa, did not reduce micellar size nor released more soluble casein with respect to treatments at lower pressures (250–550 MPa). However, these treatments led to a severe denaturation of the whey proteins, in particular of β-Lg and the minor proteins Lf and sIgA. The possibility of using high hydrostatic pressure to obtain a soluble milk fraction with a casein and whey protein composition similar to that of human milk is discussed.     

Influence of lupin-based milk alternative heat treatment and exopolysaccharide-producing lactic acid bacteria on the physical characteristics of lupin-based yogurt alternatives

In the present study we investigated the influence of heat treatment of lupin-based (LB) milk alternatives and different exopolysaccharide (EPS)-producing lactic acid bacteria on the physical characteristics of set-type LB yogurt alternatives. LB milk alternatives, obtained from protein isolate of Lupinus angustifolius cv. Boregine, were either pasteurized at 80 °C for 60 s or ultra-high temperature (UHT) heated at 140 °C for 10 s and was fermented with Lactobacillus plantarum TMW 1.460 and 1.1468, Pediococcus pentosaceus BGT B34 and Lactobacillus brevis BGT L150. Fermentation duration was strongly affected by heat treatment: different strains needed between 25 to 35 h in UHT LB milk alternative to reach a pH of 4.5 compared to 14 to 24 h in pasteurized LB milk alternative. EPS extraction revealed slightly higher amounts of EPS for UHT LB yogurt alternatives (~ 0.5–0.9 g/l; pasteurized: ~ 0.4–0.7 g/l). The more intensive heat treatment (UHT) resulted also in better rheological (apparent viscosity, hysteresis loop area, flow point, elastic, viscous and complex modulus) and textural properties (firmness, consistency, cohesiveness and index of viscosity) of the investigated LB yogurt alternatives. Furthermore, LB yogurt alternatives out of UHT milk alternative revealed a lower tendency to syneresis, measured with siphon and centrifugation method. This work contributes to the fundamental knowledge of the textural properties of LB yogurt alternatives.     

Rennet-induced coagulation of enzymatically cross-linked casein micelles

The enzymatic cross-linking of casein micelles with transglutaminase had an adverse influence on rennet-induced coagulation. Incubation with transglutaminase at 30 °C progressively reduced the levels of monomeric caseins and increased rennet flocculation time (RFT) in a Berridge test. For incubation up to 3 h at 30 °C, the reciprocal of the RFT was linearly correlated with the level of residual monomeric κ-casein, indicating that at complete cross-linking flocculation is absent. After treatment for 4–24 h at 30 °C, no residual monomeric κ-casein was detected and no rennet-induced flocculation of the casein micelle suspension was observed. Monitoring rennet-induced coagulation by diffusing wave spectroscopy revealed that transglutaminase-induced inhibition of rennet-induced coagulation of casein micelles is primarily due to an inhibition of the secondary phase of rennet coagulation, i.e., the gelation and gel-firming phase of the casein micelle coagulation. The gelation and fusion of κ-casein-depleted para-casein micelles as in normal milk appears to be absent if the casein macropeptide remains attached to the casein micelle.     

Reuterin,lactoperoxidase, lactoferrin and high hydrostatic pressure treatments on the characteristics of cooked ham

The effect of reuterin, lactoperoxidase system (LPS) and lactoferrin (LF) combined with high hydrostatic pressure (HHP) on the characteristics of sliced cooked ham during 35 days at 4 and 10 °C were investigated. Reuterin and LPS inhibited the growth of total microorganisms during 35 days at 4 and 10 °C, whereas a regrowth at 10 °C was observed when HHP was applied. Combined treatments kept total viable counts below 1.5 log cfu/g after 35 days at 10 °C. Regarding the effect of treatments on colour of cooked ham, LPS alone or in combination with HHP slightly affected L*, a* and b* values, but these changes tended to attenuate during storage. Likely, slight differences were registered in shear strength values among control and treated cooked ham. The accumulation of volatile compounds was reduced in cooked ham treated with LPS and LF in combination with HHP, even under abuse temperature conditions (10 °C).Industrial relevanceLPS applied in combination with HHP was the most effective treatment at reducing the growth of total microorganisms in refrigerated cooked ham with minor changes in its characteristics. The antimicrobial activity of such combined treatment against food-borne pathogens, which has also been reported in RTE foods, points to its usefulness to assure a safe product of sensory characteristics similar to those of untreated cooked ham.     

Inhibition of Chymosin and the Coagulation of Para-Casein Micelles by Anions

The effect of monovalent anions on the coagulation of para-casein micelles was determined by monitoring light transmission after adding different Ca salts (40 mM) to partially hydrolyzed casein micelles. The release of macro-peptide by chymosin was quantified using fluorescamine. The average rate of the chymosin-initiated coagulation of casein micelles, approximated by the reciprocal of clotting time, was determined as a function of anion type and concentration to determine the simultaneous effects of anions on chymosin velocity and para-casein micelle aggregability.Chymosin velocity and the coagulation of para-casein micelles were progressively inhibited by anions; the larger the anion, the greater the inhibition (SCN⁻ > NO₃⁻ > Br⁻ > Cl⁻). This is attributed to the binding of anions to cationic binding sites on kappa-casein and para-kappa-casein. The relative effect of the larger anions compared with that of Cl⁻ on the average rate of the chymosin-initiated coagulation of casein micelles increased with anion concentration (6 to 120 mM) but became limited at 120 mM. In the presence of 120 mM SCN⁻, NO₃⁻ and Br⁻, the average rate of coagulation was 26, 59, and 78% of that obtained with 120 mM Cl⁻.The marked sensitivity of the interactions between chymosin and kappa-casein and between para-casein micelles to anion type support conclusions that cationic sites on kappa-casein are important in the mechanism of the chymosin-initiated coagulation of casein micelles. Thus, monovalent anions may be useful to elucidate the mechanism of protein-protein interactions in food systems.     

Moisture sorption isotherms of high pressure treated fruit peels used as dietary fiber sources

High hydrostatic pressure (HHP) treatments can improve the potential of orange, mango, and prickly pear peels as food formulation fiber sources. Akaike Information Criteria differences identified Peleg and GAB as the best model alternatives to describe experimental moisture isotherms. HHP (600 MPa/10 min/22 and 55 °C) effects on moisture isotherms expressed as relative water sorption content change with respect to controls (RWSC_aw) showed that in the 0.1–0.93 a_w range, HHP improved the adsorption water retention of orange peels. The same was true for the desorption water retention for all HHP-treated fruit peels except for prickly pear HHP-treated at 22 °C and > 0.35 a_w. The area under the hysteresis curve (A_H) in the 0.15–0.51 a_w range showed that HHP increased hysteresis for all fruit peels tested. All this illustrates the HHP potential to modify the hygroscopic properties of fruit peels at lower temperature and in less processing time than conventional processes.Industrial relevanceOrange, mango, and prickly pear peels are potential food fiber formulation sources with differentiated hygroscopic and functional properties. In this study, 600 MPa treatments at 22 and 55 °C for 10 min modified the adsorption and desorption moisture retention capacity of all fruit peels tested in this study. HHP technology can improve the potential of fruit peels as dietary fiber sources with the advantage of shorter processing times and lower temperatures than conventional technologies used to treat food fibers.     

Effects of high hydrostatic pressure on physicochemical properties,enzymes activity,and antioxidant capacities of anthocyanins extracts of wild Lonicera caerulea berry

Wild Lonicera caerulea berries were subjected to five different high hydrostatic pressure (HHP) treatments (which resemble the conditions of active component extraction and commercial sterilization). The content of anthocyanins and total phenolics increased by 6.84% and 14.35% (p < 0.05), respectively after treatment at 200 MPa for 5 and 10 min. As HHP increased, a higher loss of active component was observed. The total phenolic contents did not differ significantly between the control and the 400 MPa/20 min treated group (p > 0.05); HHP processing demonstrated better sterilization effect but severely destroyed enzymes. Polyphenol oxidase (PPO) and peroxidase (POD) activity were activated at lower HHP, such as 200 MPa, and decreased at 400–600 MPa. Superoxide dismutase (SOD) maintained good stability under HHP processing. The antioxidant capacities of anthocyanins extracts of wild L. caerulea berry were evaluated by 3 different methods (DPPH assay, oxygen radical absorbance capacity assay, and cellular antioxidant activity assay).Industrial RelevanceFactors such as color, luster, and nutrition often affect consumer choice in food. However, the color and nutrition of foods tend to be destroyed during processing and storage. The demand for healthier and more nutritious food while retaining the color and flavor after processing highlights the need to develop novel and gentler technologies for fruit processing. Recently, high hydrostatic pressure (HHP) technologies have been used in different branches of the food industry. In the present study, the content of active component in blue honeysuckle fruit pulps such as anthocyanins and polyphenols showed tendency to increase and then decrease with increasing pressure at room temperature. Five different HHP treatment groups (resembling the conditions of active component extraction conditions and commercial sterilization) were compared to the control (fresh fruit) and heat-treated group to determine the effects of HHP processing on L. caerulea berry pulps. The aim of this study was to investigate the changes in active component particularly the content and composition of anthocyanins under different high-pressure treatment at room temperature; the color and physicochemical indexes were also analyzed at the same conditions. Low HHP for a long period of time (400 MPa/20 min) demonstrated better results than that with high HHP for a short time (600 MPa/10 min), as indicated by the higher contents of anthocyanins and phenols and stronger antioxidant capacities. Therefore, Low HHP conditions can be used as an auxiliary means of active component extraction. The conditions of HHP processing at low HHP for a long period of time (400 MPa/20 min) can be altered to retain active components during food processing.     

The effects of high hydrostatic pressure at subzero temperature on the quality of ready-to-eat cured beef carpaccio

We compared the application of high hydrostatic pressure (HHP) on unfrozen carpaccio (HHP at 20 °C) and on previously-frozen carpaccio (HHP at − 30 °C). HHP at 20 °C changed the color. The pressure increase from 400 to 650 MPa and the time increment from 1 to 5 min at 400 MPa increased L* and b*. a* decreased only with 650 MPa for 5 min at 20 °C. The prior freezing of the carpaccio and the HHP at − 30 °C minimized the effect of the HHP on the color and did not change the shear force, but increased expressible moisture as compared to the untreated carpaccio. HHP at 20 °C was more effective in reducing the counts of microorganisms (aerobic total count at 30 °C, Enterobacteriaceae, psychrotrophs viable at 6.5 °C and lactic acid bacteria) than HHP at − 30 º C. With HHP at 20 °C, we observed a significant effect of pressure and time on the reduction of the counts.     

Lactobacillus fermentum and Lactobacillus amylovorus as probiotics alter body adiposity and gut microflora in healthy persons

Modification of gut microflora has been reported as altering energy and lipid homeostasis, leading to changes in body composition. We evaluated whether consumption of Lactobacillus amylovorus (LA) and Lactobacillus fermentum (LF) as novel probiotics alters body adiposity through modification of gut microflora. Healthy, but overweight participants (n = 28) consumed yogurt containing 1.39 × 10⁹ colony-forming unit (CFU) microencapsulated LA, 1.08 × 10⁹ CFU microencapsulated LF, or a control yogurt using a randomized, double-blind crossover design. Body composition measurements showed that body fat mass was reduced in all treatments, with the greatest reduction from LA consumption. Bacterial distribution of gut microflora determined a significant reduction in the abundance of Clostridial cluster IV from LA consumption and significant increases in the abundance of Lactobacillus in both LF and LA treatments. The results suggest that modulation of gut microbial composition from probiotic consumption may contribute to altered energy metabolism and body composition.     

A combined treatment of UV-assisted TiO2 photocatalysis and high hydrostatic pressure to inactivate internalized murine norovirus

Human norovirus (HuNoV) is a major cause of foodborne illness associated with shellfish consumption. A solidified agar matrix (SAM) was experimentally prepared using agar solution for inactivation of murine norovirus (MNV-1) as a surrogate for HuNoV in a simulation model approach. MNV-1 was injected inside the SAM for virus internalization, and the effects of single and combined UV-assisted TiO₂ photocatalysis (UVTP) and high hydrostatic pressure (HHP) treatments were determined. The internalized MNV-1 were reduced by 2.9-log₁₀ and 3.5-log₁₀, respectively, after single treatments of UVTP (4.5 mW/cm², 10 min) and HHP (500 MPa, 5 min, ambient temperature). However, the internalized MNV-1 was reduced by 5.5-log₁₀ (below the detection limit) when UVTP was followed by HHP, indicating a synergistic inactivation effect. Analysis of viral morphology, proteins, and genomic RNA allowed elucidation of mechanisms involved in the synergistic antiviral activity of combined treatments, which appeared to disrupt the MNV-1 structure and damage both the capsid protein and genomic RNA.Industrial relevanceHHP treatment of raw oysters has proved commercially successful, but there is a less evidence available regarding the potential of HHP for inactivation of localized viruses present inside foods. A sequential combination of UV-assisted TiO₂ photocatalysis (UVTP) and high hydrostatic pressure (HHP) achieved significantly higher inactivation of localized virus compared to individual treatments due to a synergistic mechanism. An experimentally prepared model food system was found useful to simulate foods with morphological variations and unpredictable viral internalization patterns. This UVTP-HHP combined treatment for inactivation of localized MNV-1 can be useful for disinfection of raw oysters and other similar foods.     

Gelation Profiles of Yogurt as Affected by Heat Treatment of Milk

Milk was heated at 140°C for 6 s (UHT), 98°C for 1.87 min (HTST) and 85°C for 10 min (vat-pasteurized), homogenized, inoculated with yogurt culture, and incubated at 45°C. During incubation, pH, ionic Ca, and viscosity changes were continuously monitored by interfacing to a microcomputer. Samples for transmission electron microscopy were taken at selected pH intervals.Acid fermentation of milk was a multistage process comprising 1) an initial lag period of low viscosity; 2) a period of rapid viscosity change, and 3) a stage of high constant viscosity. The shapes of the viscosity versus incubation time curves were the same for all heat treatments differing only in the extent of viscosity change in stage 2. A microstructural study of UHT milk indicated that at pH 5.1, casein micelles appeared to dissociate into subparticles of approximately 30 to 40 nm diameter. By pH 4.8, subparticles reassociated into large casein aggregates of nonspecific shape and dimensions. Micellar dissociation was thought to be influenced by conversion of colloidal Ca to ionic Ca.     

Formation of whey protein/κ-casein complexes in heated milk: Preferential reaction of whey protein with κ-casein in the casein micelles

Studies of the formation of soluble κ-casein/whey protein (WP) complexes in heated (90 °C 10 min⁻¹) milk and related mixtures of proteins have been made. The use of milk samples containing different genetic variants, and having different compositions, allowed the effects of changing the natural protein balance on the formation of particles to be investigated. In addition, studies were made of the effects of addition of WP or of purified κ-casein to the milk samples. The addition of WP caused an increase in the amount and the size of the complexes, but addition of κ-casein to the milk had little or no effect on the complex formation, nor did it seem that the added κ-casein could react with the WP in the milk. Conversely, in systems where the casein micelles were absent, the purified κ-casein reacted well with WP, suggesting that in milk heated at its normal pH the WP react preferentially with the κ-casein on the casein micelles.     

High hydrostatic pressure effects on mold flora,citrinin mycotoxin,hydroxytyrosol, oleuropein phenolics and antioxidant activity of black table olives

High hydrostatic pressure (HHP) as a non-thermal technology is an effective tool for microbiologically safe and shelf-stable fruits. Mycotoxin citrinin (CIT) is a toxic secondary metabolite, especially produced from filamentous fungus Penicillium citrinum and is also produced by other species of Penicillium, Aspergillus, and Monascus that are able to develop on olive after harvest, during brine and storage of olives. Nutritional benefits of olive fruit are mainly related to phenolics such as hydroxytyrosol, oleuropein and antioxidative effects. With HHP application of olives, total mold was reduced to 90% at 25 °C whereas it was 100% at 4 °C based on Rose-Bengal Chloramphenicol Agar (RBCA). Total Aerobic-Mesofilic Bacteria load was reduced to 35–76% at 35 ± 2 °C based on the Plate Count Agar (PCA). Citrinin load was reduced to 64–100% at 35 ± 2 °C. 2.5; 10; 25; and 100 ppb of spiked citrinin in sample were degraded as %56; %37; %9; and %1.3, respectively. 2.5 ppb and less citrinin contamination in table olive were degraded more (56%). Total phenolics were increased to 2.1–2.5-fold after HHP (as mgGA/100 g). Hydroxytyrosol in olives increased on average 0.8–2.0-fold whereas oleuropein decreased on average 1–1.2-fold after HHP (as mg/kg dwt). Antioxidant activity values varied from 17.238 to 29.344 mmol Fe²⁺/100 g for control samples whereas 18.579–32.998 mmol Fe²⁺/100 g for HHP-treated samples. HHP could be used in the olive industry as non-thermal preservation.     

Okara treated with high hydrostatic pressure assisted by Ultraflo® L: Effect on solubility of dietary fibre

Okara is an abundant and inexpensive by-product from soybean, rich in total dietary fibre (> 55% dry weight), but poor in soluble dietary fibre (SDF, < 5% dw). A combined method of high hydrostatic pressure (HHP) aided by the food grade enzyme Ultraflo® L was used for SDF maximization. At atmospheric pressure, incubation time was not a key factor, and a ratio of 1:40 (enzyme:Okara, v/w) was able to saturate the enzyme within 120–150 min. When HHP plus Ultraflo® L were applied, a synergy between both treatments was observed. Thus, at 600 MPa, 0.025% Ultraflo® L and 30 min treatment, soluble carbohydrate added up to 15.64 ± 0.32%, consisting of two peaks (9.14 ± 0.18 and 0.57 ± 0.01 kDa), determined by HPLC-ELSD. The combination of HHP plus Ultraflo® L on Okara improved the solubility of the dietary fibre, making it more suitable to be used in functional foods.Industrial relevanceThere is an increasing interest in finding new prebiotics from food industrial waste. Okara has been simultaneously treated with Ultraflo® L and HHP, taking into account the industrial costs of the procedure, and adjusting the quantity of enzyme and time of treatment to the minimum, by a Response Surface Methodology and an enzymatic activity assay. Besides, an HPLC-ELSD direct analysis (High Performance Liquid Chromatography with Evaporative Light Scattering Detector) was employed to monitor soluble fibre, as an easy and fast analytical method.     

Effect of NaCl on some physico-chemical properties of concentrated bovine milk

The influence of added NaCl (75–850 mmol L⁻¹) on some physicochemical properties of 2× or 3× concentrated milk (18 or 27%, w/v, total solids, respectively) was investigated. Adding NaCl did not influence average casein micelle size, but reduced the net-negative charge on the casein micelles and milk pH. The level of soluble and ionic calcium was increased by addition of NaCl, but the level of soluble inorganic phosphorus was not influenced. Addition of NaCl shifted the maximum in the pH–heat coagulation time (HCT) profile of 2× or 3× concentrated milk to a higher pH value and certain concentrations increased the maximum HCT, probably due to the fact that NaCl reduced the extent of heat-induced dissociation of κ-casein. Added NaCl reduced the ethanol stability, with the extent of this effect increasing with the concentration of NaCl. The key-mechanism though which added NaCl induces changes in the physico-chemical stability of casein micelles appears to be through changes in the charge on the casein micelles.     

Baroprotection of vegetative bacteria by milk constituents: A study of Listeria innocua

The baroprotective effect of milk constituents on Listeria innocua 4202 treated at 350 or 500 MPa for 5 min was examined. High-pressure (HP) treatment of L. innocua 4202 (1×10⁹ cfu mL⁻¹) resulted in complete inactivation in simulated milk ultra-filtrate (SMUF), a ∼5.0 log reduction in phosphate-buffered saline and a ∼2.9 log reduction in milk. The addition of micellar casein to SMUF increased survival of the bacterium by 3 logs, compared with SMUF alone, but the protective effect was negated if the minerals associated with the casein micelles were removed. The colloidal minerals calcium (30 mm), magnesium (5 mm), citrate (10 mm) and phosphate (20 mm), suspended in SMUF increased survival by ∼3.3, ∼1.7, ∼3.3 and ∼3.5 logs, respectively. The buffering capacity of the suspending medium was found to be a key factor in microbial baroresistance. Buffering by phosphate and citrate in milk may protect microorganisms against changes in pH during HP treatment, whereas the divalent cations calcium and magnesium may protect cell membranes against HP.     

Ethanol stability of casein micelles cross-linked with transglutaminase

The influence of transglutaminase (TGase)-induced cross-linking on the ethanol stability of skimmed milk was investigated. The stability of milk against ethanol-induced coagulation increased in sigmoidal fashion with milk pH (5.0–7.5) for all samples; ethanol stability also increased upon incubation (0–24 h) with 0.05 g L⁻¹ TGase at 30 °C. In untreated milk, addition of ethanol induced a collapse of the polyelectrolyte brush of κ-casein on the micelle surface, thereby facilitating micellar aggregation. Dynamic and static light scattering measurements indicated that in TGase-treated milk, the ethanol-induced collapse of the polyelectrolyte brush was far less than in untreated milk, suggesting that the increased ethanol stability of TGase-treated casein micelles is caused by the cross-linking of the polyelectrolyte brush on the micellar surface.