Effect of beta-lactoglobulin and precipitation of calcium phosphate on the thermal coagulation of milk

The effect of beta-lactoglobulin and heat-induced precipitation of calcium phosphate on the pH dependence and mechanism of thermal coagulation of milk throughout the pH range 6.3-7.3 was studied using serum protein-free milk and sodium caseinate as models for micellar and non-micellar milk protein systems respectively. It appears that the specific effect of beta-lactoglobulin at the pH of maximum stability may be related to its ability to chelate calcium. The effect of beta-lactoglobulin at the pH of minimum stability does not appear to be directly related to heat-induced dissociation of K-casein or micellar integrity but may be due to its ability to sensitize casein micelles to heat-induced precipitation of calcium phosphate, by increasing micellar hydrophobicity. The extent of heat-induced precipitation of calcium phosphate, as a function of pH, is an inverse reflection of the pH dependence of heat stability. Micellar integrity appears to play a critical role in the heat stability of milk but for reasons not previously appreciated.     

Influence of added calcium chloride on the heat stability of unconcentrated and concentrated bovine milk

The influence of added calcium chloride (1–10 mmol/L) on the heat-induced coagulation of skim bovine milk was examined. Unconcentrated milk displayed a pH-heat coagulation time (HCT) profile with a maximum at pH 6.6 and minimum at pH 7.0. Adding calcium chloride to unconcentrated milk progressively reduced the HCT at the maximum, increased the pH at which the maximum occurred and reduced the HCT at pH > 7.0. For concentrated milk, the shape of the pH-HCT profile, that is, a maximum at pH 6.6, was not altered by added calcium chloride, but HCT was reduced progressively with increasing concentration of calcium chloride. Preheating (90°C for 10 min) shifted the maximum in the pH-HCT profile of unconcentrated milk to a more acidic pH, and addition of 5 mmol/L calcium chloride to preheated milk induced changes in heat stability similar to those noted for unheated milk. Addition of calcium chloride to milk prior to preheating strongly reduced the stability of milk against heat-induced coagulation. These data suggest that calcium has a strong destabilizing effect on the stability of milk systems against heat-induced coagulation.     

Heat stability of milk: influence of modification of lysine and arginine on the heat stability-pH profile

Several dicarbonyl compounds (glyoxal, substituted glyoxals, diacetyl and 1, 2-cyclohexanedione) had a marked stabilizing effect on the heat stability of milk, especially in the presence of urea. These reagents are believed to modify arginine more or less specifically suggesting an important role for arginine residues in heat stability. In contrast, modification of lysine residues with dansyl chloride, acetic anhydride or cyanoborohydride had little effect on maximum heat stability although it did alter the HCT-pH profile. Since diacetyl is a natural constituent of fermented milks and cheese, it may be acceptable as an additive to increase the heat stability of milk.     

Maillard Reaction Products as Encapsulants for Fish Oil Powders

The use of Maillard reaction products for encapsulation of fish oil was investigated. Fish oil was emulsified with heated aqueous mixtures comprising a protein source (Na caseinate, whey protein isolate, soy protein isolate, or skim milk powder) and carbohydrates (glucose, dried glucose syrup, oligosaccharide) and spray‐dried for the production of 50% oil powders. The extent of the Maillard reaction was monitored using L*, a*, b* values and absorbance at 465 nm. Encapsulation efficiency was gauged by measurement of solvent‐extractable fat and the oxidative stability of the fish oil powder, which was determined by assessment of headspace propanal after storage of powders at 35 °C for 4 wk. Increasing the heat treatment (60 °C to 100 °C for 30 to 90 min) of sodium caseinate‐glucose‐glucose syrup mixtures increased Maillard browning but did not change their encapsulation efficiency. The encapsulation efficiency of all heated sodium caseinate‐glucose‐glucose syrup mixtures was high, as indicated by the low solvent‐extractable fat in powder (<2% powder, w/w). However, increasing the severity of the heat treatment of the sodium caseinate‐glucose‐glucose syrup mixtures reduced the susceptibility of the fish oil powder to oxidation. The increased protection afforded to fish oil in powders by increasing the temperature‐time treatment of protein‐carbohydrate mixtures before emulsification and drying was observed irrespective of the protein (sodium caseinate, whey protein isolate, soy protein isolate, or skim milk powder) and carbohydrate (glucose, glucose/dried glucose syrup, or oligosaccharide/dried glucose syrup) sources used in the formulation. Maillard reaction products produced by heat treatment of aqueous protein‐carbohydrate mixtures were effective for protecting microencapsulated fish oil and other oils (evening primrose oil, milk fat) from oxidation.     

Chymosin sensitivity of the heat-induced serum protein aggregates isolated from skim milk

The sensitivity of heat-induced milk serum κ-casein/whey protein aggregates to recombinant chymosin was investigated in the serum phase of heated skim milk or on aggregates isolated from it. In both cases, significant amounts of caseinomacropeptide were produced after 1 h-incubation at 37 °C, as assayed by high performance liquid chromatography coupled with mass spectrometry analysis. In milk serum, the aggregates remained stable upon hydrolysis, as opposed to 10 g L⁻¹ suspensions of isolated aggregates in imidazole/Ca²⁺ buffer where visible flocculation occurred. The results are discussed in terms of variation in chymosin activity between the two systems.     

Effect of the protein, citrate and phosphate content of milk on formation of lactulose during heat treatment

Milk ultrafiltrate and milks of varying protein, citrate and phosphate concentrations were heated in sealed containers. Protein was found not to be involved in the mechanism of formation of lactulose, but increasing the protein content of milk reduced the concentration of lactulose after heating. This was considered to be due to increased condensation of lactose and lactulose with amino groups of the protein. Less lactulose was formed in milk ultrafiltrate than in skimmed milk accorded the same heat treatment, which was attributed to the buffering capacity of the milk protein in skimmed milk. Activation energies for lactulose formation in skimmed milk and in ultrafiltrate were 128 and 131 kJ/mol respectively. Citrate and phosphate catalysed the formation of lactulose. It is proposed that the formation of free lactulose in heated milk and ultrafiltrate proceeds exclusively by the Lobry de Bruyn-Alberda van Ekenstein transformation with the naturally occurring phosphate and citrate acting as base catalysts.     

Effect of Trichoderma reesei tyrosinase on rheology and microstructure of acidified milk gels

The aim of this work was to study the potential of tyrosinase enzymes in structural engineering of acid-induced milk protein gels. Fat free raw milk, heated milk or a sodium caseinate solution were treated with tyrosinases from Trichoderma reesei (TrTyr) and Agaricus bisporus (AbTyr) and the reference enzyme transglutaminase (TG) prior to acid-induced gelation. TrTyr treatment increased the firmness of raw milk and sodium caseinate gels, but not that of heated milk gels, even though protein cross-linking was detected in heated milk. AbTyr did not cross-link proteins in any of the studied milk protein systems. TG was superior to TrTyr in gels prepared of heated milk. In acidified heated milk and sodium caseinate, TrTyr and TG treatment resulted in a decrease of the pore size. Scanning electron microscopy revealed more extensive particle interactions in the heated milk gels with TG than with TrTyr.     

NMR relaxometry and differential scanning calorimetry during meat cooking

By combining simultaneous nuclear magnetic resonance (NMR) T₂ relaxometry and differential scanning calorimetry (DSC) on pork samples heated to nine temperature levels between 25 and 75 °C, the present study investigates the relationship between thermal denaturation of meat proteins and heat-induced changes in water characteristics. Principal component analysis (PCA) on the distributed ¹H NMR T₂ relaxation data revealed that the major changes in water characteristics during heating occur between 40 and 50 °C. This is probably initiated by denaturation of myosin heads, which however, could not be detected in the DSC thermograms obtained directly on the meat. In contrast, the DSC thermograms revealed endothermic transitions at 54, 65 and 77 °C, probably reflecting the denaturation of myosin (rods and light chain), sarcoplasmic proteins together with collagen and actin, respectively. Simultaneous modelling of DSC and NMR data by partial least squares regression (PLSR) revealed a correlation between denaturation of myosin rods and light chains at 53–58 °C and heat-induced changes in myofibrillar water (T₂ relaxation time 10–60 ms) as well as between actin denaturation at 80–82 °C and expulsion of water from the meat. Accordingly, the present study demonstrates a direct relationship between thermal denaturation of specific proteins/protein structures and heat-induced changes in water mobility during heating of pork.     

Manufactore of Ras cheese from ultrafiltered milk supplemented with caseinates

Whole cow's milk was ultrafiltered (conc. Factor 5), sodium and calcium caseinate were added to the milk retentate at the rate of 0.5 and 0.75% (w/w) of the milk. Ras cheese was made from milk retentate supplemented with different levels of caseinate salts, as well as unsupplemented retentates, and compared with Ras cheese made by the traditional method. Retentate supplemented with 0.5% Na and Ca caseinate was suitable for Ras cheese-making with limited whey drainage, supplementation of retentate with 0.75% Ca caseinate gave defective cheese at the end of ripening, while Na caseinate increased soluble N and free fatty acids in the cheese. Organoleptic scoring showed that supplementation with Na caseinate enhanced cheese ripening.     

Fortification of milk protein content with different dairy protein powders alters its compositional,rennet gelation,heat stability and ethanol stability characteristics

Skim milk (SM) was fortified from 3.3 to 4.1% protein using different milk protein powders: skim milk powder (SMP), native phosphocasein (NPC), calcium-reduced phosphocasein (CaRPC), sodium caseinate (NaCas) or calcium caseinate (CaCas). Compared with SMP or NPC, fortification with NaCas and CaRPC, and to a lesser extent CaCas, resulted in milk samples having higher proportions of non-sedimentable casein and calcium, and lower- and higher-levels of κ- and α_S1-casein, respectively, as a proportion of non-sedimentable casein. These changes coincided in milk samples fortified with NaCas, CaRPC or CaCas failing to undergo rennet-induced gelation, and having higher heat stability in the region 6.7–7.2 and ethanol stability at pH 6.4. The study demonstrates that the aggregation behaviour of protein-fortified milk samples is strongly influenced by the degree of mineralisation of the protein powder used in fortification, which affects the partitioning of casein and calcium in the sedimentable and non-sedimentable phases.     

Heat stability of transglutaminase-treated milk

Heat-induced coagulation of unconcentrated (9%, w/w) and concentrated (18%, w/w) reconstituted skim milk was determined after incubation with transglutaminase (TGase). Cross-linking ∼20% of κ-casein strongly increased the heat stability of unconcentrated milk at pH >6.9, presumably by preventing heat-induced dissociation of κ-casein, whereas increased heat stability of unconcentrated milk at pH 6.6–6.8 was only observed when >80% of casein was cross-linked. Treatment with TGase reduced heat stability of unconcentrated milk at pH <6.6, presumably due to the increased susceptibility of partially cross-linked casein micelles to coagulation arising from heat-induced acidification. A low degree of cross-linking increased the heat stability of concentrated milk at pH >6.8, but more extensive cross-linking progressively reduced heat stability. The degree of cross-linking studied did not increase the heat-stability of concentrated milk at its natural pH. The outcomes of this study substantiate the crucial roles of heat-induced acidification and casein dissociation in heat stability of milk.     

Emulsifying and foaming properties of protein isolates prepared from heated milks

Surface activities at the air-water interface and the emulsifying and foaming properties of sodium caseinate, conventional casein-whey protein co-precipitate prepared from milk heated at 90°C × 15 min at pH 6.6 and milk protein isolates prepared from milks heated at 90°C × 15 min at pH 7.5 or at 60°C × 3 min at pH 10.0 were determined. The surface activities of the four proteins at the air-water interface were similar, while the emulsifying capacity and emulsion stabilizing ability of casein was less than that of the milk protein isolates or the conventional co-precipitate. Fat surface areas formed on emulsification with the four proteins were similar and increased with increasing power input. Total protein adsorbed at the interface and protein load (mg protein/m₂ fat) for the emulsions stabilized by sodium caseinate and the milk protein isolate prepared from the milk heated at 90°C × 15 min at pH 7.5 were similar and lower than those for emulsions stabilized by the other two proteins. Foam overruns followed the order: sodium caseinate > milk protein isolate prepared from milk heatedat90°C × 15min, pH 7.5 > milk protein isolate prepared from milk heated at 60°C × 3 min, pH 10.0 > conventional co-precipitate, while foam stabilities followed the reverse order.     

Preparation and functional properties of protein coprecipitate from sheep milk

The optimum conditions for the production of coprecipitate from sheep milk were studied. The best percentage of calcium chloride added to milk was 0.2%, which resulted in a recovery of 97.5% of milk proteins. At low pH (4.5–5), the recovery of protein was low, but it increased at higher pH values (5.5–6.5); recovery was greatest at pH 6.5. The optimum heating temperature to obtain coprecipitate from sheep milk was 85–95°C. The functional properties of the sheep milk coprecipitate were studied. At pH values higher than 6, there were no differences between the solubility of sheep milk coprecipitate and sheep milk sodium caseinate, but the solubility of coprecipitate at pH values lower than 5 was relatively higher than those of the caseinate. At pH ≥6, the emulsion activity index (EAI) for emulsions of sheep milk coprecipitate and caseinate increased as pH increased; at all pH values, the EAI of the coprecipitate was higher than that of the caseinate. Sheep milk coprecipitate showed higher foaming ability and stability than sheep milk sodium caseinate.     

Pulse NMR of Casein Dispersions

The spin-spin relaxation time, T₂, of skimmilk, sodium caseinate dispersions and milk ultrafiltrate was measured as a function of pH and temperature using pulse proton NMR. The T₂ shows a maximum around pH = 5.2 for skimmilk, decreases with decreasing pH for sodium caseinate dispersions and is independent of pH for milk ultrafiltrate. For all systems studied T₂ increases with temperature, but, the extent of increase depends on the system. It is concluded that the T₂ of casein dispersions is mainly determined by factors determining the state of aggregation of the caseins. Apart from affecting the extent of aggregation of the casein particles, micellar calcium phosphate probably contributes also directly to the measured T₂.     

Impaired rennetability of heated milk; study of enzymatic hydrolysis and gelation kinetics

Casein micelles in milk are stable colloidal particles with a stabilizing hairy brush of kappa-casein. During cheese production rennet cleaves kappa-casein into casein macropeptide and para-kappa-casein, thereby destabilizing the casein micelle and resulting in aggregation and gel formation of the micelles. Heat treatment of milk causes impaired clotting properties, which makes heated milk unsuitable for cheese production. In this paper we compared five different techniques, often described in the literature, for their suitability to quantify the enzymatic hydrolysis of kappa-casein. It was found that the technique is crucial for the yield of casein macropeptide and this yield then affects the calculated enzymatic inhibition caused by heat treatment, ranging from 5 to 30%. The technique, which we found to be the most reliable, demonstrates that heat-induced calcium phosphate precipitation does not affect the enzymatic cleavage, while whey protein denaturation causes a very slight reduction of enzyme activity. By using diffusing wave spectroscopy, a very sensitive technique to monitor gelation processes, we demonstrated that heat-induced calcium phosphate precipitation does not affect the clotting. Whey protein denaturation does not affect the start of flocculation but has a clear effect on the clotting process. This work adds to a better understanding of the processes causing the impaired clotting properties of heated milk.     

Effects of varying time/temperature-conditions of pre-heating and enzymatic cross-linking on techno-functional properties of reconstituted dairy ingredients

The effects of varying time/temperature-conditions of pre-heating and cross-linking with transglutaminase (TG) on the functional properties of reconstituted products from skim milk, WPC and sodium caseinate was analyzed. The degree of cross-linking (DC) of skim milk proteins could be increased from 54.4% to 70.5% by varying process conditions. Thereby the water-holding capacity (WHC) increased from 10% to 20%, while the heat stability decreased. The burning-on was lower than that of the non-treated products at optimum pre-heating conditions (90 °C/30 s). Using sodium caseinate as substrate for TG the DC increased from 39.2% to 100% due to the improvement of the process. As a result the WHC increased by 30% and the heat stability up to 380%. However, the burning-on of casein increased as well. TG-treated sodium caseinate started to gel at 10% protein, whereas untreated sodium caseinate gelled not before 15% protein. The WHC of enzyme-treated whey proteins was lowered. The heat stability of WPC could be doubled by TG-treatment, and the burning-on of the products was, especially at optimum pre-heating conditions, less pronounced. The degree of denaturation of TG-treated whey proteins was 2–5% higher than that of untreated samples.     

Effect of cooking on protein quality of chickpea (Cicer arietinum) seeds

Amino acid composition and in vitro protein digestibility of cooked chickpea were determined and compared to raw chickpea seeds. Heat treatment produced a decrease of methionine, cysteine, lysine, arginine, tyrosine and leucine, the highest reductions being in cysteine (15%) and lysine (13.2%). Protein content declined by 3.4% and in vitro protein digestibility improved significantly from 71.8 to 83.5% after cooking. The decrease of lysine was higher in the cooked chickpea seeds than in the heated protein fractions, globulins and albumins. The structural modification in globulins during heat treatment seems to be the reason for the increase in protein digestibility, although the activity of proteolytic inhibitors in the albumin fraction was not reduced. Results suggest that appropriate heat treatment may improve the bioavailability of chickpea proteins.     

Emulsification of Commercial Dairy Proteins with Exhaustively Washed Muscle

The addition of dairy proteins to exhaustively washed chicken breast muscle improved the emulsion stability in heated cream layers (emulsions) containing whey protein concentrate (WPC) or whey protein isolate (WPI). The initial weight of the heated cream layers made with WPC or WPI was heavier than those for sodium caseinate (CNate) or milk protein isolate (MPI). The addition of CNate or MPI resulted in decreased emulsion stability and increased inhibition of myosin heavy chain and actin participation in the emulsion formation compared to WPC or WPI.     

Bioavailability of calcium and physicochemical properties of calcium-fortified buffalo milk

Buffalo milk was fortified with calcium at the rate of 50 mg calcium/100 ml milk using calcium chloride, calcium lactate and calcium gluconate, and the resulting decrease in pH was restored to its original value by adding disodium phosphate. The maximum heat stability of calcium-fortified buffalo milk remained slightly lower than that of unfortified milk. Calcium gluconate-fortified milk had the highest heat stability, bioavailability of calcium, partitioning of calcium in the dissolved state and viscosity, and the least curd tension compared to other fortified milk, without any adverse impact on sensory properties. The bioavailability of calcium and heat stability was lowest in the case of buffalo milk fortified with calcium chloride.     

Effect of feed texture, meal frequency and pre-slaughter fasting on carcass and meat quality, and urinary cortisol in pigs

The gelation characteristics of myofibrillar proteins are indicative of meat product texture. Defining the performance of myofibrillar proteins during gelation is beneficial in maintaining quality and developing processed meat products and processes. This study investigates the impact of pH on viscoelastic properties of porcine myofibrillar proteins prepared from different muscles (semimembranosus (SM), longissimus dorsi (LD) and psoas major (PM)) during heat-induced gelation. Dynamic rheological properties were measured while heating at 1 °C/min from 20 to 85 °C, followed by a holding phase at 85 °C for 3 min and a cooling phase from 85 to 5 °C at a rate of 5 °C/min. Storage modulus (G′, the elastic response of the gelling material) increased as gel formation occurred, but decreased after reaching the temperature of myosin denaturation (52 °C) until approximately 60 °C when the gel strength increased again. This resulted in a peak and depression in the thermogram. Following 60 °C, the treatments maintained observed trends in gel strength, showing SM myofibrils produced the strongest gels. Myofibrillar protein from SM and PM formed stronger gels at pH 6.0 than at pH 6.5. Differences may be attributed to subtle variations in their protein profile related to muscle type or postmortem metabolism. Significant correlations were determined between G′ at 57, 72, 85 and 5 °C, indicating that changes affecting gel strength took effect prior to 57 °C. Muscle type was found to influence water-holding capacity to a greater degree than pH.