How to tailor heat-induced whey protein/κ-casein complexes as a means to investigate the acid gelation of milk—a review

Acid and rennet gelation properties of sheep, goat, and cow milks: Effects of processing and seasonal variation

Gelation is an important functional property of milk that enables the manufacture of various dairy products. This study investigated the acid (with glucono-δ-lactone) and rennet gelation properties of differently processed sheep, goat, and cow milks using small-amplitude oscillatory rheological tests. The impacts of ruminant species, milk processing (homogenization and heat treatments), seasonality, and their interactions were studied. Acid gelation properties were improved (higher gelation pH, shorter gelation time, and higher storage modulus (G') by intense heat treatment (95°C for 5 min) to comparable extents for sheep and cow milks, both better than those for goat milk. Goat milk produced weak acid gels with low G' (<100 Pa) despite improvements induced by heat treatments. Seasonality had a marked impact on the acid gelation properties of sheep milk. The acid gels of late-season sheep milk had a lower gelation pH, no maximum in tan δ following gel formation, and 70% lower G' values than those from other seasons. We propose the potential key role of a critical acid gelation pH that induces structural rearrangements in determining the viscoelastic properties of the final gels. For rennet-induced gelation, compared with cow milk, the processing treatments of the goat and sheep milks had much smaller impacts on their gelation properties. Intense heat treatment (95°C for 5 min) prolonged the rennet gelation time of homogenized cow milk by 8.6 min (74% increase) and reduced the G' of the rennet gels by 81 Pa (85% decrease). For sheep and goat milks, the same treatment altered the rennet gelation time by only less than 3 min and the G' of the rennet gels by less than 14 Pa. This difference may have been caused by the different physicochemical properties of the milks, such as differences in their colloidal stability, proportion of serum-phase caseins, and ionic calcium concentration. The seasonal variations in the gelation properties (both acid and rennet induced) of goat milk could be explained by the minor variation in its protein and fat contents. This study provides new perspectives and understandings of milk gelation by demonstrating the interactive effects among ruminant species, processing, and seasonality.

Effect of mild thermal and pH changes on the sol-gel transition in skim milk

The present work aimed to improve acid and rennet milk gelation properties with mild thermal and pH changes to skim milk, with emphasis on heating temperatures below the denaturation temperature of whey proteins. We hypothesized the heat-induced, pH-dependent micellar changes, namely the shifts in casein and calcium equilibria between the micellar (or colloidal) and serum phases, result in firmer acid and rennet milk gels and reduced gelation time. Homogenized, pasteurized skim milk was adjusted to pH values in the range of 6.4 to 7.3, heated at temperatures in the range of 50 to 80°C, cooled to refrigeration temperature, and restored to native pH (pH 6.7). Then, acid and rennet gels were made by the addition of glucono-δ-lactone and chymosin, respectively. We monitored the storage modulus (G', Pa) during gel formation with small-amplitude oscillatory shear and the gelation time and maximum G' (G'max, Pa) of acid and rennet gels, were measured at 3 and 2 h, respectively. When skim milk was heated at 50°C for 15 min, there was a 58 and 163% increase in the G'max of acid and rennet gels, respectively, as the pH at heating was raised from pH 6.7 to 7.3. Increases in gel strength were greater for skim milk heated at 60°C for 15 min. There was a positive correlation between G'max of acid gels and the heat-induced casein protein exchanges between the micellar and serum phases on heating milk at pH in the range from 6.4 to 7.3 (r = 0.78). We also found positive correlations between the variation in G'max of rennet gels with the heat-induced, pH-dependent migration of casein (r = 0.83) and calcium (r = 0.80) from the micelle into the serum phase, as determined by PAGE and atomic emission spectroscopy. Under these mild heating temperatures (50 and 60°C), rennet coagulation time was significantly reduced from 45 ± 5 to 27 ± 3 min when the pH at heating was raised from pH 6.7 to 7.3. The ability to enhance milk gelation properties with a scalable pretreatment allows for the expression of novel functionality of casein.

Effects of the thermal denaturation degree of a whey protein isolate on the strength of acid milk gels and the dissociation of κ-casein

In this study, the effects of the degree of thermal denaturation of whey protein (WP) added to milk on the dissociation of κ-casein from casein micelles were investigated, since they are related to the strength of acid milk gel and its factors. Acid milk gels were prepared by heating thermally denatured WP isolate (WPI) and undenatured milk mixtures and treating them with glucono-δ-lactone as a coagulant. The strength of these gels was negatively correlated with the WPI denaturation degree and strongly positively correlated with the extent of κ-casein dissociation from casein micelles. This behavior was ascribed to the fact that α-lactalbumin (α-La) and β-lactoglobulin (β-Lg) contained in WPI denatured after heating and engaged in disulfide bond formation with each other. With an increase in the degree of denaturation and disulfide bond formation, the bonding between β-lactoglobulin and κ-casein was suppressed to decrease the amount of κ-casein-WPI complexes. When β-Lg forms SS bonds with α-La, the number of highly reactive, free SH groups decreases, which complicates the formation of SS bridges between β-Lg and κ-casein. Thus, the denaturation degree of WPI largely determined the degree of κ-casein dissociation from casein micelles and, consequently, the strength of acid milk gels. Adding WP to milk increases the strength of acid milk gel, and it can be controlled by changing the degree of thermal denaturation of the WP. Furthermore, it was clarified for the first time that the dissociation of κ-casein from casein micelles influences this effect. Further studies are needed to elucidate the structural features of κ-casein-dissociated micelles.

ISOLASI PROTEIN KACANG MERAH DAN KACANG HIJAU MENGGUNAKAN METODE ASAM BASA DIKOMBINASIKAN DENGAN PROSES ENZIMATIS

Red beans (Phaseolus vulgaris L.) and greens beans (Phaseolus raditus L.) proteins contain high amount of essential amino acids lysine and leucine. The study aimed to determine the optimum conditions for protein isolation process from red beans and green beans flour to produce the highest protein content. Additionally, an enzymatic hydrolysis was aimed to produce isolates or protein concentrates of red beans and green beans with good digestibility. The research method used was the Response Surface Methodology (RSM) Box-Behnken Design with Design Expert 10. The variables used in this process were extraction temperature (30-50°C), extraction pH (8.50-9.50), and time extraction (30-60 minutes). The results showed that the optimum conditions for the extraction of red beans protein were extraction pH of 8.60, temperature of 30°C, and time of 30.1 minutes, with the resulting protein content of 86.88±1.38% with and a validity value of 0.91. Meanwhile, the optimum conditions for the green beans protein extraction process were extraction pH of 8.83, extraction temperature of 30°C, extraction time of 30 minutes which yielded protein content of 88.27±1.08% and a validity value of 0.97. Enzymatic hydrolysis using of 3% (w/w) bromelain enzyme on red bean and mung bean protein concentrate powder was able to increase protein digestibility by 15.61 and 14.51%, respectively.

Heat-Induced Interaction of Milk Proteins: Impact on Yoghurt Structure

Heating milk for yoghurt preparation has a significant effect on the structural properties of yoghurt. Milk heated at elevated temperature causes denaturation of whey protein, aggregation, and some case gelation. It is important to understand the mechanism involved in each state of stabilization for tailoring the final product. We review the formation of these complexes and their consequence on the physical, rheological, and microstructural properties of acid milk gels. To investigate the interactions between denatured whey protein and casein, the formation of covalent and noncovalent bonds, localization of the complexes, and their impact on ultimate gelation and final yoghurt texture are reviewed. The information regarding this fundamental mechanism will be beneficial to develop uniform quality yoghurt texture and potential interest of future research.

The effect of ultrafiltration on the acid gelation properties of protein-standardised skim milk systems

In this study, we investigated the impact of ultrafiltration (UF) on the acid gelation of milk using two protein-standardised milk systems, consisting of either skim milk and retentate (SR) or permeate and retentate (PR), over different seasons in New Zealand. The composition and the physicochemical properties of the two systems before heating were comparable, whereas the levels of heat-induced α-lactalbumin denaturation and the association of the casein micelles with α-lactalbumin were significantly lower in PR than in SR. PR displayed superior acid gelation properties compared with SR, which was most pronounced in the late season. The structural modifications of the whey proteins and casein micelles that were induced by UF and the re-equilibration of calcium in the milk mixtures may have affected the acid gelation properties of the milk by influencing the denaturation and micelle association of the whey proteins. The results suggest that UF has the potential as a tool for tuning the acid gelation properties of milk.

Effect of seasonal variations on the acid gelation of milk

We investigated the effect of seasonal variations on the acid gelation properties of bovine milk in a seasonal-calving New Zealand herd for 2 full milking seasons. We tested the formation of acid gels in 2 milk systems: unstandardized skim milk and standardized whole milk (4.6% protein, 4.0% fat). For unstandardized skim milk, late-season milk acid gels had a longer gelation time and a lower gelation pH than early- and mid-season milk acid gels, but we found no consistent seasonal variation in the final storage modulus. For standardized milk, late-season milk had the most inferior acid gelation properties during the year, including the lowest final storage modulus, the lowest gelation pH, and the longest gelation time. Standardization alleviated but did not eliminate the prolonged gelation time of late-season milk. These results indicated that the physicochemical properties of seasonal milk contributed greatly to its acid gelation, independent of differences in protein content. Standardization was not adequate to stabilize the acid gelation properties of late-season milk. Desirable acid gelation properties correlated with lower glycosylated κ-casein content, lower β-lactoglobulin:α-lactalbumin ratio, lower extent of whey protein-casein micelle association, and lower total calcium and ionic calcium content. We discuss the possible effects of the correlating variables on the acid gelation properties of seasonal milk. Natural variations in the glycosylation degree of κ-casein might play an important role in acid gel structural development by altering the electrostatic and hydrophobic interactions among the milk proteins.

Effect of heat-induced κ-casein dissociation on acid coagulation of milk

In this study, the relationship between the dissociation of κ-casein from casein micelles due to heat-induced denaturation and the strength of acid milk gel was investigated. The κ-casein-dissociated micelles were fractionated by gel filtration chromatography and two-dimensional polyacrylamide gel electrophoresis, and their zeta potential and surface hydrophobicity were measured. The negative charge of the κ-casein-dissociated micelles was lower than that of native micelles, and micellar surface hydrophobicity was higher. For confirmation, the isoelectric point of the casein micelles was measured. The κ-casein-dissociated micelles were found to cohere at an earlier stage of acidification than the native micelles. These results demonstrated that the heat-induced increase in the strength of acid milk gel was partly due to the decrease in micellar surface charge and partly to the increase in surface hydrophobicity caused by the dissociation of κ-casein.

Modification of Food Systems by Ultrasound

This review describes the mechanism, operation, and recent potential applications of ultrasound in various food systems, as well as the physical and chemical effects of ultrasound treatments on the conservation and modification of different groups of food. Acoustic energy has been recognized as an emerging technology with great potential for applications in the food industry. The phenomenon of acoustic cavitation, which modifies the physical, chemical, and functional properties of food, can be used to improve existing processes and to develop new ones. The combination of ultrasonic energy with a sanitizing agent can improve the effect of microbial reduction in foods and, thereby, their quality. Finally, it is concluded that the use of ultrasound in food is a very promising area of research; however, more research is still needed before applying this technology in a wider range of industrial sectors.

Effect of the heat-induced whey proteins/κ-casein complex on the acid gelation of yak milk

The soluble whey protein/κ-casein complexes were predominant in changing the rheological properties of yak milk yoghurt.

Conventional and Innovative Processing of Milk for Yogurt Manufacture; Development of Texture and Flavor: A Review

Milk and yogurt are important elements of the human diet, due to their high nutritional value and their appealing sensory properties. During milk processing (homogenization, pasteurization) and further yogurt manufacture (fermentation) physicochemical changes occur that affect the flavor and texture of these products while the development of standardized processes contributes to the development of desirable textural and flavor characteristics. The processes that take place during milk processing and yogurt manufacture with conventional industrial methods, as well as with innovative methods currently proposed (ultra-high pressure, ultrasound, microfluidization, pulsed electric fields), and their effect on the texture and flavor of the final conventional or probiotic/prebiotic products will be presented in this review.

Regulation of milk protein solubility by a whey-derived proline-rich peptide product

The effects of a bovine whey peptide product enriched in proline (wPRP) on the solubility of milk proteins were tested under ambient conditions or following heat treatment at 75 and 100 °C, for 1 and 15 min, followed by post-incubation storage at either ambient temperature or 4 °C for up to 7 d. wPRP promoted solubilisation of milk proteins in a concentration-dependent manner without heat treatment and also after heat treatment at 75 and 100 °C, and the effect was enhanced after storage under either ambient or refrigerated storage conditions. Interactions of wPRP and milk proteins were monitored by particle size analysis and tryptic digestion and specifically linked with solubilisation of αS1 casein (αS1-Cn), which supported observed changes in milk protein solubility. The results suggested that wPRP preferably prevented or reversed physical versus covalent protein aggregation, with the relaxation of hydrophobic interactions at 4 °C providing an additive effect. This application of wPRP represents a novel approach to stabilisation of dairy proteins following thermal processing with industrial usefulness yet to be explored.