Stability Of Milk Proteins Research Articles

Application of Calcium Citrate in the Manufacture of Acid Rennet Cheese Produced from High-Heat-Treated Goat's Milk from Spring and Autumn Season.

The stability of milk proteins is affected by changes in the pH value of milk, the heating temperature, and the addition of calcium compounds or chelating agents, which can cause alterations in calcium distribution. The purpose of this study was to determine the potential of the use of calcium citrate to manufacture fresh acid rennet cheese from high-temperature-pasteurized goat’s milk (90 °C, 15 s) from the spring and autumn season and the effect of the calcium dose used on the physicochemical and organoleptic properties of the cheese. Autumn milk was found to be a richer source of total solids, confirming the effect of the production season on milk quality. The applied doses of calcium did not cause the denaturation of goat milk proteins and allowed pasteurization to take place at 90 °C for 15 s. The addition of calcium citrate resulted in a significant increase in the pH value of milk and cheese compared to the control sample. Adding 15 and 20 mg of Ca 100 g−1 to milk as citrate had the most beneficial effect on increasing protein retention in cheese in both seasons, showing a rise from 1.33% to 2.40%. The production season significantly influenced the cheese yield. The control goat cheese from the autumn season showed a 6.85% higher yield compared to the spring cheese. An increase in cheese yield was also observed as the calcium dose of milk increased. The content of micro- and microelements in cheese was affected by the production season. The addition of calcium citrate to milk resulted in a significant increase in the calcium content of cheese—from 120.83 to 147.45 mg 100 g−1 in the spring season and from 130.66 to 151.21 mg 100 g−1 in the autumn season. Increasing the dose of calcium increased the hardness of cheese samples by 1.37 N in the spring and 0.90 N in the autumn. The organoleptic evaluation showed that adding calcium to milk did not significantly affect the organoleptic characteristics of goat cheese.

The effect of transglutaminase on colloidal stability of milk proteins

Food industry is on constant search for improved, enzymatically modified forms of proteins, which are the most important functional food ingredients, capable of either introducing or modifying texture, appearance, and flavour. One of the key aspects of food design is creating new functional properties and novel food macro- and microstructures in order to provide tailored food characteristics expected by consumers. Milk proteins are among the most thoroughly studied protein complexes occurring in foods and having a wide range of functionalities. The common usage of milk proteins has also contributed to the development of techniques designed to modify and improve functional properties of milk proteins. The aim of this study was to investigate the impact of enzymatic modification induced by microbial transglutaminase (TGM) in milk proteins on the range of cross-linking as well as heat and ethanol stability of these proteins. Milk used in the study was subjected to various technological processes and then incubated with the enzyme added at an amount of 2 Units g−1 protein for 2 h at 40 °C. Modification of milk proteins by transglutaminase leads to their partial cross-linking and formation of high molecular weight polymers in the range of 55–200 kDa. The changes of cross-links between milk proteins occur most dynamically in the first hour of incubation at 40 °C. Enzymatic modification of milk proteins significantly increases the ethanol stability of raw as well as pasteurized milk and UF concentrate. Addition of transglutaminase does not change the heat stability of milk measured as the time of heat coagulation at 140 °C.Graphical abstract

Evaluation of effective storage conditions and in-vitro bioaccessibility of vitamin A from native and modified sodium caseinate -vitamin A complexes

Native sodium caseinate-Vitamin A (Vit A) complex {Sodium caseinate-Vit A complex (NaCas-VA)}, and modified sodium caseinate-Vitamin A complexes {succinylated sodium caseinate-Vit A complex (SNaCas-VA), reassembled sodium caseinate-Vit A complex (RNaCas-VA) and reassembled succinylated sodium caseinate-Vit A complex (RSNaCas-VA)} were prepared and evaluated for effective storage conditions and in-vitro bioaccessibility of vitamin A. Complexes showed higher vitamin A stability when packed in aluminium laminate pouches as compared to microcentrifuge tubes. Free vitamin A (fat soluble) showed the lowest vitamin A stability followed by NaCas-VA, SNaCas-VA, RNaCas-VA and RSNaCas-VA complexes at different storage temperatures i.e. −20 °C, 4 °C and 37 °C. All complexes and free vitamin A (fat soluble) showed the lowest stability at 37 °C followed by 4 °C and −20 °C. Stability of milk protein -Vit A complexes were ascertained at two different pH (5.0 and 7.0). In-vitro bioaccessibility of vitamin A was higher for milk protein-Vit A complexes as compared to free vitamin A (fat soluble).

Effect of dietary cation-anion difference on performance of lactating dairy cows and stability of milk proteins

Casein micelle stability is negatively correlated with milk concentrations of ionic calcium, which may change according to the metabolic and nutritional status of dairy cows. The present study aimed to evaluate the effect of dietary cation-anion difference (DCAD) on concentrations of casein subunits, whey proteins, ionic calcium, and milk heat and ethanol stability. Sixteen Holstein cows were distributed in 4 contemporary 4×4 Latin square designs, which consisted of 4 periods of 21d and 4 treatments according to DCAD: 290, 192, 98, and −71mEq/kg of dry matter (DM). The milk concentrations of ionic calcium and κ-casein were reduced as DCAD increased, whereas the milk urea nitrogen and β-lactoglobulin concentrations were increased. As a result of these alterations, the milk ethanol stability and milk stability during heating at 140°C were increased linearly with increasing DCAD [Y=74.87 (standard error=0.87) + 0.01174 (standard error=0.0025) × DCAD (mEq/kg of DM) and Y=3.95 (standard error=1.02) + 0.01234 (standard error=0.0032) × DCAD (mEq/kg of DM), respectively]. In addition, 3.5% fat-corrected milk and fat, lactose, and total milk solids contents were linearly increased by 13.52, 8.78, 2.5, and 2.6%, respectively, according to DCAD increases from −71 to 290mEq/kg of DM, whereas crude protein and casein content were linearly reduced by 4.83 and 4.49%, respectively. In conclusion, control of metabolic changes in lactating dairy cows to maintain blood acid-base equilibrium plays an important role in keeping milk stable to ethanol and during heat treatments.

UHT milk contains multiple forms of αS1-casein that undergo degradative changes during storage

Milk proteins are susceptible to chemical changes during processing and storage. We used proteomic tools to analyse bovine αS1-casein in UHT milk. 2-D gels of freshly processed milk αS1-casein was presented as five or more spots due to genetic polymorphism and variable phosphorylation. MS analysis after phosphopeptide enrichment allowed discrimination between phosphorylation states and genetic variants. We identified a new alternatively-spliced isoform with a deletion of exon 17, producing a new C-terminal sequence, K164SQVNSEGLHSYGL177, with a novel phosphorylation site at S174. Storage of UHT milk at elevated temperatures produced additional, more acidic αS1-casein spots on the gels and decreased the resolution of minor forms. MS analysis indicated that non-enzymatic deamidation and loss of the N-terminal dipeptide were the major contributors to the changing spot pattern. These results highlight the important role of storage temperature in the stability of milk proteins and the utility of proteomic techniques for analysis of proteins in food.

Interaction between whey proteins and lipids during light-induced oxidation

Dairy products are a suitable group for polyunsaturated fatty acid (PUFA) fortification, but are very susceptible to light-induced oxidation. In this study, the combined effect of light and PUFA fortification on the stability of milk proteins and on the formation of protein-lipid complexes was studied. Therefore, emulsions were prepared containing whey proteins and lipids with a different degree of unsaturation (olive, soybean, fish and algae oil). PUFA stimulated the protein oxidation process, even after complete degradation of the photosensitizer. First order reaction kinetics were established for riboflavin and tryptophan degradation, as well as the formation of N-formylkynurenine, a tryptophan degradation product. Oxidation was further characterised by degradation of the essential amino acids lysine, histidine, methionine and tyrosine. Protein-lipid adduct formation, measured fluorometrically and by the protein-bound carbonyl groups, could be linked with a decreased extractability of the unsaturated lipids from the emulsion, but also caused the formation of protein aggregates of high molecular mass.

Casein Proteins as Molecular Chaperones

Under conditions of stress, such as elevated temperature, molecular chaperones stabilize proteins from unfolding, aggregating, and precipitating. We have investigated the chaperone activity of the major milk proteins alpha(S)-, beta-, and kappa-casein with reduced insulin and the milk whey proteins, alpha-lactalbumin and beta-lactoglobulin, and compared it with that of the mammalian small heat shock protein (sHsp), alpha-crystallin, and clusterin. alpha(S)-Casein exhibited different chaperone behavior under reduction and heat stresses, i.e., chaperone activity increased with increasing temperature (as observed with alpha-crystallin), but under reduction stress, its chaperone activity increased at lower temperatures. beta- and kappa-casein had comparable chaperone ability with each other but were less effective than alpha(S)-casein. Under molecular crowding conditions, precipitation of stressed protein was accelerated, and alpha(S)-casein was a poorer chaperone. Furthermore, at slightly alkaline pH values, alpha(S)-casein was a less effective chaperone than at neutral pH. Detailed fluorescence, size exclusion chromatography, and real-time NMR studies studies indicated that the casein proteins underwent conformational changes and stabilized the partially unfolded whey proteins prior to formation of high molecular weight soluble complexes. These results are consistent with casein proteins acting as molecular chaperones in a manner similar to sHsps and clusterin.

Effect of soybean soluble polysaccharides on the stability of milk protein under acidic conditions

Soybean soluble polysaccharide (SSPS) extracted from soybean cotyledons has a pectin-like structure. SSPS has been used as a dispersion stabilizer for milk protein under acidic conditions, below the isoelectric point of milk protein, in a manner similar to high methoxyl pectin. SSPS at concentrations of less than 0.2% stabilized acidic beverages prepared with 8.0% of milk solid non-fat at pH 3.4–4.4, in contrast with pectin which needed at least 0.4% concentration. To elucidate the structure of the polysaccharide, which seemed likely to have a significant influence on its ability to stabilize acidic beverages, SSPS was digested with purified pectinases and hemicellulases, and the stabilizing effects of the digestion products on acidic beverages were analyzed. The stabilizing ability of SSPS was reduced by digestion of its neutral sugar side chains, arabinan or galactan, and was only slightly changed by digestion of its galacturonan main backbone with polygalacturonase or rhamnogalacturonase. This indicates that neutral sugar side chains, which mainly consist of arabinose and galactose residues, play a significant role in the ability of SSPS to stabilize acidic beverages. In contrast, the stabilizing effect of pectin was reduced by digestion of its galacturonan backbone, and was not changed by digestion of its neutral sugar side chains. This indicates that SSPS may differ from pectin in the mechanism by which it stabilizes milk proteins under acidic conditions.

Breastfeeding and the Working Mother: Effect of Time and Temperature of Short-term Storage on Proteolysis, Lipolysis, and Bacterial Growth in Milk

Women who breastfeed have to store expressed milk while at work for later feeding to their infants; however, storage conditions are often not optimal. Top assess microbial growth and stability of milk protein and lipid at 15 degrees C to 38 degrees C for up to 24 hours. Sixteen healthy women who breastfed exclusively, either at home (n=11) or who expressed milk for their infants (n=5), were studied during early (1 month) or late (5 to 6 months) lactation. Expressed milk was stored at 15 degrees C, 25 degrees C, and 38 degrees C for 1 to 24 hours for quantitation of pH, proteolysis, and lipolysis; bacterial growth was quantified at 0, 4, 8, and 24 hours of storage. Milk pH decreased 2 units by 24 hours of storage at all temperatures tested. Proteolysis was minimal during milk storage at 15 degrees C or at 25 degrees C for 24 hours and was apparent only after 24 hours of storage at 38 degrees C. Lipolysis was rapid, starting in the first hours of storage and progressing to 8% at 24 hours. Thus, while the greatest increment in proteolysis products was a 40% increase above baseline after 24 hours of storage at 38 degrees C, free fatty acid concentration at this storage time was 440% to 710% higher than in freshly expressed milk. Bacterial growth was restricted mainly to nonpathogens, was minimal at 15 degrees C throughout the 24 hours of storage, was low at 25 degrees C for the first 4 to 8 hours, and was considerably higher at 38 degrees C even during the relatively short period of 4 hours. Storage of human milk is safe at 15 degrees C for 24 hours, whereas at 25 degrees C it is safe for 4 hours. Milk should not be stored at 38 degrees C. Minimal proteolysis during storage suggests that milk proteins probably maintain their structure and function during short-term storage, while the marked lipolysis might slow bacterial growth during this time.

Borden Symposium on Stability of Milk Proteins in Dairy Foods

The three-dimensional structure of proteins exhibits marginal stability even under physiological conditions. Any deviation of the environment from the physiological state may result in perturbation of the protein's structure. Under appropriate conditions, the structural transition may be reversible (Figure 1). For example, for several proteins, the native structure can be reformed by refolding proteins from a random coil state. However, for many proteins, refolding in solution leads to kinetically trapped nonnative states due to intermolecular interaction. Swaisgood proposed the principle of physical isolation of nascent folding polypeptide chains and has shown that even proteins that cannot be refolded in solution to give biologically active structure can be refolded to the native state if the folding molecules are physically isolated by immobilization. In principle, any protein, even those kinetically trapped in nonnative states, can be refolded to the native state by passing through the random coil state (Figure 1). In food systems, these conditions for reformation of native structure are infrequently observed. Hence, any substantial structural perturbation due to salt, pH, alcohol, or heat treatment leads to irreversible changes in protein structure, even if a simulated physiological environment is restored. The nonnative structures thus formed have properties that are different from those of the native protein. Conse-

Studies on the heat stability of milk protein: II. Effect of exposing milk to light

SummaryIt was found that the increase in heat stability (coagulation time) that occurs when some milks are stored in light was not caused by changes in pH. These labile milks invariably gave a type A heat-stability curve, had a pH in the range corresponding to the coagulation-time minimum of the curve, and showed the expected abnormal 2-stage type of coagulation. The increase in coagulation time following exposure to light resulted from a delay in the occurrence of the first coagulation, the second coagulation time remaining unaltered. The presence of riboflavin in milk and of O2 in the headspace atmosphere during storage were necessary conditions for milk to be labile. Since light, headspace O2 and riboflavin had similar influences on rennet coagulation time it is suggested that in both instances the causative reaction, as proposed by others to account for the increase in rennet coagulation time of H2O2-treated milk, is stabilization of the heat- and rennin-sensitive methionine-phenylalaninebond in κ-casein by photo-oxidation of the methionine to methionine sulphoxide.

The stability of milk protein to heat: II. Effect on heat stability of ageing milk at different temperatures

SummaryThe effect on heat stability as measured by coagulation time, of storing separated milk at 20, 4 and −20 °C has been examined. Milk with a good coagulation (initial clots large) could be stored for at least 30 h at 20 °C, 1 week at 4 °C and 1 month at −20 °C with no significant change in coagulation time. With milks giving a poor coagulation (initial clots small), a common occurrence during storage at 20 °C was a marked progressive increase in coagulation time; the rate of increase was reduced by storage at 4 °C. The increase in coagulation time of these labile milks, which are usually obtained from cows with subclinical mastitis, may occur to the same extent in darkness as in light, may be enhanced by exposure to light or may occur only when the milk is exposed to light. From these results, together with others reported by Davies & White (1966) and White & Davies (1966) it is concluded that, in studying heat stability, milks giving a good coagulation should be regarded as in a different class from milks giving a poor coagulation.

A Test of the Heat Stability of Milk Proteins for Use in Research

Summary A test of the heat stability of milk proteins has been described which is suitable for research purposes. The necessary control of the different variables in the test has been evaluated and the reproducibility of the test established. The possible significance of the results obtained from the test was briefly discussed.