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

Heat Stability Assessment of Milk: A Review of Traditional and Innovative Methods

It is important to differentiate milk with different thermostabilities for diverse applications in food products and for the appropriate selection of processing and maintenance of manufacturing facilities. In this review, an overview of the chemical changes in milk subjected to high-temperature heating is given. An emphasis is given to the studies of traditional and state-of-the-art strategies for assessing the milk thermostability, as well as their influencing factors. Traditional subjective and objective techniques have been used extensively in many studies for evaluating thermostability, whereas recent research has been focused on novel approaches with greater objectivity and accuracy, including innovative physical, spectroscopic, and predictive tools.

Interactions between intestinal microbial fermentation products of Pleurotus eryngii polysaccharide with gut mucus

Recently, Pleurotus eryngii (P. eryngii) polysaccharide (PEP) has received a lot of attention from many researchers as the primary active substance. The PEP influences the gut microbiota in several ways, including the interaction of fermentation products with the intestinal mucus layer (IML) and intestinal epithelial cells. Herein, we characterized interactions between the IML and PEP after degradation by the gut microbes. Our results showed that fermented P. eryngii polysaccharide (FPEP) can interact with intestinal mucus (IM), and this interaction can reduce the degree of molecular aggregation of polysaccharides. At the same time, the fermentation time of FPEP also affects the interaction between the two. SEM showed that the FPEP solution tended to aggregate into larger particles, while with the addition of IM, the FPEP molecules were dispersed. Particle size measurements unveil substantial differences in the fermented polysaccharides' particle size between the group with supplementary IM (0 hours of fermentation: 485.1 ± 11.3 nm) and the group without IM (0 hours of fermentation: 989.33 ± 21.3 nm). Remarkably, within the group with added IM, the particle size reached its maximum at 24 hours of fermentation (585.87 ± 42.83 nm). Additionally, turbidity assessments demonstrate that, during the 12-hour interaction period, the 24-hour fermented polysaccharides consistently exhibit the highest OD values, ranging between 0.57 and 0.61. This work investigates the interaction between FPEP and IM, predicting the adhesion of polysaccharides to IM. Meanwhile, this provides a theoretical basis for further studies on the absorption and transport pathways of PEP and provides a novel research viewpoint on intestinal digestion and absorption.

The effect of in vitro digestion on the interaction between polysaccharides derived from Pleurotus eryngii and intestinal mucus

Pleurotus eryngii polysaccharides (PEPs) have been proven to display multiple activities through digestive system action, from which the digestion products should first interact with intestinal mucus (MUC), followed by the function of intestinal cells. Hence, possible interacting characterizations between MUC and in vitro simulated digestion products of P. eryngii polysaccharides (DPEPs) and PEP were carried out in the present study. Results showed that both PEP and DPEP could significantly interact with MUC. Moreover, digestion can modify the interaction between polysaccharides and MUC; the degree of interaction also changes with time incrementing. Viscosity could be decreased after digesting. According to the zeta potential and stability analysis result, the digestive behavior could be regular and stable between polysaccharides and MUC interactions. Following fluorescence and infrared spectra, the structure of polysaccharides and mucin might be changed by digestion between polysaccharides and MUC. The study indicates that the interaction formed between DPEP and MUC might indirectly impact the exercise and immune activities of polysaccharides and influence the transportation of other nutrients. Overall, our results, the absorption and transport pathways of PEP, can be initially revealed and may provide a novel research viewpoint on the active mechanism of PEP in the intestinal tract.

Effect of Fenugreek (Trigonella foenum-graecum L.) Seed Extracts on the Structure of Myofibrillar Protein Oxidation in Duck Meat

The effect of fenugreek (Trigonella foenum-graecum L.) seed extracts (FSEs) on the structure of duck myofibrillar protein (MP) oxidation was researched via particle size, zeta potential, Fourier transform infrared (FTIR), fluorescence spectroscopy, SDS-PAGE, and scanning electron microscopy (SEM) in the Fenton oxidation system. FSE (0.3 mg/mL) could scavenge 58.79% of the hydroxyl radical and possessed good antioxidation. FSE could retard the oxidation of MP, and the carbonyl formation and total sulfhydryl loss of MP decreased by 42.00% and 105.94%, respectively, after 4.67% of FSE treatment. SDS-PAGE results showed that 0.67% and 2.67% of FSE decreased the strength of the myosin heavy chain (MHC) and actin bands of the oxidized MP, respectively. The FSE changed the secondary structures of the MP and promoted the unfolding of the MP structure and the transformation from α-helix to β-turn. When treated with 0.67% of FSE, the hydrophobicity of the MP declined by 26.14%, and solubility was improved by 37.21% compared with the oxidation group. After 0.67% of FSE treatment, the particle size and zeta potential of the MP returned to the level of the blank group. Scanning electron microscopy revealed that FSE improved the apparent morphology of the MP. Overall, FSE had positive effects on the antioxidation of the duck MP, and it could improve the structure and characteristics of the MP. It is hoped that FSE could be considered as a natural antioxidant to retard the oxidation of the MP in meat products.

Comprehensive binding analysis of glycated myosin with furan derivatives via glucose by means of multi-spectroscopy techniques and molecular docking simulation

Myosin is an ideal binding receptor for aroma compounds and its functional properties are easily affected by glucose. The study comprehensively clarified the effects of glucose glycation-induced structural modifications of myosin on its binding ability with furan derivatives, including 2-methylfuran, 2-furfural, and 2-furfurylthiol. The results demonstrated that the binding levels of furan derivatives were obviously affected by the glycation levels of myosin due to the changes of myosin structure and surface. The increased glycation levels caused the unfolding of myosin structure and accelerated the aggregation, as were exhibited by the data of zeta potential, particle size, microstructure, and secondary structure. The glycated myosin with wrinkled surfaces favored the significant increase of hydrophobic interactions (31.59-69.50 μg), the more exposure of amino acid residues (3459-6048), the formation of free sulfhydryl groups (16.37-20.58 mmol/104g) and hydrogen bonds. These key (non)covalent linkages accounted for the generation of glycated myosin-odorants complex, including 2-furfurylthiol (29.17-47.87 %), thus enhancing the resultant binding ability as evidenced by the free furan derivatives concentrations, fluorescence quenching and molecular docking simulation analysis. The glycated myosin for 8 h bound highest concentrations of furan derivatives. The results will provide comprehensive data on the retention of aroma compounds in meat products.

Comparison of the Physicochemical Properties of Carboxymethyl Agar Synthesized by Microwave-Assisted and Conventional Methods

The microwave-assisted carboxymethylation of agar to improve its physicochemical properties was investigated. Microwave power, reaction time, and temperature, ethanol concentration, and amounts of chloroacetic acid and sodium hydroxide were assessed for their effects on synthetic yield and degree of substitution (DS). All factors were positively correlated with DS within a certain range. Using optimized conditions, samples with different DS were prepared, and the physicochemical properties of unmodified and carboxymethyl agars prepared by microwave and conventional methods were compared. Carboxymethylation significantly changed the physicochemical properties of the agar, improving gel transparency and reducing dissolution temperature, gel strength, gel hardness, molecular weight, and molecular size; DS was the key factor. Specifically, higher DS values resulted in greater changes. The microwave-assisted method significantly shortened the reaction time and preserved molecular weight, gel strength, and texture hardness of the agar. Therefore, as an environmentally friendly method, microwave-assisted synthesis shows great promise for producing carboxymethyl agar.

Age Gelation, Sedimentation, and Creaming in UHT Milk: A Review

Demand for ultra-high-temperature (UHT) milk and milk protein-based beverages is growing. UHT milk is microbiologically stable. However, on storage, a number of chemical and physical changes occur and these can reduce the quality of the milk. These changes can be sufficiently undesirable so as to limit acceptance or shelf life of the milk. The most severe changes in UHT milk during storage are age gelation, with an irreversible three-dimensional protein network forming throughout, excessive sedimentation with a compact layer of protein-enriched material forming rapidly at the bottom of the pack, and creaming with excessive fat accumulating at the top. For age gelation, it is known that at least two mechanisms can lead to gelation during storage. One mechanism involves proteolytic degradation of the proteins through heat-stable indigenous or exogenous enzymes, destabilizing milk and ultimately forming a gel. The other mechanism is referred to as a physico-chemical mechanism. Several factors are known to affect the physico-chemical age gelation, such as milk/protein concentration, heat load during processing (direct compared with indirect UHT processes), and milk composition. Similar factors to age gelation are known to affect sedimentation. There are relatively few studies on the creaming of UHT milk during storage, suggesting that this defect is less common or less detrimental compared with gelation and sedimentation. This review focuses on the current state of knowledge of age gelation, sedimentation, and creaming of UHT milks during storage, providing a critical evaluation of the available literature and, based on this, mechanisms for age gelation and sedimentation are proposed.

The effect of storage conditions on the composition and functional properties of blended bulk tank milk

The objective of this study was to investigate the effects of storage temperature and duration on the composition and functional properties of bulk tank milk when fresh milk was added to the bulk tank twice daily. The bulk tank milk temperature was set at each of 3 temperatures (2, 4, and 6°C) in each of 3 tanks on 2 occasions during two 6-wk periods. Period 1 was undertaken in August and September when all cows were in mid lactation, and period 2 was undertaken in October and November when all cows were in late lactation. Bulk tank milk stored at the 3 temperatures was sampled at 24-h intervals during storage periods of 0 to 96 h. Compositional parameters were measured for all bulk tank milk samples, including gross composition and quantification of nitrogen compounds, casein fractions, free amino acids, and Ca and P contents. The somatic cell count, heat stability, titratable acidity, and rennetability of bulk tank milk samples were also assessed. Almost all parameters differed between mid and late lactation; however, the interaction between lactation, storage temperature, and storage duration was significant for only 3 parameters: protein content and concentrations of free cysteic acid and free glutamic acid. The interaction between storage temperature and storage time was not significant for any parameter measured, and temperature had no effect on any parameter except lysine: lysine content was higher at 6°C than at 2°C. During 96 h of storage, the concentrations of some free amino acids (glutamic acid, lysine, and arginine) increased, which may indicate proteolytic activity during storage. Between 0 and 96 h, minimal deterioration was observed in functional properties (rennet coagulation time, curd firmness, and heat stability), which was most likely due to the dissociation of β-casein from the casein micelle, which can be reversed upon pasteurization. Thus, this study suggests that blended milk can be stored for up to 96 h at temperatures between 2°C and 6°C with little effect on its composition or functional properties.

Physicochemical and sensory properties of milk supplemented with dispersible nanopowdered oyster shell during storage

The current study was carried out to investigate the dispersibility of powdered oyster shell (POS), nanopowdered oyster shell (NPOS), and Zn-activated nanopowdered oyster shell (Zn-NPOS) in milk and to determine effects of adding oyster shell on the physicochemical and sensory properties of milk during storage at 4°C for 16 d. To ensure dispersibility, 10% (wt/vol) oyster shell was added to distilled water and stirred at 800 rpm for 2 h, and then the emulsifier 0.5% polyglycerol monostearate (PGMS) was added and stirred continually for 24 h. The particle sizes of POS, NPOS, and Zn-NPOS were 180μm, 389 nm, and 257 nm, respectively. The pH values of all milk samples ranged from 6.62 to 6.88 during storage, and the zeta-potential of milks with NPOS and Zn-NPOS added were more stable than that of milk with POS in low concentrations (0.5 and 1.0%, vol/vol) during storage. The L and a color values of the milks were not significantly influenced by treatment; however, the b value (yellow-blue color) significantly increased during storage after adding POS, NPOS, or Zn-NPOS. Sensory analysis revealed that sedimentation score significantly increased with POS-supplemented milk, but the NPOS- and Zn-NPOS-supplemented milks did not show sedimentation until after 8 d of storage. Based on the data obtained, we conclude that dispersible nanosized oyster shell at concentrations of 0.5 and 1.0% (vol/vol) could be supplemented to milk without significant adverse effects on physicochemical and sensory properties.

Plasmin activity in UHT milk: relationship between proteolysis, age gelation, and bitterness

Plasmin, the major indigenous protease in milk, is linked to quality defects in dairy products. The specificity of plasmin on caseins has previously been studied using purified caseins and in the indigenous peptide profile of milk. We investigated the specificity and proteolytic pathway of plasmin in directly heated UHT milk (>150 °C for <0.2 s) during 14 weeks of storage at 20 °C in relation to age gelation and bitter peptides. Sixty-six peptides from αS- and β-caseins could be attributed to plasmin activity during the storage period, of which 23 were potentially bitter. Plasmin exhibited the highest affinity for the hydrophilic regions in the caseins that most probably were exposed to the serum phase and the least affinity for hydrophobic or phosphorylated regions. The proteolytic pattern observed suggests that plasmin destabilizes the casein micelle by hydrolyzing casein-casein and casein-calcium phosphate interaction sites, which may subsequently cause age gelation in UHT milk.

Preservative action of 11S (glycinin) and 7S (β-conglycinin) soy globulin on bovine raw milk stored either at 4 or 25 °C

Considerable inhibitory antibacterial actions were exerted by the soybean 11S subunit comparable with nisin on the proliferation of total viable count, Pseudomonas count and Enterobacteriaceae count in bovine milk stored at 4 or 25 °C for 30 d and 48 h, while 7S and lysozyme were much less effective. The maximum magnitudes of bacterial reduction by 11S and nisin were in the range 2-4 log CFU/ml. The proliferation of 3 pathogenic bacteria (Listeria monocytogenes, Salmonella Enteritidis and Escherichia coli O157:H7) artificially inoculated into raw milk stored at 4 or 25 °C were particularly and significantly (P<0.05) reduced by 11S subunit and nisin (0.5% w/v), but only slightly by 7S and moderately by lysozyme. Lactose consumption, acidity development and casein degradation during storage of bovine raw milk were attenuated during storage at 4 or 25 °C and sensorial traits were better maintained by supplementation with 11S (0.5% w/v). 11S subunit may be used a safely food preservative, if permitted.