Interactive Effects of Milk Fat Globule and Casein Micelle Size on the Renneting Properties of Milk

Effect of solution plasma process on microorganism sterilization, physicochemical properties and nutrients of milk

Solution plasma process (SPP) was used for sterilizing Staphylococcus aureus (S. aureus) in raw milk (RM). The sterilization efficacy analysis and kinetics analysis showed bacterial concentration and the distance between electrodes were negatively correlated with the sterilization effect, while discharge voltage was positive. The better sterilization effect was achieved at 4 kV. The electrochemical indices analysis indicated that pH value of RM had no changed. The DO content decreased. The conductivity increased with the increasing discharge voltage. The nutrient content analysis revealed that the content of acidity, lactose, fat, and protein decreased. RM after SPP treatment exhibited higher values of sourness and slightly lower values of astringency than the control. The higher discharge voltage and narrower distance between electrodes presented the stronger effect. The structural characterization of CMs and MFGs was carried out using a laser particle sizer, FTIR, 1H NMR, XRD, and AFM. The results showed that the main chemical structure of CMs was unchanged basically. The SPP with the narrower distance between electrodes and lower discharge voltage significantly reduced the size and aggregation of MFGs at the molecular level. At 4 kV/2 mm, the particle sizes of CMs and MFGs were reduced from 238 nm and 523 nm to 224 nm and 302 nm, respectively. The average diameter of MFGs was reduced from 45 nm to 18 nm. Therefore, SPP is a potential method in the milk industry and provides a new idea for the preservation and processing of beverage.

Effect of composition, casein genetic variants and glycosylation degree on bovine milk whipping properties

This study investigated the individual and combined effects of ĸ-Casein (ĸ-CN; AA, AB, BB), β-Casein (β-CN; A1A1, A1A2, A2A2) and high and low ratios of glycosylated ĸ-CN to total ĸ-CN, referred to as the glycosylation degree (GD), on bovine cream whipping properties. The genetic variants of individual cows were identified using reversed-phase high-performance liquid chromatography (RP-HPLC) and verified through liquid chromatography-mass spectrometry (LC-MS). A previously discovered relationship between days-in-milk and GD was validated and used to obtain high and low GD milk. Whipped creams were created through the mechanical agitation of fat standardised cream from milk of different ĸ-CN, β-CN, and GD combinations, and whipping properties (the ability to whip, overrun, whipping time and firmness) were evaluated. No significant correlation was measured in whipping properties for cream samples from milks with different ĸ-CN and β-CN genetic variants. However, 80 % of samples exhibiting good whipping properties (i.e., the production of a stiffened peak) were from milk with low GD suggesting a correlation between whipping properties and levels of glycosylation. Moreover, cream separated from skim milk of larger casein micelle size showed superior whipping properties with shorter whipping times (<5 min), and higher firmness and overrun. Milk fat globule (MFG) size, on the other hand, did not affect whipping properties. Results indicate that the GD of κ-CN and casein micelle size may play a role in MFG adsorption at the protein and air interface of air bubbles formed during whipping; hence, they govern the dynamics of fat network formation and influencing whipping properties.

Influence of β-casein genotype on physicochemical properties and functionality of bovine milk

Several studies have been focused on the effect of milk protein genetic variants on milk physicochemical properties and functionality in recent years. β-casein, an important protein related to milk processibility, has been reported to have 2 main genetic variants A1 and A2, for which cows may be homozygous or heterozygous. In this study, several physicochemical properties of milk with β-casein variants A1A1, A1A2, and A2A2 from 3 collection occasions were analyzed. Higher manganese content and lower pH were found to be associated with the A1A1 variant compared with the other 2 genotypes. Better rennet and acid coagulation were found in A1A1 milk compared with A1A2 and A2A2 milk (although P > 0.05), whereas A2A2 milk was more stable to creaming compared with the other 2 genotypes, which may be linked to its smaller fat globule size. Thus, milk from cows with A1A1 genotype could be preferable for cheese making, while that with A2A2 variant can be used in formulations requiring good stability against creaming, and for example, yogurt making, where the softer yogurt texture may be easier to digest.

Impact of varying levels of pasture allowance on the nutritional quality and functionality of milk throughout lactation

The objective of this study was to examine the impact of increasing proportions of grazed pasture in the diet on the composition, quality, and functionality of bovine milk across a full lactation. Fifty-four spring-calving cows were randomly assigned to 1 of 3 groups (n = 18), blocked on the basis of mean calving date (February 15, 2020 ± 0.8 d), pre-experimental daily milk yield (24.70 ± 3.70 kg), milk solids yield (2.30 ± 0.27 kg), lactation number (3.10 ± 0.13), and economic breeding index (182 ± 19). Raw milk samples were obtained weekly from each group between March and November 2020. Group 1 (GRS) consumed perennial ryegrass and was supplemented with 5% concentrates (dry matter basis); group 2 was maintained indoors and consumed a total mixed ration (TMR) diet consisting of maize silage, grass silage, and concentrates; and group 3 consumed a partial mixed ration diet (PMR), rotating between perennial ryegrass during the day and indoor TMR feeding at night. Raw milk samples consisted of a pooled morning and evening milking and were analyzed for gross composition, free amino acids, fatty acid composition, heat coagulation time, color, fat globule size, and pH. The TMR milks had a significantly higher total solids, lactose, protein, and whey protein as a proportion of protein content compared with both GRS and PMR milks. The GRS milks demonstrated a significantly lower somatic cell count (SCC), but a significantly higher pH and b*-value than both TMR and PMR milks. The PMR milks exhibited significantly lower total solids and fat content, but also demonstrated significantly higher SCC and total free amino acid content compared with GRS and TMR. Partial least squares discriminant analysis of fatty acid profiles displayed a distinct separation between GRS and TMR samples, while PMR displayed an overlap between both GRS and TMR groupings. Variable importance in projection analysis identified conjugated linoleic acid cis-9,trans-11, C18:2n-6 cis, C18:3n-3, C11:0, and C18:2n-6 trans as the largest contributors to the variation between the diets. Milk fats derived from GRS diets exhibited the highest proportion of unsaturated fats and higher unsaturation, health-promoting, and desaturase indices. The lowest proportions of saturated fats and the lowest atherogenic index were also exhibited by GRS-derived milk fats. This work highlights the positive influence of grass-fed milk for human consumption through its more nutritionally beneficial fatty acid profile, despite the highest milk solid percentages derived from TMR feeding systems. Furthermore, this study demonstrates the proportional response of previously highlighted biomarkers of pasture feeding to the proportion of pasture in the cow's diet.

Effect of Direct Steam Injection and Instantaneous Ultra-High-Temperature (DSI-IUHT) Sterilization on the Physicochemical Quality and Volatile Flavor Components of Milk

The effects of variations in the heat treatment process of milk on its quality and flavor are inevitable. This study investigated the effect of direct steam injection and instantaneous ultra-high-temperature (DSI-IUHT, 143 °C, 1-2 s) sterilization on the physicochemical properties, whey protein denaturation (WPD) rate, and volatile compounds (VCs) of milk. The experiment compared raw milk as a control with high-temperature short-time (HTST, 75 °C 15 s and 85 °C 15 s) pasteurization and indirect ultra-high-temperature (IND-UHT, 143 °C, 3-4 s) sterilization. The results showed no significant differences (p > 0.05) in physical stability between milk samples with different heat treatments. The DSI-IUHT and IND-UHT milks presented smaller particle sizes (p < 0.05) and more concentrated distributions than the HTST milk. The apparent viscosity of the DSI-IUHT milk was significantly higher than the other samples (p < 0.05) and is consistent with the microrheological results. The WPD of DSI-IUHT milk was 27.52% lower than that of IND-UHT milk. Solid-phase microextraction (SPME) and solvent-assisted flavor evaporation (SAFE) were combined with the WPD rates to analyze the VCs, which were positively correlated with ketones, acids, and esters and negatively associated with alcohols, heterocycles, sulfur, and aldehydes. The DSI-IUHT samples exhibited a higher similarity to raw and HTST milk than the IND-UHT samples. In summary, DSI-IUHT was more successful in preserving the milk's quality due to its milder sterilization conditions compared to IND-UHT. This study provides excellent reference data for the application of DSI-IUHT treatment in milk processing.

Fabrication, characterization, and potential applications of re-assembled casein micelles

Re-assembled casein micelles (rCMs), were formulated in the 1970s as a model system to understand native casein micelles (nCMs) in milk. These early works allowed an understanding of the critical factors involved in the formation of rCMs, such as minerals (citrate, phosphate, and calcium), casein type (αs-, β-, and κ-casein) and the extent of their phosphorylation. rCMs were also used to understand the effect of treatments such as ethanol, high hydrostatic pressure and heating on the stability and integrity of the micelles. More recently, the applications of rCMs have been investigated, these include their use as a nanocarrier of bioactive molecules and as electrode-bound substrates to monitor chymosin activity by electrochemistry, to cite a few. Moreover, the potential to use rCMs in both food and non-food applications remains to be fully exploited. The advantage of choosing rCMs over nCMs as an encapsulant and a lucrative food ingredient is due to their more efficient preparation and being free from impurities. In this review, we report on the formulation of rCMs, their physico-chemical properties and their behavior under different physico-chemical treatments, along with the applications and challenges of rCMs in food systems and their industrial production as a dairy ingredient.

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.

Gastric devolution of transglutaminase-induced acid and rennet-induced casein gels using dynamic DIDGI® and static COST action INFOGEST protocols

Limited studies in the literature have compared in vitro dynamic and in vitro static protocols for modelling the gastric digestive process of food systems. This experiment explores the differences between two different in vitro approaches to the devolution of a transglutaminase-induced acid gel (TG, pH 5.1-5.3) and rennet-induced gel (RG, pH 6.5-6.7). Gels were exposed to a simulated oral phase, followed by either the dynamic DIDGI® or static COST action INFOGEST protocol to simulate gastric conditions. Protein hydrolysis was evident from 15 min onwards for TG exposed to the dynamic protocol where levels continued to increase at a steady rate. In contrast, RG exhibited a notable lag-phase before levels increased from around 60 min onwards. Under the static protocol, protein hydrolysis was observed for both TG and RG upon exposure to the gastric environment which continued to increase over time. Despite these differences, similar levels of protein hydrolysis were found for TG and RG at the gastric endpoint using either protocol demonstrating that both the dynamic DIDGI® and static COST action INFOGEST methods provide a suitable and comparable environment for the in vitro digestion of casein protein under simulated gastric conditions.

Tempering governs the milk fat crystallisation and viscoelastic behaviour of unprocessed and homogenised creams

The crystallisation behaviour of milk fat plays an important role in the functionality and sensory properties of fat-rich dairy products. In this study, we investigated the impact of tempering to 25 °C on the viscoelastic properties, particle size and thermal behaviour of 20% w/w unprocessed and homogenised creams prepared from bovine milk. The crystallisation properties were examined by synchrotron X-ray diffraction (XRD) at small (SAXS) and wide angle (WAXS) and differential scanning calorimetry (DSC). Oscillation rheology was performed to characterise the cream's viscoelastic properties. Homogenisation (35 MPa) reduced the average droplet size from 4.4 to 1.3 µm. After 24 h storage at 4 °C, milk fat structures showed triacylglycerol (TAG) 2L and 3L_{(001, 002, 003, 005)} lamellar stacking orders associated predominantly with the α and β' polymorphic forms. Tempering to 25 °C induced the complete melting of the 3L crystals and led to an irreversible loss in the elastic modulus (G') and a reduction in the viscous modulus (G'') once returned to refrigerated conditions, due to changes in the particle-particle interactions and structure of the reformed milk fat crystals. The results demonstrate that crystallisation behaviour of milk fat is influenced by droplet size and the rearrangement of triacylglycerol (TAG) upon tempering, and lead to changes in the viscoelastic behaviour of dairy products containing a high level of milk fat.

Impact of gastric coagulation on the kinetics of release of fat globules from milk of different species

The behavior of fat globules during the gastric digestion of raw and pasteurized cow, goat, and sheep whole milks was studied using a human gastric simulator. Microstructural and physicochemical analysis revealed that, initially, the coagulation of the milks in the human gastric simulator resulted in the majority of the milk fat globules being entrapped within the curd. As the digestion progressed, the proportion of fat globules entrapped within the aggregated protein matrix (curd) decreased; there was also some flocculation as well as coalescence of the fat globules within the curd. The liberation of the entrapped fat globules from the curd to the liquid phase of the chyme was strongly dependent on the disintegration and hydrolysis of the structured casein network. Surprisingly, the fat globules released (or already present) into the liquid phase of the chyme were not as extensively coalesced as those remaining within the curd. These phenomena were observed to be similar for the raw and pasteurized whole milk of all species. The pasteurized whole milks from all species formed relatively less structured coagula compared with their raw milk counterparts, leading to a greater extent of protein breakdown and, thus, higher proportions of fat release from the pasteurized milk curds. This study provides a deeper understanding of how the curd-forming properties of different mammalian milks in the gastric environment provide controlled delivery of nutrients (such as protein and fat).

Investigating casein gel structure during gastric digestion using ultra-small and small-angle neutron scattering

This study aimed to understand the structural devolution of 10% w/w rennet-induced (RG) and transglutaminase-induced acid (TG) gels in H₂O and D₂O under in vitro gastric conditions with and without pepsin. The real-time devolution of structure at a nano- (e.g. colloidal calcium phosphate (CCP) and micelle) and micro- (gel network) level was determined using ultra-small (USANS) and small-angle neutron scattering (SANS) with electron microscopy. Results demonstrate that gel firmness or elasticity determines disintegration behaviour during simulated mastication and consequently the particle size entering the stomach. Shear of mixing in the stomach, pH, and enzyme activity will also affect the digestion process. Our results suggest that shear of mixing primarily results in erosion at the particle surface and governs gel disintegration behaviour during the early stages of digestion. Pepsin diffusivity, and hence action, occur more readily in the latter stages of gastric digestion via access to the particle interior. This occurs via the progressively larger pores of the looser gel network and channels created within the larger, less dense casein micelles of the RG gels. Gel firmness and brittleness were greater in the D₂O samples compared to H₂O, facilitating gel disintegration. Despite the higher strength and elasticity of RG compared to TG, the protein network strands of the RG gels become more compact when exposed to the acidic gastric environment with comparatively larger pores observed through SEM imaging. This led to a higher degree of digestibility in RG gels compared to TG gels. This is the first study to examine casein gel structure during simulated gastric digestion using scattering and highlights the benefits of neutron scattering to monitor structural changes during digestion at multiple length scales.

Milk Products from Minor Dairy Species: A Review

Milk processing is one of the most ancient food technologies, dating back around 6000 BC. The majority of dairy products are manufactured from cows, buffaloes, goats, and sheep; their production technologies are mostly standardized and have been widely investigated. Milk and dairy products from minor species are less important under the economic point of view, but they play a fundamental social role in many marginal and poor areas. Due to scarce interest of the dairy industry, their technological characteristics and related issues have been investigated less. Recently, the increasing interest toward ethnic foods and food biodiversity is helping these minor products to emerge from the "darkness" in which they have remained for long time. Some of them are increasingly seen as useful for the valorization of marginal areas, while others are recognized as innovative or healthy foods. The present review aims to resume the most recent knowledge about these less-known dairy products. The first part summarizes the main technological properties of equine, camel, and yak milk with a view to processing. The second is a survey on the related dairy products, both the traditional ones that have been manufactured for a long time and those that have been newly developed by food researchers.

Biotechnological potential of a cysteine protease (CpCP3) from Calotropis procera latex for cheesemaking

This article reports the characterization and evaluation of the biotechnological potential of a cysteine protease purified from Calotropis procera (CpCP3). This enzyme was highly stable to different metal ions and was able to hydrolyze κ-casein similarly to bovine chymosin. Atomic force microscopy showed that the process of casein micelle aggregation induced by CpCP3 was similar to that caused by chymosin. The cheeses made using CpCP3 showed higher moisture content than those made with chymosin, but protein, fat, and ash were similar. The sensory analysis showed that cheeses made with CpCP3 had high acceptance index (>80%). In silico analysis predicted the presence of only two short allergenic peptides on the surface of CpCP3, which was highly susceptible to digestive enzymes and did not alter zebrafish embryos' morphology and development. Moreover, recombinant CpCP3 was expressed in Escherichia coli. All results support the biotechnological potential of CpCP3 as an alternative enzyme to chymosin.

Interactive effects of casein micelle size and calcium and citrate content on rennet-induced coagulation in bovine milk

Interactive effects of casein micelle size and milk calcium and citrate content on rennet-induced coagulation were investigated. Milk samples containing small (SM) and large (LM) micelles, obtained from individual Holstein cows, were modified by addition of calcium and/or citrate and milk coagulation properties were evaluated in a full factorial design. The results showed that LM milk had a higher relative proportion of casein, coagulated faster, and resulted in a stronger gel than SM milk. Addition of calcium slightly decreased casein micelle size, while addition of citrate slightly increased micelle size. Calcium addition resulted in a shorter coagulation time and the strongest gels, while citrate addition increased the coagulation time and resulted in the weakest gels. Addition of calcium and citrate in combination resulted in intermediate coagulation properties. The interactive effect of micelle size and citrate was significant for gel strength. Microstructural differences between the milk gels were consistent with the rheological properties, for example, the micrographs revealed that a more homogeneous network was formed when calcium was added, resulting in a stronger gel. A more inhomogeneous network structure was formed when citrate was added, resulting in a weaker gel. Thus, variations in casein micelle size and in calcium and citrate content influence rennet-induced coagulation in bovine milk. The calcium and citrate contents in Swedish milk have changed over time, whereby calcium content has increased and citrate content has decreased. In practical cheese making, calcium is added to cheese milk, most likely altering the role of inherent citrate and possibly influencing casein micelle size. The observed interaction effect between casein micelle size and citrate in this study, suggests that larger micelles with moderate citrate level will result in firmer gels, whereas a higher citrate content reduced gel strength more in case of large than SM. Since firmer gels are likely to retain more protein and fat than less firmer gels, this interaction effect could have implications in practical cheese production.

Symposium review: Structure-function relationships in cheese

The quality and commercial value of cheese are primarily determined by its physico-chemical properties (e.g., melt, stretch, flow, and color), specific sensory attributes (e.g., flavor, texture, and mouthfeel), usage characteristics (e.g., convenience), and nutritional properties (e.g., nutrient profile, bioavailability, and digestibility). Many of these functionalities are determined by cheese structure, requiring an appropriate understanding of the relationships between structure and functionality to design bespoke functionalities. This review provides an overview of a broad range of functional properties of cheese and how they are influenced by the structural organization of cheese components and their interactions, as well as how they are influenced by environmental factors (e.g., pH and temperature).

Advances in high frequency ultrasound separation of particulates from biomass

In recent years the use of high frequency ultrasound standing waves (megasonics) for droplet or cell separation from biomass has emerged beyond the microfluidics scale into the litre to industrial scale applications. The principle for this separation technology relies on the differential positioning of individual droplets or particles across an ultrasonic standing wave field within the reactor and subsequent biomass material predisposition for separation via rapid droplet agglomeration or coalescence into larger entities. Large scale transducers have been characterised with sonochemiluminescence and hydrophones to enable better reactor designs. High frequency enhanced separation technology has been demonstrated at industrial scale for oil recovery in the palm oil industry and at litre scale to assist olive oil, coconut oil and milk fat separation. Other applications include algal cell dewatering and milk fat globule fractionation. Frequency selection depends on the material properties and structure in the biomass mixture. Higher frequencies (1 and 2MHz) have proven preferable for better separation of materials with smaller sized droplets such as milk fat globules. For palm oil and olive oil, separation has been demonstrated within the 400-600kHz region, which has high radical production, without detectable impact on product quality.