Journal Articles
Permanent URI for this collectionhttps://mro.massey.ac.nz/handle/10179/7915
Browse
8 results
Search Results
Item Comparative lipidomics analysis of in vitro lipid digestion of sheep milk: Influence of homogenization and heat treatment(Elsevier Inc on behalf of the American Dairy Science Association, 2024-02) Pan Z; Ye A; Fraser K; Li S; Dave A; Singh HThis study investigated the changes in sheep milk lipids during in vitro gastrointestinal digestion in response to heat treatment (75°C/15 s and 95°C/5 min) and homogenization (200/50 bar) using lipidomics. Homogenized and pasteurized sheep milk had higher levels of polar lipids in gastric digesta emptied at 20 min than raw sheep milk. Intense heat treatment of homogenized sheep milk resulted in a reduced level of polar lipids compared with homogenized-pasteurized sheep milk. The release rate of free fatty acids during small intestinal digestion for gastric digesta emptied at 20 min followed the order: raw ≤ pasteurized < homogenized-pasteurized ≤ homogenized-heated sheep milk; the rate for gastric digesta emptied at 180 min showed a reverse order. No differences in the lipolysis degree were observed among differently processed sheep milks. These results indicated that processing treatments affect the lipid composition of digesta and the lipolysis rate but not the lipolysis degree during small intestinal digestion.Item Dynamic in vitro gastric digestion behavior of goat milk: Effects of homogenization and heat treatments.(Elsevier Inc on behalf of the American Dairy Science Association, 2022-02) Li S; Ye A; Pan Z; Cui J; Dave A; Singh HThe gastric digestion behavior of differently processed goat milks was investigated using a dynamic in vitro gastric digestion model, the human gastric simulator. Homogenization and heat treatment of goat milk resulted in gastric clots with highly fragmented structures. They also delayed the pH reduction during digestion, altered the chemical composition of the clots and the emptied digesta, promoted the release of calcium from the clots, and accelerated the hydrolysis and the emptying of milk proteins. The apparent density of the protein particles and the location of the homogenized fat globules changed during the digestion process, as shown in the emptied digesta of the homogenized goat milks. The effects of processing on the digestion behavior of goat milk were broadly similar to those previously reported for cow milk. However, the overall gastric digestion process of goat milk was more affected by homogenization than by heat treatments.Item Kinetics of heat-induced interactions among whey proteins and casein micelles in sheep skim milk and aggregation of the casein micelles(Elsevier Inc on behalf of the American Dairy Science Association, 2022-05) Pan Z; Ye A; Dave A; Fraser K; Singh HThe interactions among the proteins in sheep skim milk (SSM) during heat treatments (67.5-90°C for 0.5-30 min) were characterized by the kinetics of the denaturation of the whey proteins and of the association of the denatured whey proteins with casein micelles, and changes in the size and structure of casein micelles. The relationship between the size of the casein micelles and the association of whey proteins with the casein micelles is discussed. The level of denaturation and association with the casein micelles for β-lactoglobulin (β-LG) and α-lactalbumin (α-LA) increased with increasing heating temperature and time; the rates of denaturation and association with the casein micelles were markedly higher for β-LG than for α-LA in the temperature range 80 to 90°C; the Arrhenius critical temperature was 80°C for the denaturation of both β-LG and α-LA. The casein micelle size increased by 7 to 120 nm, depending on the heating temperature and the holding time. For instance, the micelle size (about 293 nm) of SSM heated at 90°C for 30 min increased by about 70% compared with that (about 174.6 nm) of unheated SSM. The casein micelle size increased slowly by a maximum of about 65 nm until the level of association of the denatured whey proteins with casein micelles reached 95%, and then increased markedly by a maximum of about 120 nm when the association level was greater than about 95%. The marked increases in casein micelle size in heated SSM were due to aggregation of the casein micelles. Aggregation of the casein micelles and association of whey protein with the micelles occurred simultaneously in SSM during heating.Item Movements of moisture and acid in gastric milk clots during gastric digestion: Spatiotemporal mapping using hyperspectral imaging(Elsevier Ltd, 2024-01-15) Li S; Dixit Y; Reis MM; Singh H; Ye ARuminant milk is known to coagulate into structured clots during gastric digestion. This study investigated the movements of moisture and acid in skim milk clots formed during dynamic gastric digestion and the effects of milk type (regular or calcium-rich) and the presence/absence of pepsin. We conducted hyperspectral imaging analysis and successfully modelled the moisture contents based on the spectral information using partial least squares regression. We generated prediction maps of the spatiotemporal distribution of moisture within the samples at different stages of gastric digestion. Simultaneously to acid uptake, the moisture in the milk clots tended to decrease over the digestion time; this was significantly promoted by pepsin. Moisture mapping by hyperspectral imaging demonstrated that the high and low moisture zones were centralized within the clot and at the surface respectively. A structural compaction process promoted by pepsinolysis and acidification probably contributed to the water expulsion from the clots during digestion.Item Structural changes in milk from different species during gastric digestion in piglets(Elsevier Inc and Fass Inc on behalf of the American Dairy Science Association, 2022-05) Roy D; Moughan PJ; Ye A; Hodgkinson SM; Stroebinger N; Li S; Dave AC; Montoya CA; Singh HThis study investigated the structural and physicochemical changes that occur in milk, a naturally designed complex structured emulsion, during gastric digestion using the bottle-fed piglet as an animal model. The gastric digestions of cow, goat, and sheep milk were compared in male piglets euthanized at different postfeeding times to collect the stomach chyme. The cow and noncow milks separated into curd (aggregated caseins) and liquid (mostly soluble whey) phases in the piglet's stomach. For milk from all the species, the curd remained longer in the stomach because of its slow disintegration, whereas the liquid phase emptied readily. The majority of the fat globules were found to be entrapped within the protein network of the curd. The rate of release of fat globules was strongly dependent on the breakdown of the surrounding protein network of the curd. The consistency of the gastric curds changed as digestion progressed, with goat and sheep milk curds having relatively softer curd consistency and less fused protein networks, especially toward the end of digestion. This might have led to the lower protein and fat retention in the goat and sheep milk curds and relatively faster gastric emptying of these nutrients from goat and sheep milk in comparison to cow milk. This in vivo study provided new and enhanced understanding of the mechanisms of the gastric digestion of milk from different species. It may have implications for developing bioinspired structures for the controlled digestion and delivery of nutrients.Item pH-dependent sedimentation and protein interactions in ultra-high-temperature-treated sheep skim milk(Elsevier Inc and Fass Inc on behalf of the American Dairy Science Association, 2023-03) Pan Z; Ye A; Dave A; Fraser K; Singh HSheep milk is considered unstable to UHT processing, but the instability mechanism has not been investigated. This study assessed the effect of UHT treatment (140°C/5 s) and milk pH values from 6.6 to 7.0 on the physical properties of sheep skim milk (SSM), including heat coagulation time, particle size, sedimentation, ionic calcium level, and changes in protein composition. Significant amounts of sediment were found in UHT-treated SSM at the natural pH (∼6.6) and pH 7.0, whereas lower amounts of sediment were observed at pH values of 6.7 to 6.9. The proteins in the sediment were mainly κ-casein (CN)-depleted casein micelles with low levels of whey proteins regardless of the pH. Both the pH and the ionic calcium level of the SSM at all pH values decreased after UHT treatment. The dissociation levels of κ-, β-, and αS2-CN increased with increasing pH of the SSM before and after heating. The protein content, ionic calcium level, and dissociation level of κ-CN were higher in the SSM than values reported previously in cow skim milk. These differences may contribute to the high amounts of sediment in the UHT-treated SSM at natural pH (∼6.6). Significantly higher levels of κ-, β-, and αS2-CN were detected in the serum phase after heating the SSM at pH 7.0, suggesting that less κ-CN was attached to the casein micelles and that more internal structures of the casein micelles may have been exposed during heating. This could, in turn, have destabilized the casein micelles, resulting in the formation of protein aggregates and high amounts of sediment after UHT treatment of the SSM at pH 7.0.Item Acid and rennet gelation properties of sheep, goat, and cow milks: Effects of processing and seasonal variation(Elsevier Inc on behalf of the American Dairy Science Association, 2023-03) Li S; Delger M; Dave A; Singh H; Ye AGelation 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.Item β-Lactoglobulin nanofibrils: Effect of temperature on fibril formation kinetics, fibril morphology and the rheological properties of fibril dispersions(Elsevier Ltd, 2012-05) Loveday SM; Wang XL; Rao MA; Anema SG; Singh HAlmost all published studies of heat-induced b-lactoglobulin self-assembly into amyloid-like fibrils at low pH and low ionic strength have involved heating at 80 C, and the effect of heating temperature on self-assembly has received little attention. Here we heated b-lactoglobulin at pH 2 and 75 C, 80 C, 90 C, 100 C, 110 C or 120 C and investigated the kinetics of self-assembly (using Thioflavin T fluorescence), the morphology of fibrils, and the rheological properties of fibril dispersions. Self-assembly occurred at all temperatures tested. Thioflavin T fluorescence increased sigmoidally at all temperatures, however it decreased sharply with >3.3 h heating at 110 C and with >5 h heating at 120 C. The sharp decreases were attributed partly to local gelation, but destruction of fibrils may have occurred at 120 C. Thioflavin T fluorescence results indicated that maximal rates of fibril formation increased with increasing temperature, especially above 100 C, but fibril yield (maximum Thioflavin T fluorescence) was not affected by temperature. At 100 C and 110 C, fibrils were slightly shorter than at 80 C, but otherwise they looked very similar. Fibrils made by heating at 120 C for 1 h were also similar, but heating at 120 C for 8 h gave predominantly short fibrils, apparently the products of larger fibrils fragmenting. Heating at 100 C gave consistently higher viscosity than at 80 C, and heating for >2 h at 120 C decreased viscosity, which may have been linked with fibril fragmentation seen in micrographs.