Journal Articles
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Item The impact of heat-set milk protein gel textures modified by pH on circulating amino acid appearance and gastric function in healthy female adults: a randomised controlled trial.(Royal Society of Chemistry, 2024-05-21) Milan AM; Menting GGA; Barnett MPG; Liu Y; McNabb WC; Roy NC; Hutchings SC; Mungure T; Weeks M; Li S; Hort J; Calder S; O'Grady G; Mithen RFModification of dairy proteins during processing impacts structural assemblies, influencing textural and nutritional properties of dairy products, and release and availability of amino acids during digestion. By modifying only pH, acid heat-set bovine dairy gels with divergent textural properties were developed to alter protein digestion. In vitro assay confirmed faster digestion of protein from a firm gel (pH 5.65) versus a soft gel (pH 6.55). We hypothesised that firm gel (FIRM-G; pH 5.6) would result in greater indispensable amino acid (IAA) appearance in circulation over 5 h and corresponding differences in gastric myoelectrical activity relative to soft gel (SOFT-G; pH 6.2). In a randomised, single-blind cross-over trial, healthy females (n = 20) consumed 150 g of each gel; plasma amino acid appearance was assessed over 5 hours. Iso-nitrogenous, iso-caloric gels were prepared from identical mixtures of bovine milk and whey protein concentrates; providing 17.7 g (FIRM-G) and 18.9 g (SOFT-G) of protein per serving. Secondary outcomes included gastric myoelectrical activity measured by body surface gastric mapping, glycaemic, triglyceridaemic, and subjective appetite and digestive responses. Overall plasma IAA (area under the curve) did not differ between gels. However, plasma IAA concentrations were higher, and increased more rapidly over time after SOFT-G compared with FIRM-G (1455 ± 53 versus 1350 ± 62 μmol L-1 at 30 min, p = 0.024). Similarly, total, branched-chain and dispensable amino acids were higher at 30 min with SOFT-G than FIRM-G (total: 3939 ± 97 versus 3702 ± 127 μmol L-1, p = 0.014; branched-chain: 677 ± 30 versus 619 ± 34 μmol L-1, p = 0.047; dispensable: 2334 ± 53 versus 2210 ± 76 μmol L-1, p = 0.032). All other measured parameters were similar between gels. Peak postprandial aminoacidaemia was higher and faster following ingestion of SOFT-G. Customised plasma amino acid appearance from dairy is achievable by altering gel coagulum structure using pH during processing and may have minimal influence on related postprandial responses, with implications for targeting food design for optimal health. The Clinical Trial Registry number is ACTRN12622001418763 (https://www.anzctr.org.au) registered November 7, 2022.Item Intragastric restructuring dictates the digestive kinetics of heat-set milk protein gels of contrasting textures(Elsevier, 2024-11) Li S; Mungure T; Ye A; Loveday SM; Ellis A; Weeks M; Singh HThe gelation of milk proteins can be achieved by various means, enabling the development of diverse products. In this study, heat-set milk protein gels (15 % protein) of diverse textures were made by pH modulation and two gels were selected for dynamic in vitro gastric digestion: a spoonable soft gel (SG, pH 6.55' G' of ∼100 Pa) and a sliceable firm gel (FG, pH 5.65; G' of ∼7000 Pa). The two gels displayed markedly different structural changes and digestion kinetics during gastric digestion. The SG underwent substantial structural compaction during the first 120 min of gastric digestion into a denser and firmer gastric chyme (26.3 % crude protein, G* of ∼8500 Pa) than the chyme of the FG (15.7 % crude protein, G* of ∼3000 Pa). These contrasting intragastric structural changes of the gels reversed their original textural differences, which led to slower digestion and gastric emptying of proteins from the SG compared with the FG. The different intragastric pH profiles during the digestion of the two gels likely played a key role by modulating the proteolytic activity and specificity (to κ-casein) of pepsin. Preferential early cleavage of κ-casein in SG stimulated coagulation and compaction of solid chyme, whereas rapid hydrolysis of αS- and β-caseins in the FG weakened coagulation. This study provided new insights into controlling the structural development of dairy-based foods during gastric digestion and modulating digestion kinetics.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.