In this present study, a three-factor Box-Behnken, response surface methodology (RSM) design was employed to optimize the skimmed milk powder (SMP)/whey protein concentrate (WPC) ratio (0.25-0.75%w/v) as a source of milk protein, inulin (1-2%w/v), and honey (4-6%w/v) for production of high-quality goat milk yoghurt (GMY). The resulting ANOVA and response surface equations revealed the significant effect (p < 0.05) of these variables on the various attributes such as total solid (%), pH, titratable acidity [(LA) % by weight], syneresis (%), DPPH (% inhibition), viscosity (m.Pa⋅s), whiteness index (WI), and overall acceptability (OA). The coefficient of determination (R2) for all response variables ranged from 0.88 to 0.99. Lack-of-fit tests resulted in non-significant F-values. The optimal conditions were determined as SMP/WPC at 0.36%w/v, inulin at 1.00%w/v, and honey at 6.00%w/v. The optimum values for total solid, pH, titratable acidity, syneresis, DPPH, viscosity, WI, and OA were 22.03, 4.46, 0.77, 6.34, 25.20, 182.30, 76.29 and 8.37, respectively with desirability value of 0.95.

BACKGROUND: Production of goat\'s milk set-style yoghurt encounters challenges in achieving the texture characteristic for this type of product, primarily due to protein composition of this milk. This study evaluated the effects of using microbial transglutaminase (mTGase) concomitantly with starter culture in the production of goat\'s milk yoghurt - a method that has not been employed with this milk type until now- indicating the potential of the enzyme to change yoghurt\'s textural properties. Goat\'s milk set yoghurts were produced from milk heated at 72 °C/30 s and 90 °C/5 min, without (G72 and G90) and with mTGase (G72TG and G90TG) and starter culture addition. Protein profiles of goat\'s milks and yoghurts were also examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Yoghurts were evaluated for rheological properties, texture, microbiological and sensory profile over 2 weeks to study the influence of mTGase, pasteurization and storage.
RESULTS: The enzyme caused significant increases of storage moduli at the end of fermentation: 8.32 ± 0.27 Pa (G90TG) and 2.89 ± 0.18 Pa (G72TG) vs. 6.13 ± 0.07 Pa (G90) and 1.27 ± 0.18 Pa (G72) without enzyme. Lower loss tangent values indicated the enhanced elastic character of the gels with enzyme. Enzyme increased yoghurt\'s firmness from 49.69 ± 2.61 g (G90) to 60.81 ± 5.29 g (G90TG) after 1 day and from 58.21 ± 0.53 g (G90) to 80.45 ± 0.59 g (G90TG) after 15 days\' storage. Enzyme improved starter bacteria survivability during storage of G72TG yoghurt.
CONCLUSIONS: mTGase can be used simultaneously with the starter culture to improve the rheological properties and texture of goat\'s milk yoghurt, without deteriorating effect on its flavour. © 2021 Society of Chemical Industry.

The overall goal was to utilize a gas chromatography spectrometry based metabolomics approach to investigate the metabolite changes in goat milk yoghurt during storage. A total of 129 metabolites were identified in goat milk yoghurt during 28 days refrigerated storage. Among 129, 39 metabolites were differentially regulated (p < 0.05) wherein 22 were upregulated (UR) and 17 were downregulated (DR). 17 (9 UR, 8 DR), 20 (11 UR, 9 DR) and 2 (both UR) differential metabolites were identified during storage period of 0-14, 14-28, and 0-28 days, respectively. Metabolic pathway analysis revealed that aminoacyl-tRNA biosynthesis, phenylalanine, tyrosine and tryptophan biosynthesis and phenylalanine metabolism altered during 0-14 days storage; while fatty acid biosynthesis, and propanoate metabolism altered during 14-28 days of storage. Metabolite-gene interaction analysis identified genes regulated by differentially expressed metabolites. Functional annotation of interacted genes in corroboration with that of KEGG pathway analysis provided the probable mechanisms that altered the metabolites during storage. These findings reveal comprehensive insights into the metabolite alterations during storage. This research provides practical information for developing goat milk yoghurt with enhanced bio-activities and would aid in future investigations into the nutritional research and isolation of functional compounds.

This research paper addresses the hypothesis that the fortification of goat milk base with whey protein concentrate (WPC) could affect both the textural and the biofunctional properties of set-style yoghurt. The effect of fortification of goat milk base with two different WPCs on thermophilic bacteria counts, proteolysis, physical and biofunctional properties of set-style yoghurts was studied at specific sampling points throughout a 4-week storage period. Fortification and storage did not influence thermophilic counts. Physical properties were affected significantly (P < 0.05) by the composition of the protein and the mineral fraction of the WPC but not by the storage. ACE-inhibitory activity was moderate in accordance to low lactobacilli counts and lack of proteolysis. DPPH-radical scavenging activity, Fe2+-chelating activity and superoxide scavenging activity were high. At 28 d an anti-inflammatory effect was observed, which was not affected by WPC addition.

Total spleen lymphocytes, lymphocyte proliferation, tumor necrosis factor-α (TNF-α), and interleukin-10 (IL-10) in spleen lymphocyte culture were studied in malnourished Wistar rats fed with goat milk yoghurt. Malnourished rats were created by using standard feed restriction as much as 50% of normal rats for 21 d. Goat milk yoghurt containing three types of microorganism e.g., Lactobacillus acidophilus, Sterptococcus thermophilus and Bifidobacterium longum derived from Lacto-B culture in powder form. After 21 d, the rats continued to receive restricted feeding and supplemented with goat milk yoghurt for 7 d. Total splenocytes were counted by hemocytometer. Splenocytes proliferation was expressed as stimulation index, whereas the TNF-α and IL-10 of spleen lymphocyte culture were measured by ELISA technique. The total number of splenocytes and stimulation index of splenocytes in moderate malnourished and normal rats supplemented with goat milk yoghurt was not significantly different. The level of TNF-α in the rat supplemented with goat milk yoghurt was lower (p<0.05) than the control group, whereas the level of IL-10 in the rat supplemented with goat milk yoghurt was higher (p<0.05) than the control group. In conclusion, goat milk yoghurt supplementation in malnourished rats could decrease TNF-α as a representation of the proinflammatory cytokine, while it increases IL-10 as a representation of the anti-inflammatory cytokine.