Physico-chemical, microstructural and rheological properties of camel-milk yogurt as enhanced by microbial transglutaminase

Abstract

Camel milk produces watery texture when it is processed to yogurt. Despite the extensive studies on microbial transglutaminase (MTGase) in dairy research, the effect of this enzyme on the properties of yogurt made from camel milk has not been studied. This study aims to investigate the impact of MTGase with and without bovine skimmed milk powder (SMP), whey protein concentrate (WPC),or b-lactoglobulin (b-lg) on physico-chemical, rheological, microstructural, and sensory properties of camel-milk yogurt during 15 days of storage period. MTGase treatment markedly reduced the fermentation time of unfortified and SMP-fortified camel milk. The fortification of camel milk without MTGase failed to give settype yogurt. The treatment of unfortified milk with MTGase enormously improved the viscosity and the body of yogurt samples. Fortification of MTGase-treated milk impacted positively on the viscosity, the water holding capacity, and the density of the protein matrix in the gel microstructure, which were influenced by the type of dairy ingredients. All MTGase-treated yogurts differed from each other in hardness and adhesiveness values. Electrophoresis results showed that the susceptibility of the individual milk proteins to MTGase varied, and there were differences among the treatments toward the enzyme. SMP-fortified yogurt was the most accepted product. Generally, the addition of MTGase preparation at a concentration of 0.4%, simultaneously with starter culture, to fortified camel milk was considered an effective tool to solve the challenges of producing set-type yogurt from this milk.

The Impact of Microbial Transglutaminase on the Quality and Antioxidant Activity of Camel-Milk Soft Cheese

“…These new fractions were formed due to the action of MTGase, which catalyzes formation of chemical crosslinks within milk proteins, so formation of polymers with high molecular weight. These results agree with other studies, in which MTGase had the same effect on camel milk proteins [39]. The decrease in the intensity of caseins bands on the gel elucidated that these proteins are preferred substrate for MTGase due to their flexible open structure [47] and due to the absence of disulphide bonds in α s1 -and β-caseins, leaving the reactive groups exposed to the enzyme [48].…”

“…higher than that of control cheese (without MTGase). The cross-linking induced by MTGase increased the free volume inside the curd matrix and produced finer protein network, causing the curd to retain more water [39] [40]. Similar results have been obtained by [21] [41], who found a marked increase in the moisture content of white cheese treated with MTGase compared with the control.…”

“…Gauche [51] reported that native whey proteins are poor substrates for TGase, denaturation process leads to exposure of more sites for TGase-specific reaction. Recently, [39] declared that thermal treatment of camel milk before reaction with MTGase (90˚C/15 min) significantly enhanced the reactivity of whey proteins towards protein cross-linking due to a series of interactions arising from heating. The effect of MTGase concentration on the milk proteins is almost the same except for the treatment that contains 80 U of the enzyme.…”

“…However, cheeses show variation of this activity due to many factors of which, the processing conditions as well as the difference in their composition and structure [26]. MTGase as a transferase forms new inter-and intramolecular ε-(γ-glutamyl) lysine bonds by cross-linking lysine and glutamine residues [11], result in the polymerization of almost milk proteins and peptides [39]. The polymerization process could decrease the antioxidant activity of the final product via inserting specific amino acid residues with antioxidant properties such as lysine in the interaction.…”

“…Also, the reduction in the antioxidant activity of MTGase-treated cheese may be due to the nature of the curd microstructure. The additional cross-linking induced by MTGase produces a dense protein network [39]. Recently, [55] have mentioned that the microstructure of the curd can influence its antioxidant properties, as the densest network showed the lowest antioxidant capacity due to the presence of protein segments not flexible enough to act as antioxidants.…”