Effect of Fortification of Yogurt-Milk with Co-Precipitate Mixtures of Casein And Flax-Seeds Protein Isolates on The Quality of The Produced Yogurt

Protein in demucilaged and defatted Flax-seeds powder dispersions was solubilized at pH 11, precipitated at pH 4.5 and freeze-dried to obtain Flax-seeds protein isolate powder (FPI). Fresh skim milk was used to precipitate acid casein at pH 4.5, and freeze-dried to obtain casein powder (Cn). Three co-precipitate mixtures of casein and Flax-seeds protein were obtained by blending cow's fresh skim milk (pH 6.6) with a solution of Flax-seeds protein isolate (3.5% protein) at different volume ratios of 90:10, 80:20 and 70:30 fresh skim milk to Flax-seeds protein solution respectively. The three mixtures were co-precipitated at pH 4.5 and freeze- dried. The powders of these mixtures called termed as Cn-FPI-10,Cn-FPI-20 and Cn-FPI-30, respectively. Six treatments of yogurt were prepared as follows: Control treatment (T1) was prepared from fresh whole cow's milk. Five fortified yogurt treatments were prepared from fresh whole cow's milk fortified with 1.5% (w/w) protein by adding protein powders from different obtained proteins, CN (T2), FPI (T3), CN-FPI-10 (T4), CN-FPI-20 (T5) and CN-FPI-30 (T6). The resultant six yogurt treatments were examined for physicochemical, microbiological and sensory properties, when fresh and during 6 days of storage period. The pH measurements during fermentation process revealed that yogurt samples fortified with co-precipitate mixtures (T4, T5 and T6) showed higher drop in pH values comparing with that of yogurt sample fortified with FPI (T3). The highest drop in pH value during fermentation process, was observed with T1 and T2. The coagulation time of the studied six yogurt treatments was 5, 5.5, 7, 6, 6 and 6 hours, for T1, T2, T3, T4, T5 and T6, respectively. The yogurt samples fortified with co-precipitate casein-Flax-seeds protein mixtures had intermediate values of syneresis % and water holding capacity %, between T2 and T3. The syneresis werez 15, 20 and 25%, for T4, T5 and T6, respectively, comparing with 20,10and 30% for T1, T2 and T3, respectively. The WHC of treatments fortified with protein mixtures were 80, 80 and 75% for T4, T5 and T6, respectively, whereas 90 and 70% for T2 and T3, respectively. After 6 days of storage period all fortified samples had higher acetaldehyde content than control sample (without fortification). Also, during storage period the highest development in acidity was observed with T1 and T2 and there was a decrease in lactose content for all treatments and no much change were observed for protein, TS, Fat and ash contents. All fortified yogurt treatments (T2, T3, T4, T5 and T6) revealed enhances in the growth of lactic acid bacteria comparing with control (T1). Yoghurt fortified with Co-precipitate of Casein and flax seeds isolates (T4 andT5) showed high organoleptic properties scores comparable to that of control treatment (T4). However the lowest acceptance scores were observed with T\u2083 and T3 and T6 treatments Compared to control (T1). Conclusively, it is recommended to use casein–flaxseed protein blends (CN-FPI) for yogurt fortification due to their effective role in improving rheological and sensory properties and enhancing the growth of lactic acid bacteria compared to the use of flaxseed protein isolate alone. It is also advised to apply mixing ratios of 90:10–70:30 to achieve an optimal balance between technological quality and sensory acceptability. Further studies are recommended to elucidate the types of interactions between plant proteins and milk proteins.