Role of gelatin in low fat stirred yogurt and its possible replacement

Abstract

Stabilizers have been widely used in yogurts to provide good stability and desirable texture, gelatin being the most preferred choice because of its multiple functions (i.e. gelling, ―melt-in-mouth‖ and surface activity). However, gelatin is not universally accepted due to some religious beliefs and potential contamination with viruses and prions. Hence, finding an alternative to gelatin has always been of considerable interest to both the dairy industry and academia. In this thesis, a systematic study on the role and replacement of gelatin type B in yogurt was carried out. The main methodologies used in this research were rheology, scanning electron microscopy, texture analysis, water holding capacity (WHC) measurement and sensory evaluation. Firstly, the effects of gelatin concentration, pH and addition of milk proteins (whey protein isolate [WPI], milk protein concentrate [MPC] and skim milk powder [SMP]) on the properties of pure gelatin were studied. Higher gelatin concentrations led to stronger gels. Rheological properties of the gelatin gels were independent of pH from 4.6 to 8.0, but the fracturability and microstructure of the gels were greatly influenced; gelation was inhibited at pH 3.0. SMP and MPC significantly enhanced the storage modulus (G′) of gelatin gels and changed their microstructure, while WPI showed a negative effect on G′ and caused no modification of microstructure. Gelatin A was briefly compared with gelatin B; results similar to those for gelatin B were obtained, except that gelatin A gel was more susceptible to extreme pH and addition of milk protein.The effect of gelatin on acid milk gels were fully characterized over four stages, which imitated the stages in the manufacturing process of yogurt. During the acidification stage (at 45 °C), the presence of gelatin (≥1%) decreased G′ of milk protein gels and a more heterogeneous microstructure was induced. During the cooling (from 45 to 10 °C) and annealing stages (at 10 °C), gelatin (≥1%) formed strand-like structures. During the heating stage (from 10 to 45 °C), gelatin strands melted and the G′ of the mixed gels tended to revert to the value at the end of the acidification stage, indicating that the changes in milk protein gels caused by gelatin after acidification are reversible. Additionally, gelatin enhanced the WHC of the gels without increasing gel firmness significantly at 0.4% concentration. Most of the previous literature has assessed gelatin replacements only on the basis the final yogurt. In this study, however, a systematic approach to compare the effect of replacers with gelatin during various stages of gelation was undertaken. One ionic (xanthan gum), two non-ionic (guar gum [guar] and locus bean gum [LBG]) polysaccharides and one type of starch were studied in acid milk gels; gels were stirred before storage in this part of the study to mimic stirred yogurt. Guar and LBG at sufficient concentrations tended to prevent milk gelation from the beginning of acidification and dramatically change the microstructure of the gels. Xanthan did not cause severe aggregation of milk proteins and the typical casein network was still obtained at high xanthan concentrations, similar to gelatin. Starch exhibited properties similar to gelatin in both microstructure and rheology aspects. Therefore, the ionic polysaccharide xanthan gum and starch were found to be more promising than non ionic ones as gelatin replacers. Since gelatin is a gelling agent, the ionic polysaccharides with gelling properties might be suitable alternatives for it. The effects of gelling polysaccharides (xanthan/LBG [X/L], carrageenan and starch) and milk proteins (WPI, sodium caseinate [NaCn] and SMP) were assessed in stirred acid milk gels. Similar to gelatin, polysaccharides tended to decrease G′ during the acidification stage and structural changes during the cooling and melting stages were observed for the gels with X/L or carrageenan. The microstructure of the gels was not modified greatly by addition of any of the polysaccharides (except carrageenan) and thin strand-like structures were observed in gels with X/L. However, none of the polysaccharides increased the WHC of milk gels as significantly as gelatin. Addition of milk proteins, on the other hand, significantly enhanced the WHC, with WPI being the most efficient. A combination of WPI and X/L or starch resulted in products close to the gelatin-containing samples. Lastly, the selected combination of WPI and X/L was further investigated and compared with gelatin in cultured stirred yogurt. WPI-X/L greatly increased WHC of yogurt and similar gel microstructure between these two yogurts was obtained. However, both instrumental and sensory evaluation showed that WPI-X/L induced stronger yogurt gel than gelatin, which was expected from the results in acid milk gels.Therefore, the methodologies used in this research provided valuable information on the mechanism of gelatin in yogurt and can be used to evaluate potential gelatin replacements in yogurt; WPI-X/L was found to be promising but further research needs to be done to optimize the concentration of the WPI-X/L combination