Pepsin-induced coagulation and in vitro dynamic gastric digestion of model infant formulae : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Manawatū, New Zealand
Loading...
Date
2021
DOI
Open Access Location
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Massey University
Rights
The Author
Abstract
The protein composition in infant formulae impacted on the protein digestion and the curd formation. This research was conducted to investigate the rheological properties of pepsin-induced coagulation and the digestion behaviours of a model infant formulae combined with different protein composition, with a focus on the effect of the casein to whey protein ratio on protein hydrolysis during gastric digestion. Four infant formulae, Sample 0, Sample 40, Sample 60 and Sample 80 were used in current study. Their compositions were classified by the ratio of casein to whey protein: 0:100, 40:60, 60:40 and 80:20, respectively. By rheological measurement of all gels of Sample 0, 𝐺', 𝐺" were slightly decreased around pH 4.0. During gastric digestion, the particle size of Sample 0 was increased, but the increasing rate was slightly decreased from 100min at pH 4.20±0.15 to 180 min at pH 2.61±0.20, which could be due to the reorganisation or partially collapse of its flocculates. At the end of gastric digestion, the pH of gastric digesta from Sample 0 reached the lowest value (pH 2.61±0.20), intact β-Lg in the digesta was detected by SDS-PAGE. For infant formulae containing casein, the gels of Sample 40, 60 and 80 induced by pepsin were significantly impacted by the ratio of casein to whey protein. With the increasing content of caseins, the final 𝐺', 𝐺* and breaking stress (σmax) of the obtained gels gradually increased at pH 4.0. For Sample 40, the 𝐺* was increased as function of pepsin concentration, the strongest gel was formed at 2.5U/mL of pepsin concentration. For Sample 60 and Sample 80, the 𝐺* of gels reached the highest value at 1U/mL of pepsin concentration, along with the highest stiffness. Moreover, the gelation behaviour of Sample 40, 60 and 80 was examed at 0 U/mL and 2.5 U/mL of pepsin concentration, and the acidification was done by the addition of GDL. For Sample 40 and 60, the breaking stress (σₘₐₓ) of pepsin-induced gels has smaller value at 2.5U/mL in comparison with acid induced gels. However, the pepsin induced gels of Sample 80 obtained larger value of σₘₐₓ in comparison with acid induced gels. The results suggested that, Sample 40 and 60 obtained gels at an earlier stage than Sample 80, in both which the extensive hydrolysis resulted in the breakdown of gel structure in the following time. Consequently, the pepsin-induced gels of Sample 40 and 60 are more susceptible to rearrangement and fracture under large deformation than Sample 80. Furthermore, the casein to whey protein ratio of infant formulae influenced the gelation time and gelation pH. At lower pepsin concentration (0-1U/mL), the infant formulae with higher casein content obtained a relatively longer gelation time and lower gelation pH, which could be due to increased number of unhydrolyzed κ-casein. At higher pepsin concentration (2.5U/mL), the formation of coagulates was observed in Sample 40, 60 and 80 above pH 6.0, and the gelation time of them was below 5 min. By analysing the result of SDS-PAGE, the formation of para-κ-casein was started at 5 min at either lower pepsin concentration (1U/mL) or higher pepsin concentration (2.5U/mL). During gastric digestion, the coagulation of Sample 60 was observed at 20 min, but the coagulation of Sample 80 was occurred between 20 min and 60 min. During gastric digestion, the ratio of casein to whey protein plays an important role in the coagulation behaviour and hydrolysis rate of Samples 40, 60 and 80. Comparing with Sample 0, smaller mean particle size of flocculates were observed in the digesta of Sample 40. It could be due to that the casein-covered oil droplets contributed to the high stability emulsion. The pH of Sample 40 reached pH 3.79±0.13 at the end of digestion, intact β-Lg and α-La was remained in the digesta, which could be due to that the pH value of it was much higher than the final pH of Sample 0 and the optimal pH of pepsin (pH 2.0). Comparing with Sample 40, Sample 60 present a greater coagulation during gastric digestion. At the end of digestion, caseins in the emptied digesta of Sample 60 was fully hydrolysed, no large curd was obtained at end of digestion. For Sample 80, large dense curds were formed and stayed in the stomach. Overall, the rheological properties and gastric digestion behaviour of infant formulae were affected by its protein composition. The ratio of casein to whey protein impacted on the pH profile, coagulation behaviour and the rate of protein hydrolysis during gastric digestion. These results provide useful information for the design and development of infant formulae by allowing greater control over the manipulation of protein bioavailability.
Description
Figure 2.15 is re-used under a Creative Commons Attribution Noncommercial License. Permission has been obtained for the re-use of Figures 2.16, 2.17, 2.18 & 2.19. Some possibly copyrighted Figures remain for the sake of clarity.