Abstract
Given the rising interest in sustainable agriculture and healthy diets, plant proteins have emerged as viable alternatives to animal-based proteins. They not only provide desirable textures to food products through forming gels but also serve as carriers for nutrients and flavors. Among plant proteins, pea proteins have been widely embraced for their nutritional value, versatility, hypoallergenic nature, and non-GMO (genetically modified organism) status. However, they lack the strength and elasticity found in animal proteins because of lower solubility and fewer disulfide bonds, particularly low content of methionine and cysteine, limiting their use in food applications. Protease and calcium are widely recognized for their ability to modify the physical and chemical characteristics of plant proteins. Nonetheless, the comprehensive understanding of how the combined effects impact the gelling capacity of plant proteins in the presence of a small quantity of carbohydrates remains a topic of ongoing investigations.In this thesis, the effects of calcium addition and limited proteolysis on the gel properties of commercial pea proteins in the presence of a small proportion of polysaccharides were investigated. Pea protein was selected as the representative plant protein. They were firstly mixed with 5-25 mM CaCl2 and 0.1% carboxymethylcellulose (CMC) to form a suspension, followed by hydrolysis by Alcalase 2.4 L for 30 s prior to heating to form gels. The degree of hydrolysis, molecular weight, surface hydrophobicity and intrinsic fluorescence of pea proteins prior to gelation were measured. After gelation, the viscoelasticity, microstructure, and molecular forces involved in gelation were specified using rheometry, microscopy, and solvent extraction, respectively. The protein digestibility of the gels was also measured to assess its potential bioavailability.
Results show that most of the protein fragments were less than 15 kDa after hydrolysis, regardless of CaCl2 concentration. However, as the CaCl2 concentration increased, the degree of hydrolysis decreased. The hydrolysates exhibited increased intrinsic fluorescence intensity and reduced surface hydrophobicity (P CaCl2 < 0.001) when compared with that of the intact proteins. In the rheological analysis, it was observed that the storage modulus (G') of pea protein hydrolyzed (PPH)-based gels exhibited a decrease when compared to those of pea protein (PP). However, the addition of CaCl2 at concentrations ranging from 5 to 15 mM led to an increase in G' for both PP and PPH-based gels. Furthermore, adding CaCl2 resulted in a reduction of the deformation strain in the creep-recovery test. Scanning electron microscopy revealed smaller protein aggregates in PPH-based gel than in PP. Also, higher CaCl2 concentrations led to a denser, less homogenous gel network. The gel dissociation assessment revealed that hydrophobic interactions and hydrogen bonds were the dominant forces in maintaining the gel structure. In vitro digestion demonstrated that the soluble protein content in PPH-based gel exceeded that of the PP counterpart by 10-30% (P interaction < 0.001). Furthermore, the addition of CaCl2 exhibited a reduction in both the soluble protein content and the degree of hydrolysis in the digesta. In summary, this study elucidates the intricate interplay of calcium addition and limited proteolysis on the gel properties of pea protein. The findings underscore the potential of optimizing the textural and nutritional attributes of plant protein-based gels, offering valuable insights for the development of plant protein products.