Effect of hemp protein hydrolysates on physicochemical properties and digestibility of starch : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

Starch is the main constituent of the human diet and is included in foods either in the gelatinized or retrograded form. This carbohydrate, produced by plants, contains amylose and amylopectin chains. Depending on the plant source, the proportion of these two glycan chains can vary within the granule, which influences the physicochemical behavior of starch in foods, impacting the textural and nutritional properties of the final food product. Over the past decades, researchers have investigated the impact of the presence and/or complexation of various biomolecules (proteins and polyphenols) with starch on its physical, chemical, and nutritional properties. There is limited research on the development of hemp protein and its hydrolysates in combination with various starch molecules, which could be a promising strategy for enhancing the nutritional and health benefits of foods. Exploring the interaction between hemp protein and different starches could lead to novel functional food ingredients with improved nutritional profiles, offering potential for increased health benefits. Firstly, this study compared the functional and nutritional properties of hemp protein hydrolysates (HPH) prepared with 0.5% Bacillus licheniformis (1 minute) and 2% pepsin protease (60 minutes). Hemp protein hydrolyzed by bacterial protease (HPHB) showed enhanced emulsifying capacity of 47.83 mL/g, compared to 45.83 mL/g for hemp protein hydrolyzed by pepsin (HPHP). On the other hand, HPHP exhibited superior oil absorption capacity of 5.46 g/g, compared to 5.18 g/g for HPHB, as well as higher foaming capacity at 31.2%, relative to 28% for HPHB. The amino acid composition, which affects the overall amino acid profile, was determined using High-Performance Liquid Chromatography (HPLC), a precise technique for separating and quantifying individual amino acids. Secondly, when the effects of HPH on the textural, particle size distribution, structural, and rheological properties of normal corn starch (NCS), waxy corn starch (WCS), and high amylose corn starch (HACS), alongside their impact on antioxidant activity and starch digestibility were examined, this study showed the addition of the HPH to the starch significantly modified the textural properties of starch gels, particularly enhancing elasticity and hardness in WCS and NCS and increasing adhesiveness in NCS and HACS. Notable effects were observed with 20% HPHP on WCS elasticity (0.09 N.s), 10% HPHB on WCS hardness (0.57 N), and 20% HPHB on WCS adhesiveness (-0.15 N) (p<0.05). Particle size distribution indicated substantial granule size increases, especially in HACS (e.g., Dx90 increased from 39.19 μm to 771 μm). FT-IR analysis revealed shifts in absorption peaks, suggesting altered hydrogen bonding interactions without new functional groups. Rheological properties varied, with increased storage (G') and loss (G") moduli in NCS-HPH mixtures and general decreases in WCS-HPH and HACS-HPH. Additionally, a significant reduction in rapidly digestible starch (RDS) was observed with 20% HPHP in HACS, while resistant starch (RS) increased by about 10% with 10% HPHB in WCS (p<0.05). Among the starches, 20% HPHP exhibited the highest antioxidant activity with 232.36 μmol TE/g for HACS, 231.60 μmol TE/g for WCS and 210.42 μmol TE/g for NCS, respectively (p<0.05).