Date of Award:

2015

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Nutrition, Dietetics, and Food Sciences

Advisor/Chair:

Dr. Donald J. McMahon

Abstract

High-protein nutrition (HPN) bars (≥ 30% protein) have limited shelf life and become excessively hard during storage. Various mechanisms have been proposed to explain the hardening. The objectives of this research were to investigate the chemistry of HPN bar hardening and propose solutions for slowing it and improving bar texture.

In phase 1, HPN bars were made containing 34% whey protein isolate (WPI) or milk protein concentrate (MPC) powder, along with either sorbitol syrup or glycerol, and vegetable shortening or cocoa butter. Substituting MPC for WPI made the bars brittle and crumbly. Using glycerol initially made bars softer but accelerated hardening. Cocoa butter increased bar hardness because of its higher solid to liquid content. Most water (~99%) in HPN bars made using sorbitol syrup is present as bound water, with ~0.9% as intermediate water and ~0.1% as bulk water. During storage bound water increased ~0.02 g/100 g of solids while intermediate water decreased, suggesting changes in state of water taking place at protein surfaces. During storage, there were changes in protein conformation indicated by an increase (~4°C) in heat denaturation temperature of β-lactoglobulin and α-lactalbumin and a 15 to 40% decrease in denaturation enthalpy.

In phase 2, various bar formulations were tested involving different proportions of proteins, lactose, glycerol, and sorbitol syrup, as well as type of lipid component, and disulfide bonds inhibition. Decreases in bar hardening occurred when MPC and WPI and sorbitol syrup and glycerol were used in combination.

In phase 3, HPN bars made with 38% protein powder as a 50:50 combinations of WPI and MPC and with 20% of sorbitol syrup substituted with glycerol, had good texture and minimal hardening during storage. Bar hardening was not caused by phase separation of protein and sorbitol, Maillard browning, or formation of inter-molecular disulfide bonds. Minimizing bar hardening requires prevention of entropy-induced protein aggregation by masking hydrophobic regions on protein surfaces and preventing formation of extended protein networks. It is proposed that preferential exclusion of cosolvents causes glycerol to be oriented at protein surfaces such that its carbon backbone masks hydrophobic regions thus avoiding a decrease in entropy of water molecules. (229 pages)

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