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Hardening and microstructure of high protein nutrition bars made using whey protein isolate or milk protein concentrate
High-protein nutrition (HPN) bars (especially those containing >30% protein) became hard during storage and have a limited shelf life, resulting in disappointment by consumers or loss of product as older products are discarded. The effect of different components on hardening of HPN bars was studied in bars containing 34% (wt/wt) whey protein isolate (WPI) or milk protein concentrate (MPC) powder, along with either sorbitol syrup or glycerol, and vegetable shortening or cocoa butter. The bars were stored at 20°C and 35°C and monitored for changes in hardness (measured using a penetration test), water activity, state of water and denaturation of whey proteins (measured using differential scanning calorimetry), and microstructure using confocal scanning laser microscopy. 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 b-lactoglobulin and a-lactalbumin and a 15 to 40% decrease in denaturation enthalpy. In bars made using WPI the protein was present as a continuous network (see Figure A) while in bars made using MPC the protein remained in separate particles of protein powder dispersed throughout the sorbitol-water cosolvent mixture (see Figure B). It is proposed that hardening of HPN bars is a result of interactions between the cosolvents and the protein surface and not because of a phase separation between protein and cosolvents as was previously hypothesized.
Keywords: protein, whey microstructure