Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Nutrition, Dietetics, and Food Sciences


Donald J. McMahon


The influence of increasing hydrolyzed protein content on the microstructure and hardness of high protein nutrition bars was investigated to determine the mechanism of hardening during storage. Bars with various hydrolyzed protein levels were manufactured using differing ratios of 0, 25, 50, 75, 100% (wt. /wt.) of partially hydrolyzed whey protein isolate (HWPI) to an intact (non-hydrolyzed) whey protein isolate (WPI) which made up approximately 38% of the total bar composition. High fructose corn syrup (HFCS) (42%) and vegetable shortening (20%) constituted the rest of the ingredients. Accelerated aging was performed by storing bars at 32 ºC for 36 d with analysis being performed every 7 d starting at d 2. Hardness was measured as the peak force to penetrate into the bars 8.5 mm using a 45º chisel blade. Microstructure was examined using confocal scanning microscopy with staining for protein and fat. The level of HWPI affected both hardness and microstructure of the bars. Bars that developed the most hardness, with hardness values of 87.6 x 102 g-force and 97.2 x 102 g-force, were those that had no added HWPI or only 25% HWPI (P < 0.05). Bars with 100% of the protein as HWPI were softest with a value of 24.6 x 102 g-force (P< 0.05) and these bars had a microstructure showing a two-phase separation of fat from the aqueous phase containing protein and sugars. The bars that exhibited severe bar hardening had a three-phase separation of the fat, protein, and sugar. The gradual separation of the protein from the sugars into two distinct phases is proposed as the mechanism causing hardening in high protein nutrition bars. The influence of different carbohydrate sources on water activity, Maillard browning, hardness, and microstructure was then investigated. Bars were formulated using either WPI or HWPI with either 70% HFCS or 70% sorbitol syrup as carbohydrate source. This resulted in four bar types, which were then aged at an accelerated rate through storage at 32 ºC and analyzed again every 7 d. Color and water activity were measured as well as hardness and the microstructure was again observed using confocal microscopy. Changing the carbohydrate component of the bars from HFCS to sorbitol syrup had a large effect on the amount of Maillard browning, no effect on the aw, and a slight effect on bar hardening and microstructure while using HWPI instead of WPI had a slight effect on browning, an effect on water activity, and a large effect on bar hardening and microstructure. The carbohydrate effect on bar hardening was not to the same degree as using HWPI. Using sorbitol with WPI reduced hardness after 35 d at 32 ºC by 25% while replacing WPI with HWPI reduced hardness by 55%. When using HWPI both the HFCS and sorbitol, bars remained soft (i.e. hardness <500 g-force) through d 27, with the HFCS increasing in hardness (P < 0.05) by d 35.


This work was revised and made publicly available electronically on July 18, 2011

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