Unpalatably, high protein nutrition (HPN) bars tend to go hard during storage. The present study aims to investigate the effects of quantity (34%, 36%, 38%, 40%, and 45%)  and quality of milk proteins: whey protein isolate (WPI) and milk protein concentrate (MPC) or a mixture of them with alcohol sugar syrups (sorbitol + glycerol), natural sugar syrups (agave, rice and honey) and glycerol combined with vegetable shortening oil on the hardness, water activity and color change in bars after accelerated storage at 35°C for 43 days in order to meet consumer demands; such as soft HPN bars and high level of protein combined with natural sugar syrups. Using MPC makes the bars brittle and crumbly. Using glycerol initially makes bars softer but accelerates hardening. Using mixture of 50:50 WPI and MPC and 80% sorbitol, agave, rice or honey mixed with 20% glycerol in bars formulations improve its cohesiveness and textural stability, and minimize bars hardness with fold minimizing (F.M) as follows: 10.4, 5.4, 1.2 and 5.8 fold of 34% protein, 7.8, 4.0, 1.1 and 4.5 fold of 36% protein, 5.0, 3.1, 1.1 and 5.0 fold of 38% protein, 4.7, 3.2, - and 5.0 fold of 40% protein, and 5.9, 4.7, - and 5.9 fold of 45% protein respectively. Hardening of HPN bars during storage was related to protein surface-solvent interactions by non-covalent interactions (hydrogen bonds and van der Waals and ionic forces not by disulphide bonds or browning reactions or water activity) with the protein layers would be with the hydroxyl groups of sugars especially of glycerol and its functioning of hydrophobic interactions between the hydrophobic layers of protein in HPN bars will tend to aggregate together resulting the hardness in bars.

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