High-protein nutrition bars: Hardening mechanisms and anti-hardening methods during storage

Abstract High-protein nutrition bars (HPNBs) contain 20–50% protein and can effectively and quickly provide the energy needed by the body for performance. They are widely used as sustenance in sports, fitness, and the military as a means of replenishing vital nutrients after physical endurance. However, the hardening of HPNBs occurs quickly during storage due to the interaction of ingredients in the bar's matrix and effects of the external environment. This will lead to deleterious changes in the qualities of the HPNBs, such as taste, and it can seriously affect their market value. This review expounds the factors affecting the hardening of HPNBs during storage, such as sugar crystallization, water migration, protein self-aggregation, phase separation, and Maillard reactions. Research advances in the current anti-hardening methods are summarized, and the characteristics, advantages, and mechanisms of various methods employed in anti-hardening strategies are also expounded. The hardening of HPNBs is a complicated process involving physical changes during the early stage and chemical reactions during the middle and late stages of the storage period. Measures such as the selection of raw materials, substitution of protein with hydrolyzed protein, protein modification, addition of polyphenols, storage conditions, and other methods have achieved promising preliminary anti-hardening results. However, the development of anti-hardening HPNB methods that enable prolonged shelf lives and maintain or enhance taste and nutritional benefits represents a crucial focus for improving the processing and storage of HPNBs.