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Water Activity is the first answer to every food science frustration...
Looking at this map, it would appear that the space between water activity 0.2 – 0.3 as the sweet spot of minimal activity. In this range, lies the most stable conditions for long term storage. Enzymatic, oxidative, and browning reactions are at their slowest and the water activity is much too low for microbial growth. Texturally this region is a barren desert, inhabited by few--hard boiled candy, crispy cookies, and maybe 30 year canned food storage.