To maintain metabolic processes and to reproduce, micro-organisms need water. Relevant here is not the total water content of baked goods, but the percentage of the free water which is available to the micro-organisms. This percentage is defined by the aw value which describes the ratio of the partial steam pressure in the foodstuff to the steam pressure of pure water.
The higher the value, the more free water is present in the foodstuff. Pure water has an aw value of 1. Micro-organisms can grow within a range of aw= 0.6 – 1.0, whereby the bacteria can first reproduce from around aw= 0.9 and mould fungi from aw= 0.80 – 0.88. But also so-called xerophilous mould fungi exist which can grow in dry conditions of aw= 0.65 – 0.75 (cf. Seibel 2001).
But it is mainly in confectionery baked goods with high concentrations of sugar that influence can be taken on the aw value via the type of sugar. Due to the use of sorbitol, glycerol or invert sugar syrup, the aw value can be lowered compared to the use of saccharose.