Water sorption and the plasticization effect in wafers

Wafers are low moisture food products whose crunchiness or crispness is considered a primary textural attribute, highly affected by the product's physical state (glassy or rubbery). The water activity-water content-glass transition relationships for commercial wafers are reported, using Gordon and Taylor's equation to model the water plasticization effect and also Brunnaver-Emmett-Teller (BET) and Guggenheim-Anderson-DeBoer (GAB) sorption models. BET monolayer moisture content was 6.2% and the moisture limit to fit this model was about 11.5%. Critical water activity and critical water content for the glass transition of the product at 20 degreesC were 0.591 and 0.118 (mass fraction) respectively. Abrupt changes in the mechanical product properties, evaluated from a three-point bend test, could be observed at these limits. Moisture levels between 6 and 11% give rise to a glassy state matrix in the product and so to acceptable product crispness. Below this range, the glassy matrix seems to turn fragile and, above this range, the product becomes rubbery.