Cakes are classified as intermediate moisture foods and may be subject to spoilage by moulds. Water activity (aw), the water that is available, unbound or free, for chemical reactions and microbial growth is a major factor that can be utilised to limit or prevent microbial growth. Cakes generally have aw values ranging from 0.65-0.9.
Mould spoilage on cakes tend to occur on the surface and work has been done to model the water vapour pressure above cakes with temperature to derive models for the mould free shelf life (MFSL) of these products. The water vapour pressure above a food is determined by several factors including temperature, the water content of the food, the solutes present and the water activity in the food.
All foods have their own equilibrium relative humidity (ERH). This is the humidity at a given temperature at which the food will neither lose nor absorb moisture to or from the atmosphere.
If the food is held below its ERH it will lose moisture and become drier; above this value, it will absorb moisture from the atmosphere. The gain or loss of water can have a major effect on a food and can influence shelf life significantly. The EHR is determined by exposing the food to carefully controlled atmospheres containing defined water vapour pressures generated using for example standard solutions of salts.
EHR and water activity (aw) are closely related. Water activity represents the ratio of the water vapour pressure of a food to the water vapour pressure of pure water under the same conditions. Water activity is expressed as a fraction. If this is multiplied by 100 then ERH is obtained. Most bacteria cannot grow below an aw of 0.86 (86 % ERH).
Cauvain and Seiler (1992) found that the logarithm of the MFSL had a linear relationship with EHR over the range 74-90% at 21° and 27°C. The equations derived (equations 1 and 2):
Equation 1. Log10 (MFSL, days at 27°C) =6.42 - (0.065 x ERH%)
Equation 2. Log10 (MFSL, days at 21°C) =7.91 - (0.081 x ERH%)
can be used to determine the shelf life of new cake products rapidly and inexpensively. These equations are available in expensive commercial software for determining the MFSL of cakes.
Cauvain, S.P. and Seiler, D.A.L. (1992). Equilibrium relative humidity and the shelf life of cakes. FMBRA Report No. 150, CCFRA, Chipping Campden, UK.
How to cite this article
Mullan, W.M.A. (2015). [On-line]. Available from: https://www.dairyscience.info/index.php/food-model/256-predicting-the-mould-free-shelf-life-of-cakes.html . Accessed: 3 December, 2016.