17 Durability of sponge cake


Long-life bakery products such as sponge cake are good examples of food where the water activity is one of the main preservation factors. These bakery products have a shelf – life of approximately 6 months regardless of a water content of between 15 to 27%. The filling of these products consists of a variety of different sugars, jams or chocolate.


The main preservation factor is the aw-value that is set in a range between 0.7 and 0.8 aw. This may be achieved by the help of different sugars (glucose, sucrose) or polyol (sorbitol). But also to spray the surface of a product with ethanol helps to control the growth of xerophilous moulds. These steps, together with a packaging under controlled atmosphere build a hurdle to microbiological spoilage


Example of a sponge cake with chocolate filling: Water activity (aw) Water content (%)


Dough Milk caramel mousse


0.755 0.785


23.2 15.4


The migration and equilibrium properties of water in combined food are an important point for the shelf life stability of the product. High baking temperatures support the level of the aw-equilibrium between the different components.


Waffles


The most important quality characteristics of waffles for ice cream are the texture (crispness) and the mechanical stability. For example, for waffles filled with ice cream there is quickly reached a water content where the texture passes from crispy to stringy and then to leathery and finally supple.


Thus water activity is better qualified to determine this water migration between the different layers as opposed to simply determining the water content.


The adjustment of the equilibrium between the different layers or components of the food is not only influenced by the baking process but also by the storage time between production and packaging.


The level of the water migration during the shelf life time of a combined food, hence the adjustment of the relative equilibrium humidity, if high enough, is what leads to rather homogenous aw-values in the different components. This simplifies the use of the water activity as an indicator for the microbial stability of combined bakery products.


To produce a stable product, yet one that still appeals to the senses of taste & texture, then monitoring during the production according to the HACCP regulations should be performed, where the aw-value measurement forms part of this control.


Bakery products and fruit


The use of fruit in bakery and confectionery has increased in popularity during the last few years. The question of how the fruit is used is mainly answered by the type and the manufacturing technology of the bakery product. The possibilities are fresh fruit and processed range of products such as jam, jellies and gel, which are mainly used for long-life bakery products.


The advantages of the processed fruit compared to the fresh fruit are:


- All season availability - Simple storage conditions - Long time shelf life - Simple handling


The main reason for a shortened shelf life of fruit is primarily caused by the microbial spoilage. Thus the environmental conditions of the micro-organisms have been made unfavourable.


The following procedures for the preservation of fruit are:


• Lowering of the water activity (aw-value) with: - dehydration


- adding sugar (osmotic effect) - freezing (transformation from water to ice)


• Temperature - heating


• pH lowering • Oxygen concentration


The Author


Markus Bernasconi (B.S, Chemistry, post-graduate course in integrated micro- systems, University of Applied Sciences, Burgdorf, Switzerland) worked at his first place of employment for about 4 years, leading research projects in the field of thin film physical and chemical vapour deposition and wet chemical wafer processing. He then changed to Mettler-Toledo Thornton, working as a project leader in R&D. In 2006, Markus Bernasconi joined Swan Analytical Instruments, working first as a product manager and afterwards as a sales and support manager. Since the beginning of Feb 2011 he is a sales engineer/key account manager and responsible for Novasina's water activity instrument portfolio.


Conclusion


Even if the measurement of water activity is not a familiar one, it must have its place in every reasonable quality assurance plan (integration into the SOP). For bakery products, the water activity is of paramount importance to avoid bad influence as changes in taste or texture, growing of moulds or microbes, which finally make the product uneatable and reduces the shelf life dramatically. Bakery products are a very good example to demonstrate, how important it is to be focussed on water activity instead of water content. The water content is very low which would assume that the water activity is the same; but this is not the case, the water activity is mostly bigger than 0.700, which allows different moulds and microbes to grow. Additionally, the water activity provides useful information regarding packaging technology due to the fact that most of the bakery products are multi-phase with liquid and solid zone and as well different water activity. Water activity is as well of relevance in other food samples, drugs and life-science products.


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Spotlight


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