Table 6.4 Recipes for comparison.
Ingredient Quantity recipe 1 Quantity recipe 2
Flour 100.0 100.0
Sucrose 117.5 125.0
Fat 35.3 35.3
Whole egg 30.7 30.7
Baking powder 4.5 4.5
Salt 0.5 1.5
Glycerol 0.5 3.0
Water 105.5 112.0
Moisture content (%) 30.6 30.6
ERH (%) 90.1 88.7
increases or decreases accordingly. Many products with high moisture contents also have high water activities, while those with low mois-ture contents have low water activities. However, it must be noted that although two bakery product recipes might have the same moisture con-tent, their water activities can be quite different. Similarly, for a given product it is possible for its moisture content to change without making a major change to its water activity. This can occur when previously undissolved materials in the product, such as sugars, go into solution in the extra water associated with the higher moisture content.
An example of two product recipes that have the same moisture con-tent but different water activities is given in Table 6.4. It is also possible for two recipes to have the same water activity with differing levels of moisture. This can lead to products with very different eating character-istics (as discussed in Chapter 5) and is an important consideration in achieving particular qualities both from the organoleptic and product-spoilage viewpoints. This particular aspect of product formulation will be discussed in more detail in Chapter 9. For the example given in Table 6.4, changes in product flavour between the two cake recipes would come from the differences in salt and sugar levels, while the addition of the humectant glycerol in recipe 2 would impact on the sen-sory evaluation of moistness (even though the moisture contents of the two recipes are equal).
Definitions of water activity and equilibrium
solution. When the atmosphere and the solution are in equilibrium, the terms awand ERH can be used interchangeably. The relationship under a defined set of conditions of atmospheric temperature and pressure is straightforward and described by the following equations:
aw= ERH 100 ERH= 100 × aw%.
Since ERH is based on the measurement of humidity, it is usual to express it as a percentage, while awhas no units. The scale for awruns from 0 to 1, with 1 representing pure water; and that for ERH runs from 0 to 100%, with 100% representing pure water. Thus, a cake with an awof 0.82 has an ERH of 82%.
As discussed in previous chapters, when soluble substances dissolve in water, the resulting solution has a lower freezing point, a higher boil-ing point and a lower vapour pressure. It is this latter change that is most relevant to aspects of water activity because Raoult’s law states that ‘the relative lowering of the vapour pressure of a solvent is equal to the mole fraction of the solute’. For an ideal solution the vapour pressure relative to that of water is equal to the mole fraction. Thus, if the vapour pressures are known then the relationship p/pogives the vapour pres-sure of a particular solution, where p and poare the vapour pressures of the solution and water, respectively. The relationship between vapour pressures, water activity and ERH is expressed as follows:
p
po = ERH 100 = aw.
Although the vapour pressure of water changes with temperature, the awof an ideal solution is independent of temperature. In practice, most solutions in bakery products are far from ideal in behaviour, and so the awvaries a little over the range of temperatures that are most relevant for food spoilage.
Water activity is a physicochemical concept first used by Scott (1957) to show that the aw rather than the moisture content determined the microbial safety of food. Water activity has been described in a number of different ways. Richardson (1986) described it as ‘the availability of
“free” water as opposed to the total moisture which includes “bound”
water’. There has been no common agreement about how much of the water present is bound and therefore unavailable in a food, or where the bound and free water is located within the food system concerned.
A relatively simple example of the uncertainty that prevails would be a mixture of water and a suitable stabiliser to form a gel. Given that we have the correct concentration of a stabiliser, we can bind quite large quantities of water to create a system that is stable but still susceptible
to mould growth (unless we take other precautions). Clearly, the water is not sufficiently bound to make it unavailable for use by microorganisms, although it is sufficiently bound to prevent it running away if the gel is not held in a container.
The concept of freezable and non-freezable water has been used as an alternative to that of free and bound. Non-freezable water would be that portion of the moisture in the food which does not freeze at 0◦C (32◦F).
Some of the issues regarding the conversion of water in foods to ice have been discussed above. The freezable/non-freezable water concept has some attraction, but studies with differential scanning calorimetry (DSC) and low-temperature nuclear magnetic resonance have shown that non-freezable water exhibits a high degree of mobility and so is not bound in the energetic sense (Ablett and Lillford, 1991).
The concepts of bound/free and freezable/non-freezable water work well when stored foods are in thermodynamic equilibrium, but since this is not the case with most foods other more appropriate means of describing the state of water in food have been sought. In composite bakery foods, the lack of thermodynamic equilibrium may be related to the properties of the individual components, e.g. cake and jam, although even at the molecular level lack of storage equilibrium may exist (e.g.
see the discussion on bread staling in Chapter 5). The current ‘models’
for the stability of foods during storage are based on the glassy/rubbery states and glass transition temperatures as discussed previously. Within this model, stability of the food is achieved when it is stored below its glass transition temperature. The development of these models for food storage stability owes much to the work of Slade and Levine (1987).
There is no direct link between the measurement of water activity as a measure of available water, the freezing point as a measure of freezable water and glass transition temperature. However, there is an indirect link in that if product formulations are ranked according to their aws, freezing points and Tgs, the order is likely to be the same with each property, i.e. the sample with the highest awwill also have the highest freezing point and the highest Tg (Cauvain, 1998). Thus, the measure-ment of the water activity of a bakery food still has a practical value for predicting its storage stability, potential changes in textural properties and moisture migration.
The relationship between awand ERH has been described above. The ERH of a product may be defined as that unique humidity at which moisture is neither lost nor gained by a product, or at which the rate of evaporation of moisture from the product equals the rate at which moisture is absorbed by the product (Cauvain and Seiler, 1992). In other words, the humidity within the product is in equilibrium with that of the atmosphere surrounding it. In practical terms, we see this if a product is wrapped in a moisture-impermeable film since, with time, a point is
eventually reached where the humidity within the product is the same as that in the atmosphere surrounding the product in its package (provided none is lost through the packaging film). At this point, the rate at which the product loses water is the same as the rate at which it gains water from the atmosphere in the package.
The ERH of a product can be measured using suitable instrumenta-tion, or it can be calculated if particular data are known for each of the ingredients present in the product formulation, along with any relevant data on the moisture losses that may have occurred during the process-ing, bakprocess-ing, cooling and storage of the product. The methods used to determine aw, ERH and moisture content of products are discussed in some detail in Chapter 7; see also Chapter 8 for the calculations.