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.
Table 6.5 Minimum water activity levels for microbial growth.
Water activity (aw) Organism
0.6 Osmophilic yeasts
0.65 Xerophilic bacteria
0.75 Halophilic bacteria
0.80 Most moulds
0.85 Most yeasts
0.91 Most bacteria
1.0 Pure water
determine the rate at which microbial spoilage will develop on bakery and most other foods are listed below:
r
Numbers of microorganisms contaminating a product (its microbial load)r
Types of microorganism presentr
Product surface water content (providing localised high moisture/awconditions)
r
Product water activityr
Storage temperaturer
Storage relative humidityr
Product pHr
Presence of inhibitors.Microorganisms contain high levels of water within their cells. They need a supply of water to maintain their life functions, e.g. feeding, growth and reproduction, just as humans do. For some microorganisms, reproduction can result in the formation of toxins that are hazardous to human health; these are commonly referred to as pathogenic organ-isms. In other cases there may be the production of unsightly, coloured blemishes (mould colonies) or off-odours and flavours, e.g. yeast fer-mentation of jams may result in a ‘winey’ odour.
The availability of water in a product can be a powerful controlling mechanism on the potential for microbial growth and is often used as a predictor of which organisms may flourish on a given food and how quickly a product can be spoiled by such microbial activity. The rela-tionship between the concentration of the liquid and its vapour pressure within the microorganisms’ cells and that of its surroundings can en-courage or disen-courage the movement of water through the cell wall membrane. As the vapour pressure (water activity) falls, the opportu-nities for microbial growth reduce and when a water activity of 0.6 or less is reached, growth is not normally possible. Table 6.5 shows typi-cal levels of water activity at which growth is suspended for different
groups of microorganisms, but it should be recognised that the opti-mal growth conditions will vary for specific organisms within a given group. For example, Seiler (1977) considered the time at which mould growth became visible on the surface of cakes stored at 27◦C (80◦F) with different ERHs and found that Aspergillus species were likely to show before Penicillium species at ERHs below 84%. When the cake ERH was 88%, colonies of both species were equally likely to become visible at similar storage times.
As a rule of thumb, bacteria need more available moisture for growth than yeasts, which in turn need more than moulds. The data given in Table 6.6 link typical water activity levels for bakery products with the types of microorganisms that are likely to grow in a given aw range.
Some microorganisms have become especially adapted to situations of low moisture availability. These are the halophilic (salt-loving), xe-rophilic (dryness-loving) and osmophilic (osmotic-pressure-loving) or-ganisms that will grow from aws of 0.75 down to 0.60. The osmophilic yeasts are often implicated when spoilage occurs in high-sugar coatings and fillings, such as jams, fondants, marzipans and marshmallows. The spoilage mainly manifests itself as ‘winey’ (fermented) or off-odours associated with the product. It may be particularly noticeable when the product package is first opened and the gases present in the pack headspace are released.
Table 6.6 Spoilage types for typical bakery products.
Water
activity (aw) Products Spoilage types
0.99 Creams, custards Bacterial spoilage, e.g. ‘rope’, mould growth and ‘chalk moulds’
0.90–0.97 Breads, crumpets, part-baked yeasted products
Bacterial spoilage, e.g. ‘rope’, mould growth and ‘chalk moulds’
0.90–0.95 Moist cakes, e.g. carrot cake Mould and yeast, bacterial spoilage, e.g. ‘rope’
0.8–0.89 Plain cakes Moulds and yeasts
0.7–0.79 Fondants Osmophilic yeasts
0.7–0.79 Fruited cakes Xerophilic moulds and
osmophilic yeasts 0.6–0.69 Some dried fruits, heavily
fruited cakes
Specialised xerophilic moulds and osmophilic yeasts, sugar-tolerant yeasts
<0.6 Biscuits, chocolate, some dried
fruits
No microbial spoilage
In some cases, although growth is initially prevented by a lower ERH, the microorganism is capable of surviving – but will be inactive – and may spring back into life when conditions again become favourable for activity and growth. For example, this may occur when wrapped products are moved from a cold to a warmer environment, e.g. from chill to room or store temperature, which can lead to condensation on the product surface within the pack. Even though the product may have had a low initial ERH, the presence of extra water on the surface can locally raise the ERH to such a level that it is now capable of supporting greater microbial growth than before. This sort of problem is exacerbated when the products are wrapped (although it can occur with unwrapped products) and often occurs when frozen products are being defrosted.
As the storage temperature is lowered, the water present in a product becomes increasingly unavailable for use by microorganisms and their growth rate slows considerably. Low-temperature storage forms the ba-sis of controlling the growth of pathogenic organisms, and for ‘high-risk’
foods (i.e. with high water activity) may be enshrined in suitable legis-lation. In the UK, the Food Hygiene (Amendment) Regulations (1990) represent one such example; although their introduction did lead to reduced microbial risks, they also introduced some adverse changes in bakery product eating qualities (Bailey, 1992; Bailey and Cauvain, 1994).
The UK sandwich-making industry has had to face a special problem in that its products often have to be kept under chill temperature condi-tions to limit microbial activity, but in doing so it stores the slices of bread at a temperature which is optimal for rapid bread staling (see Chapter 5).
These and other adverse changes in bread organoleptic properties have been overcome through the reformulation of bread recipes destined for sandwich production (Gould, 2007).
Once water is frozen in a product, it becomes largely unavailable for microbial growth. The value of using low temperatures to prevent food spoilage has been appreciated for many hundreds of years, and until the introduction of the refrigerator in the twentieth century, icehouses were used on large estates as a means of extending the supply of ‘fresh’
products throughout the winter (Beamon, 1987). For greater detail of how microorganisms grow under particular sets of defined conditions the reader is referred elsewhere, e.g. Troller (1989) and Walker (1996).