Introduction
Water activity (aw) refers to the vapor pressure of a liquid divided by the vapor pressure of pure water at a consistent temperature (Berry, 1990). It is a concept used to define the relationship between water and certain compounds (Slade and Harry, 1991). Berry (1991) defines water activity as the amount of water needed to support the growth of microbes. Microbes are organisms that include yeast, mold and bacteria (Sloan, 1977). It is measured on a scale of 0 to 1.0 (Slade et al. 1991).
Berry (1990) asserts that the water activity of baked goods can be calculated by establishing the weight of the food in relation to its dry mass. The difference between these values represents the product’s moisture content (Berry, 1990). “As the relative amount of solutes in the water increases – by direct addition and/or a drying process – the water which is free and available to microorganisms decreases. Some examples: Foods A and B may contain the same high amount of water but differ significantly in the amount of NaCl. Foods C and D may have the same amount of NaCl but different amounts of water. As NaCl in food is always dissolved in the water (up to the saturation point), the concept of available water (aw) becomes more important than simply the amount of water in a food” (Lindquist, 1998).
Water activity is directly proportional to temperature as long as several factors remain constant (Sloan, 1977). Crystalline substances like sugar and sodium carbonate are known to slow down the rate of water activity.
Bakeries use water activity to determine the shelf life of certain products. Water activity can help bakers determine the best ingredients to use in food production. This knowledge can be used to inhibit the growth of mold in products that are made using dough (Slade et al., 1991). It can also be used to reduce the rate of moisture migration in certain products (Berry, 1990).
Bakeries use water activity as a critical control point with regard to Hazard Analysis Critical Control Points (Slade et al., 1991). This process involves the periodic sampling of products, which are tested to determine their different rates of water activity (Berry, 1990). It is used to determine the appropriate water activity rates for certain pastries. These rates can be used to establish the expiration date of certain pastries. Water activity is often responsible for bacterial growth (Berry 1990). Bacterial growth can also be inhibited based on the particular rates of water activity (Slade et al., 1991). Reducing the water activity in dough causes bacterial growth levels to fall (Sloan, 1977).
Measuring Water Activity
The values of water activity can be determined through the use of capacitance hygrometers and dew point hygrometers (Berry, 1990). A capacitance hygrometer is an instrument that consists of a membrane that is placed between two charged plates (Slade et al., 1991). The device determines the equilibrium of vapor in a chamber which houses the sample (Berry, 1990).
Hazards Controlled by Water Activity
When certain baked goods are kept under certain conditions, they experience a progressive deterioration of quality (Slade et al., 1991). These changes are determined by the content of moisture in the food. Soft pastries like bread and cakes are more likely to become stale than hard pastries like biscuits and rock buns. Staling is caused by the absorption of moisture from the atmosphere (Sloan, 1977). Moisture can also be distributed from the crumb to the crust (Slade et al., 1991).
“Most foods contain a substantial amount of water – the milieu in which nutrients for microorganisms are dissolved and their biochemical reactions take place. Indeed, the growth of bacteria is always associated with an aqueous environment. Some bacteria are accorded the freedom of lakes and streams and some are trapped in drops of moisture in the soil – to give a couple examples from nature” (Lindquist, 1998). Physiochemical changes such as starch crystallization and foul odor are evident during the staling process (Sloan, 1977). Starch, which is evident in pastries, may be affected by the activity of amylase. The starch may also be insoluble.
Gelatinizing starch is a major component in industrial baking. It is a complex substance that is often responsible for microbial activity. It provides bacteria with a sufficient environment for breeding. Meat, which is often used in pies, is also affected by water activity (Berry, 1990). Meat pies are therefore vulnerable to microbial activity. They cannot be stored as well as other pastries. Their shelf life is relatively short. If they are not prepared well, they can become poisonous. They have to be baked on demand.
“Many ingredients other than flour pose a greater microbiological safety risk in the bakery. Ingredients containing meat, egg or dairy products such as fresh and synthetic creams, custards and icings (interface between icing and baked product) are the most likely sources of serious food safety hazards. Other ingredients added after baking, such as spices, nuts and fruit toppings or fillings, may also be a potential source of contamination” (Canadian International Grains Institute, 2006). Bakeries only use cream and icing on demand. Such substances are likely to decompose faster than dry pastries. The staling process is also defined by alterations in the gluten structure (Slade, 1991). This is the mechanism responsible for producing labile moisture (Slade, 1991). Staling is also defined by the retrograding starch that absorbs moisture (Sloan, 1977).
Soft pastries often contain granular particles, which are encased in a jelly-like substance. This substance is rich in amylase. Amylopectin and amylase experience structural changes staling progresses (Sloan, 1977). Moisture transfer between the starch and the gluten is also responsible for staling (Slade et al., 1991). It can lead to severe cases of food spoilage. Bakeries can suffer losses if their products do not appeal to their consumers. They can also inadvertently poison the consumers.
Water activity creates a breeding ground for microorganisms. When dough is exposed to such conditions, it becomes soft and sticky. In extreme cases, a cotton-like growth may be evident. The dough may also change color. A foul odor is usually evident. The structural integrity of the dough may also be compromised. It can lose its malleability.
“Many foods serve as good media for microorganisms. One particularly rich medium which comes to mind is milk. Organisms which are involved in the development of food products – such as fermented milk products – must not be impeded by overgrowth of undesirable organisms or unsuitable properties of the raw starting material. Relevant intrinsic and extrinsic factors can be manipulated in the proper development of different fermented foods such as sausage, sauerkraut and yogurt” (Lindquist, 1998). Baked goods often contain milk. They are therefore susceptible to microbial activity. Such pastries are vulnerable. Their shelf life is relatively short. Water activity is therefore prominent in pastries that contain milk.
They tend to decompose faster than other baked goods. Dry pastries experience this problem but to a lesser degree. Dry pastries therefore have a longer shelf life. Contaminated foods can cause vomiting, diarrhea and even death (Canadian International Grains Institute, 2006).
Solutions to the Problem
Most bakeries use humectants to avoid stalling. Humectants are substances that act against humidity. They ensure the freshness of pastries and other baked goods. They slow down the staling process by reducing the rate of moisture absorption.
Some bakeries hydrolyze the starch in their pastries to deter the activity of water. This process can significantly slow down the staling in pastries. Moisture cannot be absorbed by hydrolyzed starch. Starch can be hydrolyzed using amylase (Sloan, 1977).
Monoglycerides and diglycerides can be used to prevent the crystallization of starch particles (Slade et al., 1991). Slade et al. (1991) also cites maltodextrin as a possible solution to water activity.
Plastic bags can also be used to ensure that moisture does not reach the pastries. It is one of the most common methods of food preservation. It is fairly effective. Plastic bags significantly reduce the activity of microbes (Sloan, 1977).
The moisture evident in dough can be retained through the use of emulsifiers (Berry, 1990). Emulsifiers reduce the rate of microbial activity (Slade et al., 1991). They reduce the density of pastries. Emulsifiers also increase the volume of certain pastries (Berry, 1990). Lindquist (1998) asserts the following:
One of the oldest methods for preserving food is that of drying. As the aw of a food product is lowered, fewer and fewer organisms will be able to grow. Bacteria tend to require more available water for growth than do the yeasts, and the yeasts tend to require more available water for growth than do the molds. The aw of most fresh foods is 0.99 or greater. Most spoilage bacteria require an aw greater than 0.91 for growth while most spoilage molds can grow at aw levels as low as 0.80. It is often necessary to determine the aw of a product. This can be difficult, particularly with solid foods. One can estimate the aw of the food by the use of the salt-crystal method and then determine the aw more precisely with expensive electronic sensing devices. The salt-crystal method makes use of the fact that saturated salt solutions or salt crystals have a particular aw. Should saturated salt solutions be exposed to an atmosphere with an aw greater than that of the salt, the salt solution will take up water from the atmosphere.
For example, if a dry piece of filter paper containing crystals of sodium carbonate (whose aw we know is 0.87) is exposed to mayonnaise (aw =0.90-0.97) in a closed system (such as a sealed beaker or Petri dish), the filter paper containing the salt crystals will take up water from the more moist atmosphere created by the mayonnaise and become visibly wet. A filter paper containing crystals of sodium sulfate (aw =0.98) in the same closed system would remain dry. This is the basis of the salt-crystal method. Although it is not too sensitive nor very accurate, it does allow one to estimate the aw of a product and can make for a very interesting and instructive laboratory exercise (Lindquist 1998).
According to Berry (1990), salt can be used to increase the shelf life of certain baked goods. Salt suppresses the repulsion of ions, thereby increasing the strength of the dough (Sloan, 1975). Cooking oil is also a shortening agent which can be used to prolong the shelf life of certain pastries (Slade et al., 1991).
Slade et al. (1991) argues that sodium propionate and calcium propionate can be used to inhibit microbial activity.
According to the Canadian International Grains Institute (2006), “the baking process effectively eliminates the safety risk of microbial populations present in flour; however, to maintain acceptable levels in the flour during the milling process, other contributing factors prior to milling must be evaluated for their effectiveness in reducing counts.”
Conclusion
Microbial activity can be controlled if not completely avoided. It is safe to say that bakers can take the necessary precautions. Every bakery should adhere to the standards of food production. The Canadian international Grains Institute (2006) argues that “Post-baking contamination can be controlled with proper GMP (e.g. sanitation, employee training). Newer technologies such as MAP can be employed to increase shelf life stability. Industrial bakeries are well equipped to minimize risks, while there may be greater potential for hazards in smaller bakeries. Similarly, higher risks will occur in tropical countries as opposed to temperate ones due to climate conditions.” Berry (1990) supports the notion that all baked goods should be handled with care in order to avoid contamination. This ensures a safe and healthy environment for both the bakers and the consumers.
References
Berry, R. S. (1990). When the Melting and Freezing Points are not the same. Scientific American. 2nd Edition. Chicago: Penguin.
Canadian International Grains Institute (2006). Microbiological Safety Concerns in the Milling and Baking Industry. Web.
Lindquist, J. (1998). Availability of Water for Microbial Growth in Foods. University of Wisconsin- Madison. Web.
Slade, L. and Harry, L. (1991). Beyond Water Activity: Recent Advances Based on an Alternative Approach to the Assessment of Food Quality and Safety. Critical Reviews in Food Science and Nutrition. 3rd Edition. London: Macmillan.
Sloan, A. and Schlueter, D. (1977). Effect of Sequence and Method of Addition of Humectants and water on AW Lowering Ability in an IMF System. Journal of Food Science, 13(1) 42:94.