Water Activity’s Role in Food Safety and Quality - Food Safety Magazine
Water activity or aw is the partial vapor pressure of water in a substance divided by the standard state partial vapor pressure of water. In the field of food science, the standard state is most often defined as the Relationship to relative humidity: The relative humidity of air in equilibrium with a sample is called the Equilibrium. Water activity is inversely related to osmotic pressure; if a solution has high It is a measure of the hydrogen ion activity of a solution and is defined as the. Water activity (aw or ERH) not only determines the stability of moisture The moisture content of a product is usually defined as the percent weight of At equilibrium, the relationship between %H2O and aw of a hygroscopic This limit corresponds to the 'suction power' of the various organisms, i.e. to the osmotic pressure.
Monitoring aw is a critical control point for many food industry operations. The importance of water activity aw in food systems cannot be overemphasized. Throughout history water activity in food has been controlled by drying, addition of sugar or salt and freezing.
These methods prevent spoilage and maintain food quality. Water activity is the ratio of the partial vapor pressure of water in equilibrium with a food to the partial saturation vapor pressure of water vapor in air at the same temperature. This is equal to the relative humidity of air in equilibrium with the food. The growing recognition of the water activity principle is illustrated by its incorporation into U. The purpose of these regulations is to detail the specific requirements, critical control points and practices to be followed by industry to assure that products are produced under sanitary conditions and are pure, wholesome and safe.
New instrument technologies have vastly improved speed, accuracy and reliability of water activity measurements and are definitely a needed tool for food safety and quality. Background Throughout history man has controlled the water activity of food through drying, addition of salt or sugar and freezing such that the food becomes stable to microbial and chemical deterioration.
Food manufacturers today have the same goal of making a stable and safe product. This means that the products must be wholesome and not endanger the health of the consumer with microorganisms or their toxins.
The advantage today is in the knowledge and understanding of the importance of water activity in controlling microbial growth and thus upon the shelf life and safety of a product. The Centers for Disease Control CDC stated foodborne disease is responsible for approximately 76 million illnesses,hospitalizations and 5, deaths annually in the United States.
Known pathogens, such as Salmonella, Escherichia coli O H7, Campylobacter and Listeria monocytogenes, account for an estimated 14 million illnesses, 60, hospitalizations and 1, deaths alone. With staggering statistics like these one would not believe that the U. This decline represents at leastfewer Americans each year suffering from foodborne illness caused by bacteria since By using water activity and controlling major food risks, such as microbial contaminants, the food industry can better ensure the safety of its products.
The consequence of a microbiological failure, particularly as they relate to product recalls, can be very costly. Brand recognition and sales may ultimately suffer as a result of consumers relating the recall to other products manufactured by a particular company. In a world of increasing pressures and diminishing resources, the need to strengthen microbiological quality assurance programs has not abated. In fact, there is more pressure than ever on the management of microbiological quality.
Food safety must be controlled during the production process from beginning to end, rather than relying on detection of problems in the finished product.
Osmotic pressure - Wikipedia
Very few intrinsic properties are as important as water activity in predicting the survival of microorganisms in a food product. Scott showed that microorganisms have a limiting water activity level below which they will not grow. The lowest aw at which the vast majority of food spoilage bacteria will grow is about 0. Staphylococcus aureus under anaerobic conditions is inhibited at an aw of 0.
The aw for mold and yeast growth is about 0. Water Activity Defined Water activity is derived from fundamental principles of thermodynamics and physical chemistry. As a thermodynamic principle there are requirement in defining water activity that must be met. In the equilibrium state: It is this fact that allows the measurement of the vapor phase to determine the water activity of the sample. Water activity is defined as the ratio of the vapor pressure of water in a material p to the vapor pressure of pure water po at the same temperature.
Relative humidity of air is defined as the ratio of the vapor pressure of air to its saturation vapor pressure.
CHAPTER 3. GENERAL CONSIDERATIONS FOR PRESERVATION OF FRUITS AND VEGETABLES
When vapor and temperature equilibrium are obtained, the water activity of the sample is equal to the relative humidity of air surrounding the sample in a sealed measurement chamber. Multiplication of water activity by gives the equilibrium relative humidity ERR in percent. There are several factors that control water activity in a system. Colligative effects of dissolved species e.
- Water activity
- Osmotic pressure
Capillary effect where the vapor pressure of water above a curved liquid meniscus is less than that of pure water because of changes in the hydrogen bonding between water molecules. Surface interactions in which water interacts directly with chemical groups on undissolved ingredients e. It is a combination of these three factors in a food product that reduces the energy of the water and thus reduces the relative humidity as compared to pure water.
These factors can be grouped under two broad categories osmotic and matric effects. Due to varying degrees of osmotic and matric interactions, water activity describes the continuum of energy states of the water in a system. This same NaCl solution in a closed container will develop an equilibrium relative humidity ERH in a head space of A relationship therefore exists between ERH and aw since both are based on vapour pressure.
The ERH of a food product is defined as the relative humidity of the air surrounding the food at which the product neither gains nor loses its natural moisture and is in equilibrium with the environment. It is well known that each microorganism has a critical aw below which growth cannot occur. The so-called intermediate moisture foods IMF have aw values in the range of 0. With aw at 0. As aw increases toward the right, the probability of the food product deteriorating increases.
According to Rahman and Labuzaenzyme-catalyzed reactions can occur in foods with relatively low water contents. The authors summarized two features of these results as follows: The rate of hydrolysis increases with increased water activity but is extremely slow with very low activity. For each instance of water activity there appears to be a maximum amount of hydrolysis, which also increases with water content. The apparent cessation of the reaction at low moisture cannot be due to the irreversible inactivation of the enzyme, because upon humidification to a higher water activity, hydrolysis resumes at a rate characteristic of the newly attained water activity.
Rahman and Labuza reported the investigation of a model system consisting of avicel, sucrose, and invertase and found that the reaction velocity increased with water activity.
Complete conversion of the substrate was observed for water activities greater than or equal to 0. For water activities below 0. In solid media, water activity can affect reactions in two ways: The effects of varying the enzyme-to-substrate ratios on reaction velocity and the effect of water activity on the activation energy for the reaction could not be explained by a simple diffusional model, but required postulates that were more complex: The diffusional resistance is localized in a shell adjacent to the enzyme.
At low water activity, the reduced hydration produces conformational changes in the enzyme, affecting its catalytic activity. The relationship between water content and water activity is complex. An increase in aw is usually accompanied by an increase in water content, but in a non-linear fashion.
This relationship between water activity and moisture content at a given temperature is called the moisture sorption isotherm. These curves are determined experimentally and constitute the fingerprint of a food system. Methods are based on the colligative properties of solutions. Water activity can be estimated by measuring the following: Vapour pressure Freezing point depression of a liquid Equilibrium relative humidity of a liquid or solid Boiling point elevation Dew point and wet bulb depression Suction potential, or by using the isopiestic method Bithermal equilibrium Hair hygrometers 3.
Thus, measuring the vapour pressure of water in a food system is the most direct measure of aw. The food sample measured is allowed to equilibrate, and measurement is taken by using a manometer or transducer device as depicted in Figure 3.
This method can be affected by sample size, equilibration time, temperature, and volume. This method is not suitable for biological materials with active respiration or materials containing large amounts of volatiles.
The water activity can be estimated using the following two expressions: Osmotic pressure is defined as the mechanical pressure needed to prevent a net flow of solvent across a semi-permeable membrane. For an ideal solution, Equation 3 can be redefined as: For non-ideal solutions, the osmotic pressure expression can be rewritten as: The temperature at which the dew point occurs is determined by observing condensation on a smooth, cool surface such as a mirror.
This temperature can be related to vapour pressure using a psychrometric chart. The formation of dew is detected photoelectrically, as illustrated in the diagram below: