Significance of Water Activity in Food Quality

The importance of water activity aw in food systems cannot be overemphasized. Throughout history water activity in food has been controlled by optimizing its usage either through drying, addition of sugar, salt or freezing. These methods prevent spoilage and maintain quality of food. Water activity is the ratio of the partial vapor pressure of water in equilibrium with a food: to the partial saturated 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 water activity of a food describes the energy state of water in the food, hence it’s potential to act as a solvent and participate in chemical/biochemical reactions including growth of microorganisms. This is an important property that is used to predict the stability and safety of food with respect to microbial growth, rates of deteriorative reactions and chemical/physical properties. The growing recognition of the water activity principle is illustrated by its incorporation into FDA and USDA regulations, GMP and HACCP requirements, and most recently in NSF International Draft Standard 75. 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 for consumption. New instrument technologies have vastly improved speed, accuracy, and reliability of water activity measurements which are definitely a needed tool for food safety and quality.

Keywords: Water activity, food quality, browning reaction, water borne disease, food borne disease.

Introduction

Throughout history man has controlled the water activity of food through optimizing its use either by drying, addition of salt , sugar or 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.

Water is necessary for microbes to grow, but microbes cannot grow in pure water. A measurement of the availability of water is aw or water activity. The aw of pure water is 1.0 while that of a saturated salt solution is 0.75. Most spoilage bacteria require a minimum aw of 0.90. Some bacteria can tolerate an aw above 0.75 as can some yeasts and most molds. Most yeast requires 0.87 water activity. An aw of 0.85 or less suppresses the growth of organisms of public health significance.

The Centers for Disease Control (CDC) stated food borne disease is responsible for approximately 76 million illnesses, 325,000 hospitalizations and 5,000 deaths annually in the United States. Known pathogens, such as Salmonella, Escherichia coli O157:H7, Campylobacter and Listeria monocytogenes, account for an estimated 14 million illnesses, 60,000 hospitalizations and 1,800 deaths alone. With staggering statistics like these one would hardly believe that the United States has the safest food supply. Based on Food Net surveillance data from 1997 to 1999, illness from the most common bacterial food borne pathogens declined nearly 20 percent. This decline represents at least 855,000 fewer Americans each year suffering from food borne illness caused by bacteria since 1997. By using water activity and controlling major food risks, such as microbial contaminants, the food industry can better ensure the safety of its products.

In India Food-borne parasitic zoonoses have a major impact on the health and economy in developing countries in the tropics and sub-tropics. Complex socio-economic and socio-cultural factors impact on the maintenance of parasitic zoonoses. In addition to human disease, some of these parasites are responsible for economic loss to livestock production. Throughout India, problems of food-borne parasitic zoonoses differ because of varied food habits. Other factors, however, such as unhygienic living conditions, lack of education, poor personal hygiene, poverty and occupation, also contribute to the dissemination of parasitic infections (1).

Gugnani discussed the role of some emerging food and water borne pathogens came in to the knowledge since last decades due to changes in demography, food habits, food technology, commerce, water sources and environmental factors. He listed some important emerging food and water borne bacterial pathogens include Listeria monocytogenes, Campylobacter jejuni, Yersinia enterocolitica, Salmonella enteritidis, Escherichia coli O157: H7, Vibrio cholerae biotype E1 Tor Serotype 0139, Vibrio parahaemolyticus and Aeromonas hydrophila, A. sobria, and A. caviae . Describe the prevalence, ecological relationships of these organisms, their transmission through food, water and other environmental sources, and role of their virulent factors in the pathogenesis of infections and their public health significance (2).

In Southeast Asia the Fasciolopsiasis an endemic is a snail-transmitted, intestinal, food-borne parasitic zoonosis caused by a trematode, Fasciolopsis buski, which also infects farm pigs. Fasciolopsiasis remains a public health problem despite changes in eating habits, alterations in social and agricultural practices, health education, industrialization, and environmental alterations. The disease occurs focally and is most prevalent in school-age children. In foci of parasite transmission, the prevalence of infection in children ranges from 57% in mainland China to 25% in Taiwan and from 50% in Bangladesh and 60% in India to 10% in Thailand. Control programs implemented for food-borne zoonoses are not fully successful for fasciolopsiasis because of century-old traditions of eating raw aquatic plants and using untreated water. Fasciolopsiasis is aggravated by social and economic factors such as poverty, malnutrition, an explosively growing free-food market, a lack of sufficient food inspection and sanitation, other helminthiases, and declining economic conditions (3).

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. Water activity’s usefulness as a food quality and safety measurement has been published when it become evident water contents could not adequately account for microbial growth fluctuations .The water activity aw concept has served the microbiologist and food technologist form decades and is the most commonly used criterion for safety and quality (4).

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 (5). Water activity, not water content, determines the lower limit of available water for microbial growth. The lowest aw at which the vast majority of food spoilage bacteria will grow is about 0.90. Staphylococcus aureus under anaerobic conditions is inhibited at an aw of 0.91, but aerobically the aw level is 0.86. The aw for mold and yeast growth is about 0.61 with the lower limit for growth of mycotoxigenic molds at 0.78 aw (6,7). Table – 1 lists the water activity limits for growth of microorganisms significant to public health and examples of foods in those ranges.

Table – 1: Water Activity and Growth of Microorganisms in Food*

S.No.

1.

Range of aw: 1.00 – 0.95

Microorganisms Generally Inhibited by Lowest awin This Range: Pseudomonas, Escherichia, Proteus, Shigells, Klebsiella, Bacillus, Clostridium perfringens, some yeasts

Foods Generally within This Range: Highly perishable (fresh) foods and canned fruits, vegetables, meat, fish, and milk

2.

Range of aw: 0.95 – 0.91

Microorganisms Generally Inhibited by Lowest awin This Range: Salmonella, Vibrio parahaemolyticus, C.botulinum, Serratia, Lactobacillus, Pediococcus, some molds, yeasts (Rhodotorula, Pichia)

Foods Generally within This Range: Some cheeses (Cheddar, Swiss, Muenster, provolone), cured meat (ham)

3.

Range of aw: 0.91 – 0.87

Microorganisms Generally Inhibited by Lowest awin This Range: Many yeasts (Candida, Torulopsis, Hansenula), Micrococcus

Foods Generally within This Range: Fermented sausage (salami), sponge cakes, dry cheeses, margarine

4.

Range of aw: 0.87 – 0.80

Microorganisms Generally Inhibited by Lowest awin This Range: Most molds (mycotoxigenic penicillia), Staphyloccocus aureus, most Saccharomyces (bailii) spp.,Debaryomyces

Foods Generally within This Range: Fruit juice concentrates, sweetened condensed milk, syrups

5.

Range of aw: 0.80 – 0.75

Microorganisms Generally Inhibited by Lowest awin This Range: Most halophilic bacteria, mycotoxigenic aspergilli

Foods Generally within This Range: Jam, marmalade

6.

Range of aw: 0.75 – 0.65

Microorganisms Generally Inhibited by Lowest awin This Range: Xerophilic molds (Aspergillus chevalieri,A. candidus, Wallemia sebi), Saccharomyces bisporus

Foods Generally within This Range: Jelly, molasses, raw cane sugar, some dried fruits, nuts

7.

Range of aw: 0.65 – 0.60

Microorganisms Generally Inhibited by Lowest awin This Range: Osmophilic yeasts (Saccharomyces rouxii), few molds (Aspergillus echinulatus, Monascus bisporus)

Foods Generally within This Range: Dried fruits containing 15-20% moisture; some toffees and caramels; honey

8.

*Adapted from Beuchat, 1983 (7)

Water activity definition

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. These requirements are; pure water (aw = 1.0) is the standard state, the system is in equilibrium, and the temperature is defined In the equilibrium state:

μ= μo +RT ln (f/fo)

Where: μ (J mol-1) is the chemical potential of the system i.e. thermodynamic activity or energy per mole of substance; μo is the chemical potential of the pure material at the temperature T (0K); R is the gas constant (8.314 J mol-1 K-1); f is the fugacity or the escaping tendency of a substance; and fo is escaping tendency of pure material (8). The activity of a species is defined as a = f/fo. When dealing with water, a subscript is designated for the substance,

aw = f/fo

aw is activity of water, or the escaping tendency of water in system divided by the escaping tendency of pure water with no radius of curvature. For practical purposes, under most conditions in which foods are found, the fugacity is closely approximated by the vapor pressure (f ≈ p) so;

aw = f/fo ≈ p/po

Equilibrium is obtained in a system when µ is the same everywhere in the system. Equilibrium between the liquid and the vapor phases implies that µ is the same in both phases. 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. 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 100 gives the equilibrium relative humidity (ERH) in percent.

aw = p/po = ERH (%) / 100

Water activity is a measure of the energy status of the water in a system. There are several factors that control water activity in a system. Colligative effects of dissolved species (e.g. salt or sugar) interact with water through dipole-dipole, ionic, and hydrogen bonds. 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 un-dissolved ingredients (e.g. starches and proteins) through dipole-dipole forces, ionic bonds (H3O+ or OH-), Vander-walls forces (hydrophobic bonds), and hydrogen bonds. 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 metric effects.

Due to varying degrees of osmotic and metric interactions, water activity describes the continuum of energy states of the water in a system. The water appears “bound” by forces to varying degrees. This is a continuum of energy states rather than a static “boundness”. Water activity is sometimes defined as “free”, “bound”, or “available water” in a system. Although these terms are easier to conceptualize, they fail to adequately define all aspects of the concept of water activity.

Water activity is temperature dependent. Temperature changes water activity due to changes in water binding, dissociation of water, solubility of solutes in water, or the state of the matrix.

Although solubility of solutes can be a controlling factor, control is usually from the state of the matrix. Since the state of the matrix (glassy vs. rubbery state) is dependent on temperature, one should not be surprised that temperature affects the water activity of the food. The effect of temperature on the water activity of a food is product specific. Some products increase water activity with increasing temperature, others decrease aw with increasing temperature, while most high moisture foods have negligible change with temperature. One can therefore not predict even the direction of the change of water activity with temperature, since it depends on how temperature affects the factors that control water activity in the food.

As a potential energy measurement it is a driving force for water movement from regions of high water activity to regions of low water activity. Examples of this dynamic property of water activity are; moisture migration in multi domain foods (e.g. cracker-cheese sandwich), the movement of water from soil to the leaves of plants, and cell turgor pressure. Since microbial cells are high concentrations of solute surrounded by semi-permeable membranes, the osmotic effect on the free energy of the water is important for determining microbial water relations and therefore their growth rates.

Measurement of water activity for product quality

The water activity scale extends from 0 (bone dry) to 1.0 (pure water) but most foods have a water activity level in the range of 0.2 for very dry foods to 0.99 for moist fresh foods. There is no device that can be put into a food that directly measures the water activity. Water activity is in practice usually measured as equilibrium relative humidity (ERH).

The water activity aw represents the ratio of the water vapor pressure of the food to the water vapor pressure of pure water under the same conditions and it is expressed as a fraction. If we multiply this ratio by 100, we obtain the equilibrium relative humidity (ERH) that the foodstuff would produce if enclosed with air in a sealed container at constant temperature. Thus a food with a water activity aw of 0.7 would produce an ERH of 70%.

New instrument technologies have vastly improved speed, accuracy and reliability of measurements. Reliable laboratory instrumentation is required to guarantee the safety of food products and enforce government regulations. Two different types of water activity instruments are commercially available. One uses chilled mirror dew-point technology while the other measures relative humidity with sensors that change electrical resistance or capacitance. Each has advantages and disadvantages. The methods vary in accuracy, repeatability, speed of measurement, stability in calibration, linearity, and convenience of use.

Advantages chilled -mirror dewpoint sensors

The Importance of the concept of water activity cannot be over emphasized; Water activity is a measure of the energy status of the water in a system (9). More importantly the usefulness of water activity in relation to microbial growth, chemical reactivity and stability over water content has been shown in fig. 1.

Microbial Growth

Water is a critical activity factor that determines the self life of products. Water activity, not water content, determines the self life of the products. Water activity, not water content determine the lower limit of available water for microbial growth, While temperature, pH and several other factors can influence whether an organism will grow in a product and the rate at which it will grow .Water activity is often the most important factor .The lowest aw at which the vast majority of spoilage bacteria will grow is about 0.99. The aw for molds and yeast growth is about 0.61 with the lower limit for growth of mycotoxigenic molds at 0.78 aw.

Chemical and Biochemical reactivity

Water activity influence not only microbial spoilage but also chemical and enzymatic reactivity. Water may influence chemical reactivity in different ways. It may act as a solvent, reactant or change the mobility of the reactants by affecting the viscosity of the system. Water activity influences non-enzymatic browning, lipid oxidation, degradation of vitamins, enzymatic reactions, protein de-naturation, starch retrogradation (Fig1).

Physical Properties

In addition to predicting the rates of various chemicals and enzymatic reactions, water activity affects the textural properties of foods. Foods with high aw have a texture that is described as moist, juicy, tender and chewy. When the water activity of these products is lowered, undesirable textural attributes such as hardness, dryness, staleness and toughness are observed. Low aw foods normally have texture attributes described as crisp and crunchy while at higher aw the texture changes to soggy. Also, water activity affects the flow, caking and clumping properties of powders and granules.

Controlling moisture migration

Water activity is an important parameter in controlling water migration of multicomponents products .Some foods contains components at different water activity levels, such as cream filled snack cakes or cereals with dried fruits. Undesirable textural changes are often the results of moisture migration in multicomponent foods. Moisture will migrate from the region of high aw to the region of lower aw, but the rate of migration depends on many factors. For example, moisture migrating from the higher aw dried fruit in to the lower aw cereal causes the fruit to become hard and dry while the cereal becomes soggy.

Free Water Vs bound water

The water activity of a food describes the energy state of water in the food, and hence it’s potential to act as a solvent and participate in chemical/biochemical reactions and growth of microorganisms (10).

Water activity instruments measure the energy status (Sometimes referred to as free, unbound or active water) of the water present in a sample. A portion of the total water content present in the sample is strongly bound to specific sites on the components in the sample. Many preservation process attempt to eliminate spoilage by lowering the availability of water to microorganism. Reducing the aw also minimizes other undesirable chemical changes occurring during storage. The process used to reduce the aw include techniques like concentrate, dehydration, humecants, freezing and freeze drying. These techniques control spoilage by making water unavailable to microorganisms, because water present in varying energy status, analytical methods that attempts to measure total moisture in the sample don’t always agree. Water activity tells the real story (11).

Chilled mirror dew point theory

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 (12).

In a chilled mirror dewpoint system, water activity is measured by equilibrating the liquid phase water in the sample with the vapor phase in the headspace of a closed chamber and measuring the relative humidity of the headspace. In the Aqua lab (13) (Water activity Measurement Instrument Name) a sample is placed in a sample cup which is sealed against a sensor block inside the sensor and an infreared thermometer. The dewpoint sensor measures the dewpoint temperature of the air and the infrared thermometer measures the sample temperature. From these measurements the relative humidity of the headspace is computed as the ration of dewpoint temperature saturation vapor pressure at the sample temperature. When the water activity of the sample and the relative humidity of the air are in equilibrium, the measurement of the headspace humidity gives the water activity of the sample. The purpose of the fan is to speed equilibrium and to control the boundary layer conductance of the dew point sensor (14).

Speed and accuracy

The major advantages of the chilled mirror dewpoint method, which is a primary method approved by AOAC (15) International are speed and accuracy. Chilled mirror dewpoint is a primary approach to measurement of relative humidity based on fundamental thermodynamic principles .Since the measurement is based on temperature determination, chilled mirror instruments make accurate (±0.003 aw ) measurements in less than 5 minutes .For some applications, fast reading allow manufacturers to perform at-line monitoring of a product’s water activity. Processing changes can then be made during production. With Aqua Lab’s chilled mirror technology (13), temperature control is unnecessary for most applications but available if required.

Water Activity -accepted and approved

Water activity is an important property. It predicts stability with respect to microbial growth, rates of deteriorative reaction and physical properties. The growing recognition of measuring water activity is illustrated by the U.S. Food and Drug Administration’s incorporation of the water activity principle in defining safety regulations. The purpose of the regulation are to details the specific requirements and practices to be followed by industry to assure that products produced under sanitary conditions and are pure, wholesome and safe .In the past, measuring water activity was a frustrating experience. New instrument technology, like Aqua lab, have vastly improved speed, accuracy and reliability of measurements (13 ).

Application of the water activity concept

From the above definition it is easily understood how water activity is useful in predicting food safety and stability with respect to microbial growth, chemical/biochemical reaction rates, and physical properties. By measuring and controlling the water activity of foodstuffs, it is possible to;

a) Predict which microorganisms will be potential sources of spoilage and infection.

b) Maintain the chemical stability of foods.

c) Minimize non-enzymatic browning reactions and spontaneous autocatalytic lipid oxidation reactions.

d) Prolong the activity of enzymes and vitamins in food.

e) Optimize the physical properties of foods such as texture and shelf life (12, 16). 

The growing recognition of the water activity principle is illustrated by its incorporation into FDA and USDA regulations, GMP and HACCP requirements, and most recently in NSF International Draft Standard 75. Water activity is an important critical control point for risk analysis as defined by the HACCP concept. These regulations and requirements are based on the current FDA Food Code definition of potentially hazardous foods. Potentially hazardous foods are those that require temperature control because they support the rapid and progressive growth of pathogenic microorganisms. Potentially hazardous food does not include items with water activity values of 0.85 or less, food with a pH level of 4.6 or less, food in unopened hermetically sealed containers, those that maintain commercial sterility under non-refrigerated storage and distribution, or those in which rapid and progressive growth of pathogenic microorganisms cannot occur.

Conclusion

Prevention will continue to be of prime importance for food safety not only for the the processor and government perspective but consumer’s point of new too. many companies are already using HACCP voluntarily, in addition to the mandatory meat, poultry and seafood programs. Included in GMP and HACCP programs are steps requiring early warning system, risk assessment, improved detection/control methods, and improved inspections and compliance. Monitoring water activity is a critical control point for many manufacturers and should be incorporated into many other food safety programs. The number one priority is protecting the consumer. Recalls will cost millions of dollars/Rupee in product losses and operational delays, along with losses to consumer confidence and company reputation. Incorporating water activity testing and other science based analyses into a food safety program helps ensure the highest quality and safest food supply. In a nutshell The simplest way to control water activity is with a process which drives off water-cooking, baking or dehydration. The highest -heat processes also use the lethal properties of heat, while dehydration or freeze drying only work by lowering the aw to a level that curbs growth. It is true that water activity is a critical measurement in determining the shelf-life and safety of foods and other substance (pharmaceutical, neutracutical and cosmoceutical products)