Moisture determination is one of the most important and most widely used measurements in the processing and testing of foods. Since the amount of dry matter in a food is inversely related to the amount of moisture it contains, moisture content is of direct economic importance to the processor and the consumer. Of even greater significance, however, is the effect of moisture on the stability and quality of foods. A food substance, grain for instance, that contains too much water is subject to rapid deterioration from mold growth, heating, insect damage, and sprouting. The rate of browning of dehydrated vegetables and fruits and of oxygen absorption by egg powders increases with an increase in moisture content (Coultate 2002).
Moisture determination is important in many industrial problems, for example, in the evaluation of materials’ balance or of processing losses. One must know the moisture content (and sometimes its distribution) for optimum consistency, and production of bread with the best grain, texture, and freshness retention. Moisture content must be known in determining the nutritive value of a food, in expressing results of analytical determinations on a uniform basis, and in meeting compositional standards or laws. Finally it is often desirable to weigh samples for analytical determinations on a given moisture basis. This is especially important if the measured analytical parameter does not vary in a linear or simple manner with an increase in dry matter content (Meyer, 1968).
The moisture content of foods vary widely. Fluid dairy products (whole milk, nonfat milk, and buttermilk) contain 87-91% water; various dry milk powders contain about 4% water. Cheeses have immediately water contents ranging from about 40% in Cheddar to 75% in cottage; the water content of butter is about 15%, of cream 60-70%, and of ice cream and sherbet around 65%. Pure oils and fats contain practically no water, but processed lipid-rich materials may contain substantial amounts of water (from about 15% in margarine and mayonnaise to 40% in salad dressings).
Some fresh fruits contain more than 90% water in the edible portion. Melons contain 92-94% , citrus fruits contain more than 86-89%, and various berries 81-90% water. Most raw tree and vine fruits contain 83-87%. Fresh fruit juices and nectars contain 85-93% water; the water content is lowered in concentrated or sweetened products. (Meyer, 1968).
Moisture Content and Texture
At high moisture contents, when the amount of moisture exceeds that of solids, the activity of water is close to or equal to 1. When the moisture content is lower than that of solids, water activity is lower than 1. Below moisture content of about 50 percent, the water activity decreases rapidly and the relationship between water content and relative humidity is represented by the sorption isotherms. The adsorption and desorption processes are not fully reversible; therefore, a distinction can be made between the adsorption and desorption isotherms by determining whether a dry product’s moisture levels are increasing or whether the product’s moisture is gradually lowering to reach equilibrium with its surroundings, implying that the product is being dried (Weaver, 1996).
Even at low water activities, sucrose may be hydrolyzed to form reducing sugars that may take part in browning reactions (Labuza et al. 1970). Browning reactions are usually slow at low humidities and increase to a maximum in the range of intermediate-moisture foods. Beyond this range, the rate again decreases. This behavior can be explained by the fact that, in the intermediate range, the reactants are all dissolved, and that further increase in moisture content leads to dilution of the reactants.
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 (Weaver 1996).
In addition to predicting the rates of various chemical and enzymatic reactions, water activity affects the textural properties of foods. Foods with high water activity have a texture that is described as moist, juicy, tender and chewy (Bourne, 1987). When the water activities of these products are lowered undesirable textural attributes such as hard, dry, stale and tough are used. Foods with low water activity normally have texture attributes described as crisp and crunchy, while at higher water activity the texture changes to soggy. Also, water activity affects the flow, caking and clumping properties of powders and granulations.
Water activity is an important parameter in controlling water migration of multi-component products. Some foods contain components at different water activity levels, such as cream filled snack cakes or cereals with dried fruits. By definition water activity defines that moisture will migrate from the region of high aw to the region of lower aw, but the rate of migration depends on many factors. Undesirable textural changes are the result of moisture migration in multi-component foods. For example moisture migrating from the higher aw dried fruit into the lower aw cereal, causes the fruit to become hard and dry while the cereal becomes soggy (Brandt, 1996).
Freezing of water
According to Margarette McWilliams, pure water freezes at 0C. The heat released when a liquid is transformed into a solid is called the Latent Heat of Fusion (80 cal/g of water.) However, the presence of solute depresses the freezing point of water. The extent of their effect on a liter of water is 1.860C per mole of non ionizing solute. Ice crystals will begin to form at -1.860C in a sucrose solution containing 342g. sucrose/liter. The corresponding effect is 1.860C per mole of ions formed in the case of an ionizing solute. Therefore, a mole of sodium chloride lowers the freezing point of a liter of water 3.720C, and a mole of CaCl lowers the freezing point of a liter of water 5.580C. The effects do not differ for volatile & nonvolatile solutes. Added to that, ice is formed as pure water goes through the two-step crystallization (McWilliams, 1993).
When water freezes, it expands nearly 9 percent. The volume change of a food that is frozen will be determined by its water content and by solute concentration. Highly concentrated sucrose solutions do not show expansion. Air spaces may partially accommodate expanding ice crystals. The expansion of water on freezing results in local stresses that undoubtedly produces mechanical damage in cellular materials. Freezing may cause changes in frozen foods that make the product unacceptable. Such changes may include destabilization of emulsions, flocculation of proteins and loss of textural integrity (Cook, 1955).
Added to that, an article, from the website psgrill.net, entitled, “The Many Sides of Sugar” stated that, besides imparting a sweet taste, sugar performs many culinary feats. Ice cream products rely heavily on sugar. Besides adding sweetness, sugar performs a remarkable job. Sugar lowers the freezing point of cream to make a colder product.
Consequently, Coultate, 2002 cited that, in aqueous solutions some water molecules are quite tightly bound to sugar molecules.
In milk, as it is more diluted, the freezing point will raise closer to zero. The current official freezing point limit (-0.525 degrees Horvet or -0.505 degrees C), (Wong et al, 1988).
Moreover, according to the website Food Science, added water can occur in milk due to both unintentional (e.g., poor system drainage) and intentional addition. Added water can be detected in milk by measuring its freezing point. The freezing point is slightly less than that of pure water and relatively constant. Typical milk generally has a freezing point below minus 0.542 degrees Hortvett, (°H is a scale used almost exclusively for milk freezing point, a derivative of degrees Celsius). When water is added to milk, the freezing point increases approximately 0.005°H for every 1% water added. NY State uses a cut-off of -0.530°H or higher (less negative) as cause for investigation. Added water reduces the value of the milk by diluting the protein and other milk components that will influence product yields. Added water in fluid milk can also dilute the sweetness, potentially resulting in a “flat” taste.
Moisture Content and Water Activity
The water activity of a food describes the degree to which the water is “bound” in the food and hence its availability to act as a solvent and participate in chemical/biochemical reactions and growth of microorganisms. It is an important property that can be used to predict the stability and safety of food with respect to microbial growth, rates of deteriorative reactions and chemical/physical properties (Caurine, 2005).
The single most important property of water in a food is the water activity. The water activity of a food describes the energy status of water in a food and hence its availability to act as a solvent and participate in chemical or biochemical reactions. Water’s ability to act as a solvent, medium and reactant increases with increasing water activity. (Hui, 2006)
The concept of water activity (aw) is an important property for food safety. It predicts food safety and stability with respect to microbial growth, chemical/biochemical reaction rates, and physical properties. Controlling water activity is an important way to maintain the chemical stability of foods. Non-enzymatic browning reactions and spontaneous autocatalytic lipid oxidation reactions are strongly influenced by water activity. Water activity can play a significant role in determining the activity of enzymes and vitamins in food. Finally, aw plays a significant role in the physical properties such as texture and shelf life of foods.
Microorganisms have a limiting water activity level below, which they will not grow (Baumann 2006). 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 molds and yeasts growth is about 0.61 with the lower limit for growth of mycotoxigenic molds at 0.78 aw (Beuchat 1981).
Water and Enzymatic Activity
Moisture content and water activity are of major importance in affecting the progress of chemical and microbiological spoilage reactions in foods. Bacterial growth is virtually impossible below a water activity of 0.90. Molds and yeasts are usually inhibited between 0.88 and 0.80 although there are some osmophilic yeast strains that grow at water activities down to 0.65. (Baumann 2006)
Most enzymes are inactivated when the water activity falls below 0.85. Such enzymes include amylases, phenoloxidases and peroxidases. However, lipases may remain active at values as low as 0.3 or even 0.1 (Loncin et al. 1968). Examples of the effect of water activity on some enzymatic reactions have been given by Loncin (1968). In the region of monomolecular adsorption, enzymatic reactions did not proceed at all or at a greatly reduced rate, whereas in the region of capillary condensation the reaction rates increased greatly. It was found that in reactions in which lipolytic enzyme activity was measured it was of great importance how the components of the food system were put into contact. Separation of substrate and enzyme could greatly retard the reaction. Also the substrate has to be in liquid form, e.g., liquid oil could be hydrolyzed at water activity as low as 0.15 but solid fat was only slightly hydrolyzed.
Enzyme and protein stability is influenced significantly by water activity due to their relatively fragile nature. Most enzymes and proteins must maintain conformation to remain active. Maintaining critical water activity levels to prevent or entice conformational changes is important to food quality. Most enzymatic reactions are slowed down at water activities below 0.8. But some of these reactions occur even at very low water activity values. This type of spoilage can result in formation of highly objectionable flavors and odors. Of course, for products that are thermally treated during processing, enzymatic spoilage is usually not a primary concern (Baumann 2006).