How Does Water Content Affect The Stability Of Water?

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Water, being the main component of foods, is an important diluent for other water soluble food components such as carbohydrate, protein, fiber, minerals and vitamins. Most fresh foods contain a large amount of water, normally more than 70%, while fresh fruits and vegetables can contain up to 90 to 98% of water (Gowen, 2012). Water in foods can be categorized into two classes, free water and bound water. Free water is freely available for chemical or biological reactions as well as for microbial growth. Meanwhile, bound water is unavailable for these reactions because it is chemically bounded to other solid matrix (Derossi et al., 2011).

Water content and water activity play a great role in determining the quality and stability of food. The
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Region one represents a water activity range of 0 to 0.2. Water is strictly bounded to the solid matric of food in this region, hence restricts the mobility of chemical reagents and reduces the rate of degradative reactions. Meanwhile, region two has a water activity range from 0.2 to 0.7. Water progressively becomes available for some biological and chemical reactions in this region such as Maillard reaction and enzymatic activities. However, it is still unavailable for microbial growth, except the growth of some molds. In region three (water activity range from 0.7 to 1.0), water molecules are weakly bounded to solutes, so they are freely available for microbial and chemical reactions (Derossi et al., 2011). Normally, food deterioration due to microbial growth is not likely to occur at water activity lower than 0.6 (Chirife, 1994). However, some chemical reactions and enzymatic changes may occur at considerably low water activities. The typical deteriorative changes occur in low-moisture foods are enzyme-catalyzed changes, non-enzymatic browning and oxidation. Enzymatic changes and non-enzymatic browning are found maximum at an intermediate-moisture level. Meanwhile, the rate of oxidation is found higher at low water content. The rate may go through a minimum with an increase in water activity, and subsequently reduce at higher water activities (Labuza, Tannenbaum and …show more content…
This process commences when a moisture-rich material is immersed in a hypertonic solution. The concentration gradient between the hypertonic solution and the intracellular fluid of material drives the water removal from the material (Phisut, 2012). In an ideal osmotic situation, only water molecules can permeate through the semi-permeable membrane, however, for living biological units, their cell membranes can stretch and expend under certain conditions, hence, some solute molecules are allowed to pass through the membrane (Toreggiani, 1993). Due to this reason, OD of biological unit will give rise to two major simultaneous counter-current flows (Figure 2.2). The first flow is water outflow from food into hypertonic solution, while the second flow is diffusion of solutes from osmotic media into food (Phisut, 2012). These two major counter-current flows are accompanied by the third flow of natural solutes from food into osmotic medium. Even though the leach of these natural solutes such as vitamins, minerals, organic acids and sugars are considered quantitatively negligible, but numerous findings showed it significantly affect the nutritional quality of the

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