In food preservation, dehydration is a common used approach. It is one of the oldest but also a simple and safe way to preserve food. It is process to remove moisture or water away from food. In a low moisture environment, yeast, bacteria and mold cannot grow (Andress and Harrison, 2006). Therefore, dehydration is helpful to minimize the growth of microorganism that is a major reason of food deterioration. At the meantime, drying also reduces the activity of enzymes that cause degradation of some nutrients. For fruit and vegetables that contain high level of water, they can go bad easily if inappropriate storage approach is applied. It was estimated 25-30% of total fruits and vegetable produced is wasted due to spoilage (Ramaswamy and Marcotte, 2006). Furthermore, dehydrated food is usually smaller and lighter and thus is convenient to transfer or store.
There are several ways to achieve dehydration in industrial production, such as heat, dry air, air movement, in the sun, in the oven and using a food dehydrator (Ramaswamy and Marcotte, 2006). For some kinds of food, pretreatment such as blanching is required. Blanching which is a process of briefly precooking of food in steam or boiling water is helpful to reduce activity of enzymes, to skill spoilage organisms and to shorten drying time. However, beside the time of storage, color, nutrient, taste and texture are also important points that should be considered in preservation (Salunkhe and Kadam, 1998). Whether the pretreat process will influence the quality of final product should be carefully assessed. Besides, dehydrated food requires rehydration before cooking and eating and whether the rehydrated food can maintain the same consistency as the original fresh food should also be considered.
Dehydration is a process to remove water from food. This is helpful to reduce growth of microorganisms and to inhibit enzyme activity to achieve food preservation. The approaches commonly used include heat, dry air, air movement, in the sun, in the oven and using a food dehydrator. During the process of drying, a series of physical changes happen to the food. Moisture content, surface morphology such as roughness (toughening of texture) and color (lighter or darker) will change.
However, some invisible changes are more influential to the food. In some cases, the approaches to dehydrate are not sufficient to achieve desired level of dehydration. Blanching is a widely used pretreat of dehydration of vegetables that is effective in destroying enzyme activity Besides, blanching also serves role to inhibit growth of microbial population, to facilitate following processing and to stabilize color and taste (Pan et al., 2005). However, over blanching can lead to excessive structure damage and makes the food tasteless.
Based on calculation of moisture content on the dry basis, it was observed that water account the largest proportion of both carrot and potato samples. Actually, blanching causes damage to cell membrane and cytoplasm, the whole cell structure is damaged and the membrane becomes permeable (Ramaswamy, H. S., Marcotte, 2006). Therefore, in the treatment of carrot samples, blanching leads gradual loss of carotenoids and leaching of soluble solid.
This is the reason why the color of blanching and over blanching samples is pale than the untreated samples. Besides, the heat and steam in blanching also contribute to the alteration of texture since it can change the crystallinity of cellulose and make the cells loss its vigor. This is the reason why blanching and over blanching samples were much softer than the untreated samples. Furthermore, structure damage is an on-going process during blanching, this explains why blanching samples have darker color and more flexible structure than the over blanching samples. In the potato groups, the situation is similar. Since potato contains high level of starch that can gelatinize around 60-70?, blanching makes the potato samples have a translucent appearance.
Water activity refers to the partial pressure of water above the food surface divided by the pure component vapour pressure of water at the same temperature as the sample. The level of water activity also refers to the water molecules available for pathogenic bacteria and biochemical reactions. According to Bhandari and Adhikari (2008), water activity is associated with food stability and safety with respect to rate of chemical and biochemical reaction, microbial growth and physical properties. The following figure 1 shows the level of water activity and correspondent degradative reactions rates and microbial growth limits.
Based on this figure, it can be concluded that low water activity can inhibit growth of microorganisms that is a potential source of food spoilage, optimize the physical properties such as texture, moisture migration and shelf life, maintain the chemical stability of food, reduce the spontaneous autocatalytic lipid oxidization and nonenzymatic browning reactions. The low moisture level contributes to the low level of water activity. Based on Graph 1 and 2, it can be seen that moisture content on the dry basis of all carrot and potato samples has significantly declined after dehydration. At the same time, the measured water activity of dehydrated carrot and potato samples also dropped significantly, from 0.979 to 0.313 and 0.896 to 0.400 respectively. The required water activity for different microorganisms can be generally concluded as in the following table 14.