PROPERTIES OF WATER RELEVANT TO POST HARVEST
Content
PROPERTIES OF WATER RELEVANT TO POST HARVEST The properties and their constants • • • • • • • • • • • Melting point at 101.3kpa (1atm) Boiling point at 101.3kpa (1atm) Heat of fusion at 0°c Heat of vaporization at 100°c Density (kg/liter) Viscosity (pa.s) 0.00 °c 100°c 6.012 kj (1.436 kcal)/mol 40.63 kj (9.705 kcal)/mol
at 20°c (0.998203) at 0°c (0.999841) at 0°c ice (0.9168) at 20°c (1.002×10‾³) at 0°c (1.787×10‾³)
Surface tension against air (N/m) at 20°c (72.75×10‾³) at 0°c (75.6×10‾³) Heat capacity(j/kg .k) at 20°c (4.1819) at 0°c 4.2177) Thermal conductivity (j/m .s.k) at 20°c (5.983×10²) at 0°c (5.644×10²) Dielectric constant static at 20°c (80.36) at 0°c (87.9) Has unusual tendency to expand on freezing
APPLICATIONS The properties of water are used in many aspects of food processing because of the technicality associated with the processing activity and also to satisfy soaring consumer demands. Water is an important constituent of most foods of which its presence may determine the state of the food, that is whether good or bad. Water holding capacity is the ability of a matrix of molecules to entrap large amount of water. Water holding capacity has a profound effect on food quality. Water activity is the availability of water in food for microbial, enzymic and chemical reactions which leads to deterioration. Foods with high water activity contributes to its faster spoilage. Reduced water activity permits the combination of several mild control
mechanisms which results in the preservation of food without substantial loss of nutritional properties. Heat of vaporization property allows blanching of most fruits and vegetables without much damage. Blanching is used to destroy enzymatic activity in vegetables and some fruits prior to further processing and preservation. Blanching is done by passing food through an atmosphere of saturated steam or a bath of hot water. Rate of evaporation in some processing activity is used to pre- concentrate foods prior to drying , freezing or sterilization. It increases the solid content of food hence preserves it by reduction in water activity. It also changes the flavor and color of food. All these are achieved if rate of evaporation property of water is used in processing. Heat of fusion: This property is mostly used by roadside sellers to preserve their fruits. The heat of fusion of water releases enormous amount of heat on freezing, saves crops from freezing heating by spraying them with water. Density of water. Considerable difference in density of water and ice may result ijn structural damage to foods when they are frozen. The density of ice changes with temperature resulting in the stresses in frozen foods. This creates unpleasant appearance of food due to the stretches caused by the change in density. Thermal conductivity: A reduction in moisture content of food causes a substantial reduction in thermal conductivity. This has important implications in processing operations which involves conduction of heat through food to remove water, this could be through freeze drying. A reduction in atmospheric pressure also influences the thermal conductivity of the food. Freezing point: this is used in pasteurization. After pasteurization, pasteurized products are cooled further by cold water if necessary chilled water in a cooling section. Vapor pressure of water aids in steam sterilization of food. After sterilization, the containers are cooled with water, steam is rapidly condensed. The water helps to maintain pressure in the containers which is necessary for operation, that is pressure cooling. Specific heat capacity: Foods with high water content heat up and cools down slowly. Because water requires so much energy to change from liquid to gas and so water serves as an ideal medium for removing heat from foods under hot conditions.
Boiling point: Hot water is used for pasteurization if food is packaged in glasses. High polarity because of this property water can dissolve most substances and in doing so enables minerals and other chemicals to react in foods. This property aids in food processing. Dissociation energy (4.614×10²kj/mol) electrolytes of water dissociates into charged ions. The slightly negative oxygen of water is attracted to positive sodium and the slightly positive hydrogen of water are attracted to negative chlorine to form a solution which is used in food preservation. Salty water removes water from the skin of fruits thereby reducing water activity.
at 20°c (0.998203) at 0°c (0.999841) at 0°c ice (0.9168) at 20°c (1.002×10‾³) at 0°c (1.787×10‾³)
Surface tension against air (N/m) at 20°c (72.75×10‾³) at 0°c (75.6×10‾³) Heat capacity(j/kg .k) at 20°c (4.1819) at 0°c 4.2177) Thermal conductivity (j/m .s.k) at 20°c (5.983×10²) at 0°c (5.644×10²) Dielectric constant static at 20°c (80.36) at 0°c (87.9) Has unusual tendency to expand on freezing
APPLICATIONS The properties of water are used in many aspects of food processing because of the technicality associated with the processing activity and also to satisfy soaring consumer demands. Water is an important constituent of most foods of which its presence may determine the state of the food, that is whether good or bad. Water holding capacity is the ability of a matrix of molecules to entrap large amount of water. Water holding capacity has a profound effect on food quality. Water activity is the availability of water in food for microbial, enzymic and chemical reactions which leads to deterioration. Foods with high water activity contributes to its faster spoilage. Reduced water activity permits the combination of several mild control
mechanisms which results in the preservation of food without substantial loss of nutritional properties. Heat of vaporization property allows blanching of most fruits and vegetables without much damage. Blanching is used to destroy enzymatic activity in vegetables and some fruits prior to further processing and preservation. Blanching is done by passing food through an atmosphere of saturated steam or a bath of hot water. Rate of evaporation in some processing activity is used to pre- concentrate foods prior to drying , freezing or sterilization. It increases the solid content of food hence preserves it by reduction in water activity. It also changes the flavor and color of food. All these are achieved if rate of evaporation property of water is used in processing. Heat of fusion: This property is mostly used by roadside sellers to preserve their fruits. The heat of fusion of water releases enormous amount of heat on freezing, saves crops from freezing heating by spraying them with water. Density of water. Considerable difference in density of water and ice may result ijn structural damage to foods when they are frozen. The density of ice changes with temperature resulting in the stresses in frozen foods. This creates unpleasant appearance of food due to the stretches caused by the change in density. Thermal conductivity: A reduction in moisture content of food causes a substantial reduction in thermal conductivity. This has important implications in processing operations which involves conduction of heat through food to remove water, this could be through freeze drying. A reduction in atmospheric pressure also influences the thermal conductivity of the food. Freezing point: this is used in pasteurization. After pasteurization, pasteurized products are cooled further by cold water if necessary chilled water in a cooling section. Vapor pressure of water aids in steam sterilization of food. After sterilization, the containers are cooled with water, steam is rapidly condensed. The water helps to maintain pressure in the containers which is necessary for operation, that is pressure cooling. Specific heat capacity: Foods with high water content heat up and cools down slowly. Because water requires so much energy to change from liquid to gas and so water serves as an ideal medium for removing heat from foods under hot conditions.
Boiling point: Hot water is used for pasteurization if food is packaged in glasses. High polarity because of this property water can dissolve most substances and in doing so enables minerals and other chemicals to react in foods. This property aids in food processing. Dissociation energy (4.614×10²kj/mol) electrolytes of water dissociates into charged ions. The slightly negative oxygen of water is attracted to positive sodium and the slightly positive hydrogen of water are attracted to negative chlorine to form a solution which is used in food preservation. Salty water removes water from the skin of fruits thereby reducing water activity.
