Water Activity Lecture Notes PDF

Water activity Introduction ▪ Food chemistry is the study of chemical processes and interactions of components of foods. ▪ It covers the basic composition, structure and properties of foods and the chemistry changes occurring during processing and utilization ▪ It also covers the chemistry o...

Water activity Introduction ▪ Food chemistry is the study of chemical processes and interactions of components of foods. ▪ It covers the basic composition, structure and properties of foods and the chemistry changes occurring during processing and utilization ▪ It also covers the chemistry of water, carbohydrates, proteins, lipids, vitamins, minerals and enzymes. Basic constituents of food Food is any substance usually composed of carbohydrates, fats, proteins, water, etc that can be eaten or drunk by human. These constituents of food are divided into: Major: Carbohydrates, Fats , Proteins Minor: Vitamins Minerals Enzymes Pigments Flavors Acids There is also an important constituent is water Classification of foods according to their functions in the body into:- 1) Energy Yielding Foods (EYF), this group includes foods rich in carbohydrates and fats as 1 gm. of CHO provide the body with 4 K-calories 1 gm. of fat provides the body with 9 K-calories of energy. 2) Body Building Foods (BBF), they are foods rich in proteins as Milk, Meat, Egg & Fish. They are rich in proteins of high biological value. These proteins have all the essential amino acids in correct proportions for the synthesis of body tissues. 3) Protective food (PF), they are classified into foods rich in vitamins, minerals & proteins of high biological value. They are classified into 2 groups: ✓ Food high in vitamins, minerals and proteins of high biological values such as milk, egg, fish and liver. ✓ Food rich in vitamins and minerals only such as green leavy vegetables and some fruits  Major Food Constituents 1-WATER ▪ Water is an essential constituent of any foods. ▪ It may occur as an intracellular or extracellular component in vegetable & animal products which acts as a dispersing medium or solvent in foods. ▪ The presence of water in food is described as the moisture content or water activity (aw) of the food. ▪ Moisture refers to the amount of water present in food ▪ water activity (aw) refers to a form in which water exist in the food. Forms of water in food There are 3 forms 1. Free water 2. Absorbed water 3. Bound water Free water Most water in food is called free water Free water is lightly entrapped and therefore easily pressed from food It acts as a dispersing agent and solvent It can be removed from drying the food Absorbed water Water that is attached to the surface of molecules like polysaccharides and proteins. It is not readily squeezed out of the food Bound water The amount of water in body tissues which are bound to macromolecules or organelles. In food science this form of water is practically unavailable for microbiological activities so it would not cause quality decreases or pathogen increases. It can be frozen only at very low temperatures Chemically Each form of water is H2O but differences exist in physical and chemical conditions in which the water can exist The form in which water occurs in foods determines the physical prosperities of the food Ex. Milk and apples contain the same amount of water but have different physical structures Water Activity (aw) Water Activity (aw) is the amount of free water in a sample that is not bounded and therefore free for microbial growth. Water Activity is the measure of the availability of water molecule to enter into microbial, enzymatic or chemical reactions It can be represented by the symbol aw The availability determines the shelf life of food Water activity ▪ Water activity has an important role in food preservation Each microorganism has a critical aw below which it cannot grow Ex. pathogenic MOs cannot grow at water activity below 0.8 Mould and yeast not below 0.62 ▪ Decreased aw retards the growth of microorganisms, slow enzymes catalyzed reactions ▪ aw of food is decreased by adding additives with high water binding capacity (humectants) as common salts,glycerol and succrose  aw is calculated as ratio of the water vapor pressure of the substance divided by the vapor pressure of pure water at same temperature Vapor pressure can be measured by using a manometer. aw = P / P0 P is the vapor pressure of water in the substance, P0 is the vapor pressure of pure water at the same temperature. Regarding the forms of water, bound water is inversely related to water activity  as the % of bound water in a food increases the aw decreases The pure distilled water has a water activity of exactly 1. Temp should be constant as vapor pressure at 25 C differ from 35C aw = P / P0 Water activity meter (Manometer) Water activity is based on a scale from 0.0 to 1.0 The relationship between (RH) of atmosphere and (RH) of food (aw): is a measured of relative humidity (RH) by multiplying (aw) by 100 (RH)=(aw) × 100 Relative humidity is expressed as percentage , while water activity is expressed as a fraction Relative humidity (RH) of the atmosphere in equilibrium with the food (RH) What is the (ERH) ?! (ERH) of a product is the relative humidity of the air surrounding the food at which the product neither gains nor loses its natural moisture and it is in equilibrium with the environment (RH) Water migrates from areas of high aw to areas of low aw. For example, if honey (aw ≈ 0.6) is exposed to humid air (aw ≈ 0.7) the honey will absorb water from the air.  Microbial growth is directly linked to water activity In general water activity, aw represents the water which is made available for the microbial action. Microbial growth is directly linked with water Activity Water activity is a measurement of the availability of water for biological reactions. It determines the ability of micro-organisms to grow. If water activity decreases, micro-organisms with the ability to grow will also decrease. higher water activity substances tend to support more microorganisms bacteria usually require water activity values of at least 0.91 and fungi at least 0.6. Every microorganism has a limit of water activity below which it will not grow Higher aw substances tend to support more microorganisms. aw Microorganism Foods 1.0 - 0.95 Bacteria Meat, fish, sausage, milk 0.95 - 0.91 Bacteria Cheese, cured meat (ham), fruit juice conc. Fermented sausages (salami), dry cheeses, 0.91 - 0.87 Yeasts margarine Juice conc., syrups, flour, fruit cakes, honey, 0.87 - 0.80 Molds jellies, preserves 0.30 - 0.20 No microorganism Cookies, crackers, bread crusts proliferation Difference between moisture content and water activity Moisture content The amount of water in your food and ingredients. How much water is in a product. It influences the physical properties of a substance, including weight, density, viscosity, conductivity, and others. It is generally determined by weight loss upon drying. Water activity explains how the water in your food will react with microorganisms. A measure of how much of that water is free, unbound, and thus available to microorganisms to use for growth. It is therefore important with regard to food safety  Water activity and Shelf life of Foods 1- Water activity is an important consideration for food product design and food safety. If a product is kept below a certain water activity, then mold growth is inhibited. This results in a longer shelf-life. 2- Water activity is used in many cases as a critical control point for Hazard Analysis and Critical Control Points (HACCP) programs. Samples of the food product are periodically taken from the production area and tested to ensure water activity values are within a specified range for food quality and safety. 3- Water activity has an important role in food preservation Each microorganism has a critical aw below which the growth cannot occur - For e.g.: Pathogenic microorganisms cannot grow at aw below 0.86, Yeast & molds are tolerant & usually no growth occurs at or below 0.62 4- aw is important in foods and it is a major factor in food spoilage & safety Decreased aw retards the growth of microorganisms, slows enzyme catalyzed reactions & retards non enzymatic browning  5- Dehydration is the oldest form of food preservation Water is a facilitator of biochemical deterioration of foods The sun drying of meat and fish has been traces to the beginning of recorded history. Drying relies on removing water, thus making it unavailable for microbial growth. a)_Dry foods are much more stable than wet foods, because any water remaining in them has low activity. None of the dangerous pathogenic bacteria associated with food, such as Clostridium or Vibrio spp. which cause botulism and cholera, can multiply at water activity values below about 0.9. Thus, drying or providing sufficient water-binding humectants is an effective method of preventing the growth of food-poisoning bacteria. Only osmophilic yeast and some molds can grow at water activities in the range 0.6 to 0.65. Thus, by reducing the water activity below these values, foods are microbial stable. That is, unless the packaging is such that the food absorbs moisture and spoilage can occur. For example, when condensation occurs within a hermetically sealed package subject to rapid cooling. b) Freezing removes water from the food matrix by forming ice crystals. Although the ice crystals remain in the food, the remaining water which is in contact with the food matrix becomes concentrated with solutes and it’s aw becomes low. Freezing is therefore akin to drying and this is the rationale for preserving food by freezing. Most micro-organisms cease functioning below the water activity of about 0.7. There are various chemical reactions that proceed, and may be accelerated, at low values of water activity. c) Salting or curing has the same effect. A saturated solution of common salt has a water activity of close to 0.75. Thus, by adding sufficient salt to foods, the water activity can be lowered to a level where most pathogenic bacteria are inactivated but the moisture content remains high