Water Activity (Aw) PDF

WATER ACTIVITY (Aw) Water content alone is not reliable indicator of perishability. Relates to the intensity : water associate with non-aqueous constituents. Bound water would be less able to support degradative activities like growth of m/o or hydrolytic chemical reactions. Intrin...

WATER ACTIVITY (Aw) Water content alone is not reliable indicator of perishability. Relates to the intensity : water associate with non-aqueous constituents. Bound water would be less able to support degradative activities like growth of m/o or hydrolytic chemical reactions. Intrinsic property of the sample.  a w = p/po where Definition: equilibrium property of water at given temperature and moisture content =relative vapour pressure (RVP) at that temperature p is partial pressure of water vapour at the surface of the product po is saturation pressure, or the partial pressure of water vapour above pure water at same temp. p/po = RVP = relative vapour pressure aw = ERH/100 Where ERH is Equilibrium Relative Humidity property of the atmosphere in equilibrium with the sample RVP can be determined by placing it in a closed chamber for a time sufficient to achieve apparent equilibrium (constant weight) and then measuring either pressure or relative humidity  Water activity is not a measure of free water vs bound water aw =0.8 does not mean 80% free and 20% bound. It just means that the vapor pressure is lower than that of pure water due to some interactions  Water activity 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 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. Not to predict even the direction of the change of water activity with temperature, how temperature affects the factors that control water activity in the food. Moisture Content Defined Quantitative measure of amount of water Empirical measurement with no standard Primary Method: Loss on Drying Wet Weight - Dry Weight x100 Dry Weight (db) or Wet Weight (wb) Primary Method: Titration 3Z + ROH + I 2 + H 2O → 3ZH + + ROSO3− + 2 I − Microbial growth Cheese & Cracker System Experiment 1 - Cracker is equilibrated over saturated NaCl in a sealed container Cheese & Cracker System Initial Final Moisture Content 4% 20% Water Activity 0.30 0.75 Cheese & Cracker System Experiment 2 - Cheese is equilibrated over saturated NaCl in a sealed container Cheese & Cracker System Initial Final Moisture Content 60% 30% Water Activity 0.90 0.75 Cheese & Cracker System Experiment 3 - Cracker and Cheese are placed together in a sealed container 20% Moisture 30% Moisture Which way does water move?  Moisture Sorption isotherm water activity of a food material-measure of the change in free energy of water in a food material. change in free energy arises both from the entropy of mixing (ΔSmix) and the enthalpy (ΔHmix) of water–solute interactions in the food material constructing an inverse plot of the water content of a food as a function of aw: possible to assess the thermodynamic status of water in a food material relate that to chemical and physical changes microbial spoilage of foods  Moisture Sorption isotherm Constructed: resorption (or adsorption) method, completely dry food material is incubated in controlled humidity chambers at constant temperature. Various saturated salt solutions to create various humidity atmospheres inside the chambers. Sample is kept in the humidity chamber until it reaches a constant weight (typically several days) net gain in weight of the sample at equilibrium at a given aw (or relative humidity) represents the water content of the sample (g water/g dry sample) at that aw Study questions What are the factors that control water activity system? How does water structure influence behaviours and properties of water? What are the various functions or actions of water in foods? Moisture Sorption isotherm Shapes and positions of MSIs of food materials depend: composition of the food material phase states of the components MSIs fall into 3 categories: crystalline materials J-type isotherm foods containing highly hygroscopic components complex foods containing polymeric materials  Moisture Sorption Isotherms The relationship between water activity and equilibrium moisture content of a sample at a specified temperature is called the moisture sorption isotherm.  What is the significance of the change of the monolayer moisture content with temperature? MSI- cornflakes Adsorption (resorption) Desorption Application: stability storage Packaging and handling systems  Moisture Sorption Isotherm (MSI) Icing Creme Filling Cake Wood Pulp Granola Bar Milk Powder Each product has its own unique 50 moisture sorption isotherm – due 45 to different interactions between 40 Moisture Content (% d.b.) the water and the solid 35 components at different moisture 30 contents. 25 20 15 10 5 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Water Activity  Interpretation of MSI energy status of water , chemical and physical changes and microbial growth in foods nonlinear relationship between water content and water activity, sigmoidal shape suggests that water exists in different coupled states in foods at different water content levels can be divided into three regions  Classification of water states in foods Class of Description Proportion of Water Typical 90% (Wet Basis) Moisture Content Food Constitutional An integral part of nonaqueous constituent than 0.70, Sorbitol and Sucrose experience phase changes and become hygroscopic Mannitol is non-hygroscopic over the entire water activity range  Powder Flow and Caking Caking is water activity, time and temperature dependent process. Free flowing powder is transformed into lumps and eventually an agglomerated solid. Problem is ubiquitous in the food and pharmaceutical industries. Water activity and food stability  Chemical/Biochemical Stability In general, reaction rate increases as water activity increases from the monolayer value. Common reactions Loss of related to chemical stability: Crispness Powder Maillard Browning Caking Collapse Lipid Oxidation Hydrolysis Nutrient Degradation Enzymatic Reactions Enzymatic Stability  Chemical/Biochemical Stability Lipid Oxidation A minimum reaction rate at a water activity of about ~0.3 aw Rate increase from 0-0.3aw and 0.4-0.8aw  Chemical/Biochemical Stability Maillard reactions are a complex series of reactions involving free amines (amino acid) carbonyl groups (reducing sugars) Maximum loss of amino nitrogen occurs at 0.60-0.70 aw Prevent Mold with aw  Water activity application Moisture migration Product formulation Microbial growth Determine texture Chemical stability  Controlling Water Activity Dehydrate Product Additives Edible films and coatings Ingredients involved in water binding Keep water from migrating Humectants: between the different salt – (NaCl) components in a composite sugars – (glucose, fructose, sucrose, samples. syrups) glycols – (glycerol, PEG, propylene They are located on the surface glycol) or as thin layers between several amino acids – (glycine, alanine) parts within the product. polymers – (starch, gums) acids – (citric acid, lactic acid) Anticaking agents