Food Science Water Properties

Water Objectives

• Students should be able to differentiate between water content and water activity at the end of the chapter.
• Students should be able to explain various terms related to water and water activity, such as equilibrium relative humidity.
• Students should be able to explain how water activity affects the rates of chemical reactions in foods and microbial growth.
• Students should be able to draw and explain water sorption isotherms, including desorption and adsorption.
• Students should be able to describe the different types of water present in foods.

Introduction

• Water is a crucial component of many foods.
• It acts as a dispersing medium and solvent, particularly in emulsified products like margarine.
• Understanding water's properties is important because it strongly influences the chemical and microbial changes in food.

Physical Properties of Water and Ice

• Molecular weight: 18.0153
• Melting point (at 101.3 kPa): 0.00°C
• Boiling point (at 101.3 kPa): 100.00°C
• Critical temperature: 373.99°C
• Critical pressure: 22.064 MPa
• Triple point temperature: 0.01°C
• Triple point pressure: 611.73 Pa
• ∆H_vap at 100°C: 40.647 kJ/mol
• ∆H_sub at 0°C: 50.91 kJ/mol
• ∆H_fus at 0°C: 6.002 kJ/mol

Temperature-Dependent Properties

• Values for density, vapor pressure, heat capacity, thermal conductivity, thermal diffusivity, compressibility, and permittivity are provided for both water and ice at different temperatures.

Water Molecules Arrangement

• Diagrams illustrate the ordered structure of frozen water, the semi-ordered arrangement of liquid water, and the random structure of water vapor.
• Water molecules change in geometric patterns based on their current physical state. This illustrates the change in density as water transitions from ice to liquid water and then to the gaseous state.

Introduction (Food Perishability)

• Food perishability is strongly related to moisture content.
• Reducing moisture content through concentration or drying processes decreases perishability.
• Moisture content alone isn't a perfect indicator of perishability. Different foods with similar moisture content can have vastly different shelf lives.
• Some foods with low moisture content are unstable (like peanut oil), while others with higher moisture content are more stable (like potato starch).

Introduction (Deterioration)

• Food deterioration is caused by microorganisms, chemical reactions, and enzymatic reactions.
• Water availability for these processes is a key factor in food stability, measured by water activity.

Water Activity

• Water activity (a_w) is a measure of how effectively water participates in chemical, physical, or microbiological reactions within a food.
• a_w is defined as the ratio of the partial pressure of water above the food to the vapor pressure of pure water at the same temperature.
• a_w = p/p_o where p = vapor pressure of water above the food, and p_o = vapor pressure of pure water at the same temperature.
• Tightly bound water has a water activity of zero as it cannot participate in reactions.
• Water activity greatly affects the rate of chemical reactions, and microbial growth in foods, resulting in different stability rates in different foods.
• Water activity of a food in equilibrium with its surroundings is also defined as the equilibrium relative humidity (ERH) of the surrounding atmosphere.
• aw = ERH/100

Water Activity & Food Stability

• Water activity affects the rate of many chemical reactions in foods and the rate of microbial growth.
• Different organisms have different optimal levels of water activity, and various rate graphs of reaction rates in foods as a function of moisture content are illustrated for many different reactions.

Microbial Growth

• Minimal water activity (a_w) levels required by different microorganisms vary.
• Some microorganisms thrive even at lower water activity levels.
• Many microbial growths decrease or stop below specific values of water activity.

Microbial Growth (continued)

• Optimal water activity ranges for most microorganisms are stated.
• Microbial stability is strong, particularly in dehydrated products.
• Immediate moisture foods (IMF), such as jam, preserve well due to altered water activity levels.

Microorganism Inhibition

• Various microorganisms have varying water activity requirements for growth.
• Some microorganisms are inhibited by lower water activities, while others thrive in drier conditions.
• These minimum a_w values are listed for various microorganisms.

Enzymic Activity

• Enzymic activity virtually ceases below a specific level of water activity.
• a_w is important for enzyme reactions as the mobility of reactants is maximized at higher moisture levels.
• An exception to this rule is lipases, which can be active in low-moisture environments.

Condensation Reactions

• The Maillard reaction, a crucial non-enzymatic browning reaction in foods, is greatly affected by the level of water activity.
• Optimum reaction rates occur within a specific water activity range (0.5–0.7).
• Higher water activity leads to a reduced browning rate due to dilution effects.

Oxidation Reactions

• Lipid oxidation, which degrades food, is affected by a_w.
• Water activity levels affect the antioxidants in foods. Low water activity leads to very fast oxidation rates due to free radicals.
• Maximum stability occurs at an a_w level that blocks oxygen's access and prevents further lipid oxidation.
• At higher water activity, oxidation speeds up.

Importance of Water Activity

• Measuring and controlling a_w in foods allows predicting spoilage sources.
• Maintaining chemical stability (preventing browning reactions and lipid oxidation) is aided by accurate a_w monitoring.
• Optimal enzyme activity and vitamin retention is maintained by controlling water activity.
• Optimizing food physical properties, such as texture and shelf life, is also possible through managing water activity.

Water Sorption Isotherm

• Plots water content versus water activity at a constant temperature.
• A sigmoidal plot, common in many foods, is shown, along with a J-shaped isotherm found in some sugar-based foods.

Water Sorption Isotherm (continued)

• Isotherms are unique for each food and temperature.
• Measuring isotherms involves either adding water to a dry sample or removing water from a wet one.
• Adsorption and desorption curves aren't mirror images due to hysteresis, which is illustrated in the diagrams and graphs.

Water Zones

• Three zones (Zone 1, Zone 2, and Zone 3) are identified on water sorption isotherms, which reflect different types of water binding and behaviours.
• Zone 1 corresponds to monolayer water that is strongly bound and stable.
• Zone 2 involves multilayered water bound to soluble components, characterized as capillary water.
• Zone 3 corresponds to free water readily available for various reactions and processes.

Reaction Rates in Foods (Water Activity)

• The table shows how different reaction rates in foods are affected by the level of water activity in monolayer water, capillary water, and free water.
• The rates of various reactions are much lower in a zone with lower water activity.

Uses of Water Sorption Isotherm

• Predicting the rate and extent of drying
• Determining optimal frozen storage temperatures.
• Determining moisture barrier properties required in packaging materials.
• Determining the water activity level at which foods are stable.
• Allowing predictions of the effect of moisture content changes on water activity and food stability.

This quiz covers essential concepts related to water in food science, including differentiation between water content and water activity. Students will learn the impact of water activity on chemical reactions and microbial growth, while also mastering water sorption isotherms and types of water in foods.