Water activity is crucial for food stability. It measures how much water is available for microbial growth and chemical reactions. Lower water activity means less free water, which can slow down spoilage and extend shelf life.

Understanding water activity helps food scientists create safer, longer-lasting products. It impacts everything from texture and flavor to microbial growth and enzymatic activity. Controlling water activity is key to food preservation and quality control.

Water Activity and Relative Humidity

Measuring Water Availability in Foods

• Water activity (aw) quantifies the availability of water in a food system
  • Ranges from 0 to 1, with pure water having an aw of 1
  • Calculated as the ratio of the water vapor pressure of the food to the vapor pressure of pure water at the same temperature
  • Lower aw values indicate less available water and greater food stability
• Equilibrium relative humidity (ERH) is the relative humidity of the air surrounding a food when the food and air are in moisture equilibrium
  • Expressed as a percentage, ERH = aw × 100
  • Foods with high aw will have high ERH values (fresh fruits and vegetables)
  • Foods with low aw will have low ERH values (crackers, cookies, and dried fruits)

Factors Influencing Water Activity

• Solutes dissolved in the aqueous phase of a food (sugars, salts) reduce the aw by binding water molecules
  • Hygroscopic solutes (honey, molasses) can dramatically lower aw
  • Non-hygroscopic solutes (sucrose) have a lesser effect on aw
• Capillary forces in porous foods (breads, cakes) can also reduce aw by physically trapping water in small pores
• Temperature affects aw, with higher temperatures generally increasing aw due to increased molecular motion and water vapor pressure

Microbial and Enzymatic Stability

Water Activity and Microbial Growth

• Microorganisms require water to grow and reproduce, with each species having a minimum aw for growth
  • Most bacteria require aw > 0.91, while some fungi can grow at aw as low as 0.6
  • Lowering aw below these thresholds prevents microbial growth and spoilage
  • Examples: Dried meats (jerky) and dried fruits have low aw, inhibiting microbial growth
• Pathogenic bacteria (Salmonella, E. coli) generally require higher aw values (> 0.95) to grow
  • Lowering aw is an effective way to control foodborne pathogens
  • Example: Salting and drying processes used in meat and fish preservation

Water Activity and Enzyme Activity

• Enzymes are proteins that catalyze chemical reactions in foods, often leading to quality deterioration
  • Enzymatic browning (cut apples turning brown) and lipolysis (rancidity in oils) are examples
• Enzyme activity is influenced by aw, with most enzymes requiring aw > 0.8 for optimal activity
  • Lowering aw can slow or prevent enzymatic reactions, extending shelf life
  • Example: Dehydrated fruits and vegetables have low aw, minimizing enzymatic browning
• Some enzymes, such as lipases and proteases, can remain active at lower aw values
  • Additional preservation methods (heat treatment, pH control) may be necessary to fully inactivate these enzymes

Water Activity and Lipid Oxidation

• Lipid oxidation is a major cause of quality deterioration in high-fat foods, resulting in off-flavors and rancidity
• The rate of lipid oxidation is influenced by aw, with intermediate aw values (0.4-0.8) often leading to the highest oxidation rates
  • At low aw (< 0.4), there is insufficient water to support the chemical reactions involved in oxidation
  • At high aw (> 0.8), water can act as a barrier, separating reactive species and slowing oxidation
• Controlling aw is an important strategy for preventing lipid oxidation and maintaining food quality
  • Example: Properly dried nuts and seeds have low aw, minimizing lipid oxidation and extending shelf life

Chemical Reactions and Physical Changes

Water Activity and Maillard Reaction

• The Maillard reaction is a complex set of chemical reactions between reducing sugars and amino acids, resulting in brown colors and distinct flavors
  • Occurs during high-heat processing (baking, frying) and extended storage
  • Examples: Browning of baked goods, caramelization of sugars, and development of roasted flavors in coffee and nuts
• The rate of the Maillard reaction is influenced by aw, with intermediate aw values (0.6-0.8) often leading to the highest reaction rates
  • At low aw (< 0.6), there is insufficient water to support the chemical reactions
  • At high aw (> 0.8), the dilution effect of water slows the reaction
• Controlling aw is important for managing the Maillard reaction and achieving desired product characteristics
  • Example: Baked goods with intermediate aw (cakes, breads) develop rich, brown colors and flavors due to the Maillard reaction

Water Activity and Texture Changes

• Water plays a critical role in determining the texture of foods, influencing properties such as crispness, softness, and chewiness
• Changes in aw can lead to significant texture changes, often due to moisture migration or physical transitions
  • Moisture migration from high aw regions to low aw regions can cause softening of crisp foods (crackers becoming stale) or hardening of soft foods (bread staling)
  • Physical transitions, such as glass transition and crystallization, can also be influenced by aw
    • Example: Amorphous sugars in hard candies can absorb moisture and recrystallize, leading to a grainy texture
• Maintaining stable aw is crucial for preserving desired textures and preventing quality deterioration
  • Example: Proper packaging and storage conditions help maintain the crisp texture of potato chips by preventing moisture absorption

Food Quality and Preservation

Water Activity and Shelf Life

• Water activity is a key determinant of food shelf life, influencing the rates of various deteriorative reactions
  • Lowering aw can significantly extend shelf life by slowing or preventing microbial growth, enzymatic reactions, and chemical changes
  • Examples: Dried foods (pasta, rice, beans) and low-moisture snacks (chips, crackers) have long shelf lives due to their low aw
• Shelf life testing often involves monitoring aw changes over time, along with other quality parameters
  • Accelerated shelf life testing (ASLT) can be used to predict shelf life by storing foods at elevated temperatures and/or humidity levels
  • Example: ASLT of a snack food might involve storing samples at 35°C and 75% relative humidity to simulate long-term storage conditions

Preservation Methods Based on Water Activity

• Many traditional and modern food preservation methods rely on controlling aw to extend shelf life and ensure food safety
• Drying is one of the oldest and most widely used methods for lowering aw
  • Sun drying, hot air drying, and freeze-drying are common techniques
  • Examples: Dried fruits, jerky, and instant coffee
• Concentration processes, such as evaporation and membrane filtration, can also be used to lower aw
  • Commonly used in the production of sweetened condensed milk, fruit juice concentrates, and sugar syrups
• Formulation strategies, such as the addition of humectants (glycerol, sorbitol) or solutes (sugar, salt), can be used to lower aw
  • Examples: Jams, jellies, and cured meats
• Hurdle technology involves combining multiple preservation methods, often including aw control, to achieve synergistic effects
  • Example: The combination of low aw, low pH, and preservatives in the production of shelf-stable condiments (ketchup, mustard)