Water Activity and its Influence on Food Reactions

1. FOOD CHEMISTRYFSTC 605 • Instructor: Dr. Steve Talcott • Office: 220F Centeq A • Phone: 862-4056 • E-mail: stalcott@tamu.edu • Course website: https://aglifesciences.tamu.edu/talcottlab/

2. Reactions Driven byWater in Foods

3. Water Activity • It is now generally accepted that Aw is more closely related to the physical, chemical, and biological properties of foods and other natural products than is total moisture content. • Specific changes in color, aroma, flavor, texture, stability, and acceptability of raw and processed food products have been associated with relatively narrowAw ranges.

4. Free vs Bound Water • What is the best way to describe Aw • Water Activity • Free water, available for reactions • Bound water, not available for reactions

5. Water • True, water must be “available” in foods for the action of both chemical and enzymatic reactions. • This “available” water represents the degree to which water in a food is free for: • Chemical reactions • Enzymatic reactions • Microbial growth • Quality characteristics • Related to a simple loss of moisture • Related to gel breakdown • Food texture (gain or loss) • Food color

6. Water • The amount of “free” water, available for these reactions and changes is represented by Water Activity. • As the percentage of water in a food is “bound” changing from its “free” state, the water activity decreases • Water Activity is represented by the abbreviation: Aw • Aw = P/ Po • P = Vapor pressure of a food • Po = Vapor pressure of pure water (1.0) • Vapor pressure can be represented as equilibrium RH • Is based on a scale of 0.0 to 1.0 • Any food substance added to water will lower water activity….so, all foods have a water activity less than 1.0

7. The Truth Behind Aw…? “Bound” Water is Not Totally Immobilized Water • Water activity is commonly described as either “free” or “bound” • Food products are defined by the amount of free water they contain. • This concept is relatively easy to understand. • However, this does not describe all aspects of water activity. • “Free” water is not subjected to any force that reduces its free energy state • Therefore ALL water in food is “bound” water. • So the question really should be how tightly is the water bound? • This is what Aw really measures. • Tightly bound water requires more energy to remove, therefore a lower Aw. • Also, “bound” water is not totally immobilized. • Chemical and microbiological reactions can still occur in what we may call “matrices” that contain only bound water. • A true measure of water activity tell the “real” story, and does not delineate between water that may or may not be bound.

8. Water Sorption Isotherm Type I Hydration Type II Absorbed Type III Free Lipid oxidation Moisture Content Isotherm Moisture Content Relative Reactivity NEB Molds Enzyme activity Yeast MO Water Activity

9. Influences on Water Activity • Foods will naturally equilibrate to a point of equilibrium with its environment • Therefore, foods can adsorb or desorb water from the environment • Desorption is when a “wet” food is placed in a dry environment • Desorption implies that the food is attempting to move into equilibrium • Dehydration is the permanent loss of water from a food • In both cases, the Aw decreases • Desorption is generally a slow process, with moisture gradually decreasing until it is in equilibrium with its environment. • Adsorption is when a “dry” food is placed in a wet environment • As foods gain moisture, the Aw increases • The term “hygroscopic” is used to describe foods or chemicals that absorb moisture • A real problem in the food industry (lumping, clumping, increases rxn rates)

10. Aw in Low Moisture Foods • Water activity and its relationship with moisture content help to predict and control the shelf life of foods. • Generally speaking, the growth of most bacteria is inhibited at water activities lower than 0.9 and yeast and mold growth prevented between 0.80 and 0.88. • Aw also controls physiochemical reactions. • Water activity plays an important role in the dehydration process. Knowledge of absorption and desorption behavior is useful for designing drying processes for foods.

11. Freezing Foods Freezing Point Super-cooling Latent heat of Crystallization

12. Drying Curve of a Food Water that is easily removed Water that is difficult to remove

13. Moisture Migration and Control in a Multi-Domain Food • Control of moisture content and moisture migration is critical to the quality and safety of multi-domain foods (foods that contain multiple moisture or Aw levels). • Examples of multi-domain systems are: • Dry cereal with semi-moist raisins • Frozen pizza crust with sauce • Ice cream in a cone • A pastry with a fruit filling • Chocolate or hard candy with liquid centers • A cheese and cracker snack • The crust and crumb of a loaf of bread right after baking. • Moisture loss or gain from one region to another region will continuously occur, striving to attain equilibrium. What to do? • Add an edible layer between domains • Change the water activity of the food ingredients • Change the diffusivity of the water • Change the viscosity in the two phases to slow water migration • Food quality and safety become critical factors; shelf life is dramatically altered. • Moisture loss or gain from one region to another will continuously occur until equilibrium is reached with the surrounding food components, its package, or the environment.

14. Edible Moisture Barriers: How to Assess of their Potential and Limits in Food Products Shelf-Life Extension

15. Moisture Control • Control of moisture transfer inside foods and its environment is a major challenge • Film-forming compounds create moisture barriers with optimized functions • Due to nonlinear water sorption isotherms, different diffusivities, and physical food states; modelling transport phenomena through edible barriers is complex.

16. Edible Coatings for Fresh-Cut Fruits

17. Edible Coatings for Fresh-Cut Fruits

18. Edible Coatings for Fresh-Cut Fruits

19. Edible Coatings for Fresh-Cut Fruits

20. Edible Coatings for Fresh-Cut Fruits

21. Beyond water activity: Recent advances based on an alternative approach to the assessment of food quality and safetyLouise Slade , Harry Levine & David S. Reid

22. Aw is the relative vapor pressure of water in the headspace above a food • Aw is easily measured and may be related to the shelf life of foods, so it is a strongly entrenched concept • However, the Aw concept is not universally useful or applicable, so a practical approach is needed. • Research has shown the shortcomings of Aw. • Serious problems can arise when Aw is used as a predictor of food quality and safety. • An alternative approach to the technological challenges of moisture management should emphasize three fundamental principles:

23. 1st: Real food systems are never equilibrium, so the kinetics of water must be considered. • 2nd: There are inter-relationships among the moisture content of a food, time, and temperature….so shelf life prediction is challenging • 3rd: With respect to freezing or drying to control Aw, the conditions of time, temperature, and moisture must be measured for each solute in the food or added to the food

24. Glass Transition Temp (Tg)Process in which a polymer changes on cooling to a polymer glass…..or when a polymer glass changes on heating to a polymer meltIt is a gradual and reversible transition in amorphous materials from a hard and relatively brittle "glassy" state into a viscous or rubbery state as the temperature is increased.