Water Activity 101: Master The Basics

Water activity is not an intuitive concept for most people. But once you understand it, it can give you the power to control moisture in food. Preventing microbial growth is just the start.

Get the essentials of water activity condensed in this 20-minute webinar. You’ll learn:

-What water activity is
-How it’s different from moisture content
-Why it controls microbial growth
-How understanding water activity can help you control moisture in your product.

Presenter
Mary Galloway has been a lead scientist in the METER Food Research & Development lab for eight years. She specializes in using and testing instruments that measure water activity and its influence on physical properties. She has worked with many customers to solve their moisture-related product issues and frequently gets the chance to answer the question, “What is water activity?”

1. WATER Activity 101: Mastering the basics Mary Galloway Application Scientist METER Group, Inc. USA

2. Contents What is Water Activity How is it different from Moisture Content How does Water Activity control Microbial Growth How can Water Activity help overcome Moisture Related Challenges

3. Product Formulation Microbial Growth Product Stability

4. Water activity defined Energy is a driving force for processes to happen More Energy = More Processes (work) High energy state Low energy state (more stable)  

5. Water activity defined Water activity (a w ) is the measure of the energy status of water in a system Higher activity = More energy = Water can do more (microbial growth, moisture migration, chemical & physical reactions) Differences in a w will dictate how moisture will move Higher a w Lower a w  

6. Water Activity Defined m = m o + RT ln (f/ f o ) Gas Constant Temperature Fugacity Chemical Potential of the pure substance Chemical Potential  

7. Water Activity Defined Fugacity (f/f )= the escaping tendency of a material f/f = p/p p/p = a w p/p = Vapor pressure of water above sample at 𝓧 °C Vapor pressure of pure water at 𝓧 °C

8. Water Activity Defined a w = p/p Requirements Equilibrium Constant Temperature and Pressure

9. Moisture Content Defined Quantitative measure of the amount of water Primary Method: Loss on Drying Primary Method: Titration Empirical measurement with no standard

10. Water activity vs. Moisture content Water Activity Energy Qualitative Driving force Known standards (Salt solutions) Unit-less Moisture Content Amount Quantitative Not a driving force Empirical measurement with no standard. Must define wet basis or dry basis (LOD)

11. 18 %MC 5 %MC

12. 18 %MC 0.60 a w 5 %MC 0.60 a w

13. Water Activity: Control microbial growth Control moisture migration Avoid caking and clumping Formulate profitable products Control chemical reaction rates Model dry ingredient mixing Predict effects of temperature abuse Achieve optimal texture Conduct shelf life testing Predict packaging needs Moisture Content: Adjust texture at a given water activity Determine ingredient concentrations Determine nutritional content Labeling requirements Applications For each

14. Moisture Sorption Isotherm Each product has its own unique relationship between a w and moisture content. Moisture S orption I sotherms can be used to determine: Determine critical water activities Effects of formulation changes Model dry ingredient mixing Predict effect of temperature abuse Shelf life predictions

15. Water Activity Applications Microbial Growth Moisture Migration Product Formulation Determine Texture/Flow Properties Chemical Stability

16. Water Activity Mode of Action a w = 0.95 a w = 0.90 Water moves out to lower a w and turgor pressure is lost a w = 0.93 a w = 0.90 The microbe tries to adapt by altering its membrane make-up or by reducing its a w to maintain turgor pressure. It will produce or transport in small solutes to reduce its a w . AA’s Polyols Sugars AA’s Polyols Sugars a w = 0.93 a w = 0.90 Unable to reach equilibrium with its surroundings, the microbe will remain in the lag phase with no growth or will begin sporulation and go dormant, which can only be reversed by ideal environment conditions.

17. Microbial Growth * Optimum conditions for pH, temperature, nutrients, competing species Microorganism Minimum a w Clostridium botulinum E 0.97 Pseudomonas fluorescens 0.97 Escherichia coli 0.95 Clostridium perfringens 0.95 Clostridium botulinum A, B 0.94 Salmonella spp. 0.95 Vibrio parahaemoliticus 0.94 Bacillus cereus 0.93 Listeria monocytogenes 0.92 Bacillus subtilis 0.91 Staphylococcus aureus (anaerobic) 0.90 Staphylococcus aureus (aerobic) 0.86 Minimum a w Limit of Growth for Bacteria

18. Microbial Growth

19. Water Activity Case Study Microbial Growth – Moldy Pecans A pecan grower dried to a 4 %MC. Not sure how dry is ‘dry enough’ to inhibit microbial growth A water activity of 0.68 relates to 4.0 %MC. A water activity of 0.68 is below the microbial limit for mold Moisture content measurement was only accurate to ±0.5 %, meaning 4 %MC could actually be closer to 4.5 %MC and the water activity exceeded the safe range for mold growth Moisture content is not a reliable quality specification

20. Case Study caking and clumping Moisture Migration Dry soup manufacture processed mix to a 3% MC. Received new pepper to add to mix, which was also at 3% MC. Mixed dry soup mix and pepper together and left to sit. Entire batch had clumped. Water activity of soup mix was 0.28, but water activity of pepper was 0.69, which was above the critical water activity of the soup, causing moisture migration If they had measured water activity before mixing, they could have avoided the caking and clumping issue Water activity can be used as quality specification

21. CASE STUDY PRODUCT FORMULATION All 3 components have same water activity 3 components have very different moisture contents Each component has a unique texture Maintaining the unique texture and quality of each component is made possible by formulating to the same water activity

22. Case Study Product Formulation A pet food manufacturer produced to 6.5% moisture content. Why? Because at that level they had never had spoilage problems or shelf life concerns. He had a safe product. Then, they measured the water activity of their product. The water activity of the product was 0.40 aw, well below any microbial limits. With a maximum moisture content for pet food of10%, could they safely increase moisture content and water activity to increase their profit margin? After considering the critical water activity for the pet food, a new water activity of 0.60 was set as an acceptable limit. From the isotherm data, a water activity of 0.60 corresponded with a moisture content of 9.5%. Both values were within safety and regulatory limits. By understanding water activity, the manufacturer was able to consistently increase their profit without compromising quality or safety

23. Chemical/Biochemical Stability There is a general effect of water activity on reaction rates. Common reactions: Maillard Browning Lipid Oxidation Hydrolysis Nutrient Degradation Enzymatic Reactions Enzymatic Stability

24. conclusion METER is always happy to provide application and technical support Water Activity Energy Qualitative Driving force Water Activity is critical for: Microbial Safety Physical Stability Product Formulation Chemical Stability Predicting Shelf Life Moisture Content Amount Quantitative Not a driving force