Sari Bornstein November 18, 2010 Thursday PM E.Y. Lab #6: Water Activity- Food Preservation by Means of Moisture Control

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1 Sari Bornstein November 18, 2010 Thursday PM E.Y. Lab #6: Water Activity- Food Preservation by Means of Moisture Control PURPOSE/OBJECTIVE: The purpose of this lab was to measure the water activity of two types of food, dehydrated potato flakes and dog food, when put in different moisture/salt environments (made of saran rape, plastic bowls and aluminum cups) to see how their moisture would change in relation to their environments. INTRODUCTION: Water activity is an important topic in understanding food moisture and the spoilage of food. Microbes, including yeast, mold, pathogens and bacteria use water to grow and thrive. Water activity content of food and measures the availability of water to vaporize. Therefore, foods with very high water activity are susceptible to such growth of microorganisms. And foods with low water activity are less available to act as a solvent, reactant or medium for microorganisms. This makes food more stable. However, not all foods are favored when dried, or contain minimal water content. This is an important topic for scientists and food processors because when making food, it s important to put enough water in the food that it s not unfavorable or dry, but don t put too much water to make the food grow pathogens within a few days and cause consumers sick when eating these foods. This is how foods are often recalled. Therefore, it s important to find a happy medium of water content that has the right moisture but will not spoil in a few days time. PROCEDURE: The procedure followed for the experiment if found in FST 101A- Food Composition Laboratory Manual (Fall 2010) pages No modifications were performed. 1

2 DATA/GRAPHS: Table 1: Raw data of the initial moisture of dog food and weight of dog food used in each salt environment Solute aw Dish weight (g) Dish + sample, initial (g) Dish + sample, final (g) % water content (initial) K- acetate Mg(NO3) NaCl KCl KNO Table 2: Calculated data of the change of weight of dog food in each salt environment based on the change of moisture Solute aw Initial sample Final sample Dry sample % moisture, final % moisture, change K- acetate Mg(NO3) NaCl KCl KNO Table 3: Raw data of the initial moisture of dog food and weight of dried potato flakes used in each salt environment Solute aw Dish weight (g) Dish + sample, initial (g) Dish + sample, final (g) % water content (initial) K- acetate Mg(NO3) NaCl KCl KNO

3 Table 4: Calculated data of the change of weight of dried potato flakes in each salt environment based on the final % moisture Solute aw Initial sample Final sample Dry sample % moisture, final K- acetate Mg(NO3) NaCl KCl KNO Final % moisture vs. a w of Dog Food (IMF) Final % moisture a w Figure 1: The aw of each salt environment versus the final % moisture of the dog food for each environment 3

4 % moisture change vs. a w of Dog Food (IMF) % moisture change a w Figure 2: The aw of salt in each environment versus % moisture change of the dog food (IMF) in each environment Final % moisture vs. a w of Potato Flakes Final % moisture a w Figure 3: The aw of salt in each environment versus final % moisture of the potato flake in each different environment 4

5 CALCULATIONS: Dry sample weight =(1- Xwater) x WTinitial = ( ) x 4.01 = 3.16 grams = = % moisture final WTfinal WTdry WTdry 100% = % moisture change WTfinal WTinit WTdry 100% = 3.10g 3.16g 3.16g 3.10g 4.01g 3.16g 100% = % 100% = % RESULTS/DISCUSSION: What we did in this lab was create an atmosphere where the moisture of the food in the environment can reach equilibrium with its environment, which can provide clues to the percent moisture and water activity of those foods. Because different salts and amounts of water were used to create each environment, each environment had a different effect on that particular food due to the water activity of the salty environment versus that of the food, in addition to the percent moisture of the food versus its surrounding. Depending on which foods either absorbed or desorbed moisture from their environment can state that foods water activity in relation to the environments water activity. The relative humidity (RH) was achieved by placing different salts having different water activities into an environment where either dog food or potato flakes were placed. Because of the difference in water activity of the salts, some salt environments provided a more moister environment, and some less moist. Depending on how moist the environment was compared to the moisture of the food in the environment, either the food lost or gained moisture, telling us how well these foods absorbs or desorbs water in a more to less moisture environment. It was obvious to why the dog food and potato flakes were so different in results. Overall, dog food, which is an intermediate moisture food, lost moisture to its environment to achieve equilibrium while the dehydrated potato flakes gained moisture from their environment to achieve an equilibrium. This means that potato flakes were an absorption isotherm and dog food was a desorption isotherm. Because potato 5

6 flakes contained zero moisture, they had to gain moisture from their environment to achieve an equilibrium where both the food and the surrounding above the food were equal in moisture. Salts with a low water activity (ex. K- acetate) produced potato flakes containing less moisture due to there not being much moisture to react with the potato flakes. And salts with a high water (ex. KNO3) activity changed the moisture of the potato flakes significantly due to there being so much access of water unreacted with the salt, that s its able to dissipate into the environment and be absorbed by the potato flakes, which initially lack any kind of moisture. Dog food was the opposite considering it had already contained an initial moisture of 21.1%. Therefore in the environment with salts of a low water activity, the dog food lost more moisture to its environment than in the environment with salts of a higher water activity. The salt with the highest water activity, KNO3, had such a high water activity that is nearly provided a more moisture environment that caused the dog food to gain moisture. Therefore as the water activity of the environment (or salt) decreased, the dog food lost moisture to its environment so equilibrium could be reached. For potato flakes, as the water activity of the environments (or salt) increased, the potato flakes obtained more moisture from its environment so equilibrium could be reached. However, this lab contained many sources of errors that would provide inaccurate results. First, the environments made with saran wrap and plastic bowls were sufficient environments in our case, but a more controlled environment would be needed to provide more accurate results. First, if the saran wrap wasn t applied immediately after the food was placed in the environment, moisture from the air could alter the results. Plus, saran wrap is porous, so if this experiment was carried out for a longer period of time, results would be less accurate. Also, errors in measuring the salts and foods could occur that would lead to inaccuracy. Lastly, if the containers containing either the dog food or potato flakes was knocked over from other groups, amounts of food were lost in the process of weighing. This happened to my lab partner and I for the potato flakes in the K- acetate environment. Even though this was the only environment in which potato flakes lost moisture, our results showed that that food lost more moisture than intended due to us losing the weight of actual potato flakes. This is why our graph is altered in Figure 3, but the overall result was still seen. 6

7 CONCLUSION: In this lab I learned about the water activity of foods and how the aw effects how well those foods react with other environments or other solutes within the food. Overall, this lab was very successful in that we created an environment that provided us with somewhat accurate results, even though it was very easy to knock over others containers. However, the environments could have used some alterations to provide even higher results of accuracy. But for our purposes and for the length of time we kept the foods in their environments, this seemed to be a very useful method. QUESTIONS: 1) The dehydrated potato flakes would be considered an absorption isotherm. This is because the potato flakes started at 0% moisture and after being in the environments for approximately one week, the potato flakes increased in % moisture overall. An exception is in the K- actetate chamber where the moisture seemed to decrease for the potato flakes. As the water activity increased in each successive container containing the different salts, this created environments with slightly more moisture in each environment. Therefore the potato flakes increased their moisture as the moisture in the environment increased to create equilibrium between the food and the environment. To measure desorption of the environment, we know that the potato flakes are in equilibrium with the environment, so we can assume whatever moisture the potato gained is the amount of moisture the environment lost. 2) In Figure 2, where the line intersects the x- axis is where the % moisture change is zero. At this point, the water activity is equal to about 86%. Considering this is relatively high and above 60%, it s unstable against microbial growth. Between 20-30% is stable and this is well above 30% moisture. 3) The dog food is a desorption isotherm because the dog food initially had 21.1% moisture, and as it was placed in the different environments, it lost moisture to the environment to create a moisture equilibrium. As the water activity of the salts in each environment decreased, the dog food lost moisture to the environment to equal out the equilibrium. In each environment, the dog food decreased in moisture except in the environment containing KNO3. This is because KNO3 had the highest water activity, so it 7

8 had a greater affinity to the water, so it didn t need moisture from the dog food to create equilibrium. This is why the moisture of the dog food didn t decrease for this one environment, but did so for the other four environments. 8