The importance of water activity in food

Pei'an translation

Water activity is an important parameter for food stability and safety prediction in terms of microbial growth and food deterioration. For centuries, people have been controlling the water in food by drying, freezing, adding sugar or salt, using this method to preserve food or control food safety.

Water activity is a measure of the energy state of water in a system (or a measure of how water is "bound"), so it can be a solvent and added to chemical reactions, biochemical reactions, and microbial growth.

To better understand this concept, let's assume that there are two tanks of water, one box of 10,000 gallons, and the other box of one gallon. How do the two tanks move? The volume of water does not have any effect. Stress is the only influencing factor. Lifting a tank containing 1 gallon of water to the top of the mountain, regardless of volume, a gallon of water will flow to the low pressure water below the mountain. Similarly, water content cannot predict the direction of water migration, but water activity can tell you the answer.

One of the goals of food safety is to prevent the growth of harmful microorganisms and produce toxins. The growth of these microorganisms has a limit on the water activity below which the harmful microorganisms will not grow. Water activity, not water content, determines the minimum level of microbial growth. Most food-degrading bacteria grow when the water activity is above 0.9.

In addition to the relationship between microbial and water activity, water activity also affects other aspects of food microbes such as sporulation, germination and mycotoxin growth.

Water activity not only affects the deterioration of microorganisms, but also has a certain relationship between chemical reaction and enzymatic reaction and water activity. Water can act as a solvent or reactant or alter the change in reactants by affecting the viscosity of the food system. Water activity affects the rate and extent of non-brown enzymatic reactions, lipid oxidation, vitamin degradation, enzymatic hydrolysis, protein denaturation, starch denaturation, and flour settling.

As the water activity increases, the probability of non-enzymatic browning reaction will also increase, and the water activity will reach a maximum when it is between 0.6 and 0.7. Although affected by different mechanisms, lipid oxidation rate can be minimized when there is an intermediate range of water activity and varies between the highest and lowest. These reactions can lead to variations in taste and odor. The degradation of water-soluble vitamins in food systems increases with increasing water activity. The stability of enzymes and proteins is significantly affected by water activity due to their relative friability. Water activity also affects starch gelatinization temperature and retrogradation process.

In addition to predicting the probability of various chemical reactions and enzymatic reactions, water activity also affects the structural structure of the food. Foods with high water activity values ​​are described as large, juicy, soft foods. When the water activity of these foods is reduced, unexpected changes in the tissue can occur, such as drying out, hardening, and staleness. Foods with low water activity values ​​should often be of this type: crunchy, brittle, and high water activity foods that make them structurally absorbent. Dry, cereals and flours such as biscuits, cookies, potato chips, and popcorn lose their crunchy taste when water activity increases. Excessive or rapid drying or repeated absorption of moisture by using GLASSY MATERIALS can lead to effects that you do not want: for example, foods lose their fragility and are prone to breakage.

Water activity is an important parameter in controlling moisture migration in foods with multiple ingredients. Some foods contain some substances with different water activities, such as oats with candied fruit. Unless the water activity is controlled, the water will migrate from the fruit juice with higher water activity to the oatmeal with low water activity, causing the fruit to become dry and hard, while the oats become wet.

Water activity is also an important factor affecting the stability of powders and dehydrated products during storage. Controlling water activity is a property of powder products that maintains the structure, texture, stability, density, and water holding properties of the product, which are important attributes in processing, handling, packaging, and storage.

Water activity also determines the shelf life of the product. The key value of water activity can be determined based on factors such as the microorganisms in the food and the texture, taste, appearance, aroma, nutrition and cooking quality of the food. The moisture exchange law of the food through the package and the rate of change of aw towards a critical value of the aw limit determine the shelf life of the product.

Water activity guidelines are introduced in the US FDA's GMP regulations to define food safety rules. The purpose of the GMP Code is to elaborate on industry specific requirements and practices that should be followed to ensure that food is produced in a hygienic environment, ensuring food purity, hygiene and food safety.

Measuring water activity in food in the past is a time consuming and difficult process. But new instrumentation technology dramatically increases the speed, accuracy and reliability of measurements.

The dew point method is a basic vapor pressure measurement method that has been used for decades. Dew point instruments use mirror condensation technology to be precise, fast and easy to use. A _w measurement range commercial hygrometer between 0.030 to 1.000, accuracy ± 0.001, 0.003 ± error, measurement time is generally 5 minutes.

Using the Aqualab water activity meter, the sample is equilibrated in a sealed chamber containing a mirror, optical sensor, and infrared temperature sensor. When equilibrium is reached, the relative humidity of the air in the chamber is the same as the water activity of the sample. A thermoelectric cooler can precisely control the temperature of the mirror surface. The optical reflectivity sensor accurately detects at which point the condensation first appears, and the thermocouple on the mirror accurately detects the temperature of the dew point. An infrared thermometer measures the temperature of the sample. The dew point and sample temperature are used to determine water activity.

Calibration and routine maintenance of Aqualab instruments can be accomplished using a range of pre-mixed standard salt solutions supplied with the instrument.

Every food producer needs to know what happens when their products are on the shelf. Resistant storage means no mold, but it also contains many other aspects of food quality. As an example of a well-known manufacturer of wheat bran cereals in the United States, producers and consumers of wheat bran cereals hope that the cereals will be crisp and chewy, when consumers have a hard-core raisin It was also extremely surprising when a tooth broke down, and the producer was soon surprised by the lawsuit. Fortunately not all moisture migration problems are due in court, but if you produce and sell discrete materials, you need to know where the moisture migrates when the product is on the shelf.

A fruitcake maker wants to predict the state of her fruitcake over time. She didn't want the cake to get wet when the fruit pieces dried and hardened, so she measured the moisture content. The cake contains 30% water and the fruit moisture content is 50%. She knew that the water would reach equilibrium, so she thought that the water would migrate from the high-humidity component (fruit) to the low-humidity component (cake).

Unfortunately, she was wrong. When she is concerned about the moisture content and is convinced of it, a completely different game that follows the water activity is going on. If she still doesn't know the rules of the game, she will finally be surprised by the end of the example – the cake is drier and the fruit is damp, because although the moisture content is so, the cake has a higher water activity than the fruit.

In this case, the moisture content is only a distracting item, and attention to this numerical ending is nothing more than teasing. Indeed, unless other forces are applied, the water will reach equilibrium. But equilibrium is only achieved when the Gibbs free energy in each part of the system is consistent. Water activity is a measure of Gibbs' free energy, while moisture content is independent of water energy. To better understand the concept, imagine the two boxes of water that were mentioned before, one box containing 10,000 gallons is almost full, and the other box is only one gallon almost empty. How will the water move? Knowing the moisture content of the two tanks of water is completely misleading, and the volume of water is not important. Water flows from a place with high pressure to a place with low pressure, and it does not flow from a place to a small place. If we lift this nearly empty tank above the full tank to increase its pressure, the last gallon of water will also run from the empty tank.

Similarly, water activity, not moisture capacity, predicts how moisture will migrate in the product. Fruit cake makers can develop recipes that have the same water activity in cakes and fruit slices. Then, when the cake is stored and sold, it will not cause any surprises due to moisture. It is a safe, delicious and storage-resistant product.

One solution to the problem of moisture migration is to increase or decrease the water activity in the dispersed components until they contain the same water activity value. You can also prevent the diffusion process that occurs between components by increasing the viscosity of the components. An edible barrier, like a conical ice cream with a layer of chocolate on the outside to prevent moisture migration. Sometimes water activities are different and cannot be balanced and require independent packaging.

Based on the catalogue business, Hotel Chocolat, a chocolate chain that has since grown into online retail and commercial street sales, is a well-known chocolate producer in the UK. The company has always attached great importance to high quality chocolate and is now developing a range of filled chocolates. However, the shelf life of the filler is much shorter, unlike chocolate, which usually has a shelf life of 12 months or longer. To ensure the safety of this new type of filled chocolate, Hotel Chocola's development lab purchased a Decagon's Aqualab Series 4TE water activity analyzer.

The Aqualab Series 4TE used by Hotel Chocolat is a laboratory-grade instrument that gives accurate water activity experiments with extremely fast results. It has a resolution of ±0.001aw and an accuracy of ±0.003aw with a built-in temperature balance system.

Adam Geileskey, Technical Manager of Hotel Chocolat, explains why they chose Aqualab Series 4TE. “It is widely considered to be a very cost-effective, very good product. Other chocolate manufacturers are already using this product. Without Aqualab we Can't start our new product development project to create this series of filled chocolates."

The Aqualab Series 4TE not only provides high resolution and high accuracy, but its use is also very simple, Mr. Geileskey said, “For the first time, its operation, cleaning and calibration are very intuitive, and it It requires minimal maintenance and is completely trustworthy."

Hotel Chocolat now has a line of filled chocolates for sale, and many more are under development. Mr. Geileskey added, “Once production is keeping up with development, we will also use Aqualab to control product quality. It is easy to operate, testing is very fast, and can immediately play the role of product development and quality control.”