I don't know if you noticed it. Water is actually an important ingredient in food. It plays a key role in the production and preservation of food. In this regard, Douglas Adams made the above precise summary in his "Guide to the Galaxy Tour." Indeed, as far as the modern food industry is concerned, what everyone mainly does on weekdays is actually dealing with water.
Everyone must understand the first important aspect. Since water itself is relatively inexpensive relative to other raw materials, any business wants to make the product as watery as possible when conditions permit—this is how much it costs to sell the cheapest raw materials at the price of a normal product. Buying and selling! Mixing water with milk and filling it with pork are typical cases.
This is why the countries of the world have set quality standards for various products. Take milk as an example. Roughly speaking, there are about 30 grams of protein, 40 grams of fat, 50 grams of lactose, and about 10 grams of minerals in one litre of milk. In addition, the remaining 900 grams of water are all water. The Chinese National Standard specifies that the non-fat milk solids in pasteurized milk should not be less than 8.1%, in order to ensure that the proportion of water cannot be too large. If this is not the case, all milk companies will certainly want to add more water today and add a little water tomorrow. It will not be long before everyone buys milk that is almost like tap water.
As for the second important aspect, it may be a little more complicated to understand. We must note that moisture plays many important roles in food at the same time. On the one hand, changes in the water in the food can directly affect the texture and taste of the food. The next day's Baguette is so much more delicious than just baked. In the final analysis, the distribution of water in it changes. On the one hand, the growth and reproduction of microorganisms are inseparable from water. The presence of water can also lead to or participate in chemical reactions that degrade foods.
Water is the source of life. This life refers not only to humans but also to all kinds of microorganisms that cause food spoilage. If food has too much water content, it will be prone to spoilage. The effect of water on food preservation has long been known. Since ancient times, people will store the newly harvested foods in the sun and store them. At first, everyone just used the solar energy without spending money to spread the grain in the market, and it is not necessary to study how much the grain is dried. After entering the modern industrial society, the situation is not the same. The food manufacturer of “Kanmen†will calculate a bill: Too tan, on the one hand to pay more, and on the other hand, to sell less water. Therefore, they called up a group of scientists and wanted to understand how much water contained in the food was more conducive to storage.
However, after studying the relationship between water content in food and the growth and reproduction of microorganisms, people were surprised to find that there was no correlation between the two. Some foods have a high moisture content, but they are not prone to bacteria. Some foods have a low moisture content, but they are still prone to spoilage. It was not until 1953 that Australian William James Scott discovered that it was not the simple food moisture content but the water activity that was associated with the growth of microorganisms.
What is water activity? If we pour water in a glass and then turn it over, under normal circumstances we will find that the fresh water has fallen to the ground. However, if we pour a large amount of sponge into the cup and then pour it in the same amount of water, the cup may still crack after it is inverted, but there will always be a part of the water that is absorbed by the sponge and it will not come out. We can say that the spilled water is free water; and the part of the water that stays in the sponge is bound by the sponge. Water activity is a similar concept that points out how much free water in food can be used biologically.
If we only measure the moisture content of a certain food, we only need to put it in the oven for a few hours, and then compare the weight before and after drying to know the water content. How to measure the water activity?
At a certain temperature, putting a pot of water in a confined space, the water molecules will continuously enter the air from the water and will continue to return to the water from the air. As long as the time is sufficient, there will be a dynamic balance between the two, and the evaporation and condensation of water vapor will coincide. At this time, the partial pressure of this water vapor in the air is the saturated vapor pressure of water at this temperature. At the same temperature, we put a certain kind of food in a small confined space where water vapor will also reach such a dynamic equilibrium, so as to obtain another partial pressure of water vapor. Dividing this partial pressure of water vapor by the saturated vapor pressure of water gives the water activity of this food at this temperature.
The water activity of pure water is 1, and the more the water activity in food is smaller than 1, the more difficult it is to use the water inside.
A high water content does not mean that there is more water available. At room temperature, a soft cheese usually has a water content of about 50%, and a cup of saturated salt water has a moisture content of about 74%. The former has a water activity of 0.95 and the latter has a water activity of only 0.75.
Later, people slowly discovered that not only the growth and reproduction of microorganisms is related to water activity, but also many other factors that can cause food deterioration are also closely related to water activity. Such as enzymatic reactions, hydrolysis, browning, fat oxidation and so on. The reaction rate of these factors is largely reduced with the decrease of water activity. Only the oxidation of fat first decreases with the decrease of water activity, but when the water activity is small to some extent, it rises again. With this information in mind, “tricks†merchants can artificially control the water activity of foods to a reasonable extent, thus finding the best balance between “sell water as a product†and extend the shelf life of food. .
Most fresh foods have a water activity of close to 1 and are therefore particularly susceptible to spoilage. Although water activity has not been discovered for a long time, people have already begun to intentionally or unintentionally use the characteristics of water activity to preserve foods based on accumulated experience.
Dry dehydration is a method that effectively reduces water activity. By removing the moisture in the food, both the food shelf life and the transportation cost can be extended. After all, although the water is cheap, it is very heavy. Instead of transporting large amounts of water in long-distance food, it is better to dehydrate first, and then add water to reduce it. Like milk powder, dried fungus and so on.
Salt or sugar is another way to reduce water activity. The previously mentioned saturated salt water has a water activity of only 0.75. Under this water activity, most microorganisms are difficult to grow and reproduce. The pickles, cured meats, and candied fruits we eat all actually use this principle.
In addition, freezing is actually a common method of reducing water activity. After water becomes ice, it is not only that microorganisms cannot grow and multiply, but lacks moisture as a medium, and even many chemical reactions are difficult to continue. This is also the reason why frozen foods can be stored for a long time.
Of course, water activity is not a panacea. Nowadays, many complex foods pursue different textures, both inside and outside, and the water is not in equilibrium. Just like the outer surface of the fried chicken is very dry, but the inside is fresh and juicy. How to measure the water in this kind of food? Activity itself is a big problem because the measurement process needs to reach the entire environment's moisture balance. There are also many foods that cannot be used to make more work on water activity in order to ensure the inherent taste. For such foods, it is necessary to rely on heat sterilization, suitable packaging materials, and appropriate food additives to extend the shelf life.
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