Water activity control in food and its effect on microorganisms

, the moisture activity of common foods 1, the moisture activity of microbial growtheach microorganism has its lowest, best, highest moisture activity (see Table 1, Table 2)Yeast and mold can grow at low water levels, but 0.85 is the safe limit for pathogen growth0.85 is based on the minimum moisture activity of Staphylococcus aureus to produce toxins0.85 or more of moisture activity foods require refrigeration or other measures to control the growth of pathogens (see Table 3)Foods with a moisture activity of 0.60 to 0.85 are medium-moisture foods (see Table 4), which do not require refrigeration to control pathogens, but have a limited shelf life due to corruption caused by major yeasts and moldsFor most foods with a moisture activity of 0.6 or less, there is a longer shelf life and no need to be refrigerated, these foods are called low-moisture foodsmost raw meats, fruits and vegetables are highly hydrated foods (see Table 3, water activity is higher than 0.85)It's worth noting that bread, which most people think is a dry, shelf-stable productIn fact, it has a fairly high moisture activity, it is only due to the pH, water activity of the multiple barriers, and make it safe, and mold is easier to grow than pathogens, in other words, it becomes dangerous before the mold turns greenproducts with some unique flavors, such as soy sauce, look like high moisture products, but because of salt, sugar or other ingredients combined with moisture, their moisture activity is very low, their moisture activity is around 0.80 (see table 4)food can be divided into three categories according to its moisture activity (see Table 3, Table 4, Table 5)Some medium and low moisture activity foods are naturally low moisture activity, such as molasses and flour, and do not have to control moisture activity when processedOther medium and low moisture live foods, if the pickle, pickled fish, strawberry jam, biscuits, soy sauce and noodles, start ingested high moisture activity food, after processing, water activity decreased2, the minimum moisture activity necessary for microbial growth3, water activity and microbial growth in foodtwo, reduce moisture activity methodcontrol water activity in two steps the first step, scientifically set a dry, salinned or processed formula that guarantees water activity of 0.85 or less, and then strictly execute it the second step, it is advisable to test the moisture activity of the finished product sample There are two traditional ways to reduce moisture in food , namely drying, salting or sugar-combining water molecules 1, dry dry is one of the oldest methods of food anti-corrosion In addition to anti-corrosion, drying produces the food's own characteristics, as is fermentation Many parts of the world are still drying with open air, and there are generally four basic drying methods (1) Hot air drying: for solid foods such as vegetables, fruits and fish (2) Spray Drying: for fluids and semi-fluids such as milk and bone broth (3) vacuum drying: used for fluidified juice (4) freeze-drying: for a wide range of products 2, add salt or sugar another way to reduce moisture activity in food is to add salt or sugar examples of this type of food are soy sauce, jam and pickled fish, which do not require very special equipment For fluid or semi-fluid products, such as soy sauce or jam, processing control with formula For solid foods such as fish or smoked ham, dry with salt, i.e in a salt solution or immersed in salt water (1) Salt storage food by salt collection can not only inhibit the growth and reproduction of microorganisms, but also give them a new flavor, so it has the effect of processing The anti-corrosion effect of salt mainly lies in increasing the osmotic pressure, so that the cell protoplasm concentration occurs mass wall separation, reduces the moisture activity, is not conducive to the growth of microorganisms, reduces dissolved oxygen in water, and inhibits the growth of good-gas microorganisms the adaptability of various microorganisms to salt concentration is greatly different Salt-loving microorganisms such as red bacteria, joint yeast genus and Gram-positive cocci can still grow in solutions with higher concentrations of salt (more than 15%) The general corrupt microorganisms such as colorless bacteria are about 5% salt concentration, and the growth of pathogens such as Clostridium difficile spoofe is inhibited when the salt concentration is 7% to 10% General mold has a strong tolerance to salt, such as some penicillin strains in 25% of the salt concentration can still grow because of the different adaptability of various microorganisms to salt concentration, so the salt concentration determines the microbial microbiome that can grow For example, when the salt concentration in meat is less than 5%, it is mainly bacterial reproduction, the salt concentration is more than 5%, there is more mold, the salt concentration is more than 20%, the main growth of microorganisms is yeast (2) sugar storage is also a storage method to inhibit microbial growth by increasing the permeable pressure of food and reducing moisture activity general microorganisms inhibit their growth when the sugar concentration exceeds 50% However, some highly permeable yeast and mold, in sugar concentration swells up to more than 70% can still grow Therefore, there are limitations only to increase the sugar concentration, but if a small amount of acid (such as vinegar) is added, the resistance of microorganisms will be significantly reduced for example, jam products, because its raw fruit contains organic acids, in the processing of the addition of sucrose, and heated, in the osmotic pressure, acid and heating and other three factors under the joint action, it is easy to kill yeast and mold, can get very good preservation According to the fence factor theory, moisture activity (Aw) is an important fence factor that can control food safety by reducing moisture activity Of course, we can also determine the moisture activity of the product, to determine the variety of microorganisms in the product needto be controlled, in the process development of targeted control programs, but also can easily estimate the shelf life of the product