Significance of water activity monitoring in food quality and safety control
Author: Pei 'an Company
Bread is the staff of life. Food safety is a major event related to the national economy and people's livelihood, and has become an important area of great concern to government departments, scientific and technological circles and consumers. Food-borne diseases are on the rise in the world, and vicious food pollution incidents occur one after another, and trade disputes caused by food safety are constantly occurring around the world. These problems are important factors that affect the economic development, international trade and national reputation of all countries (China is no exception). Since the reform and opening-up, while basically solving food safety, especially as a new member of WTO, China's trade with other countries in the world will increase day by day. Food safety has become a key factor affecting the competitiveness of agriculture and food industry in China.
Globally, food-borne diseases caused by microorganisms are still the number one food safety problem. According to the statistics of the World Health Organization, 70% of the hundreds of millions of food-borne patients in the world every year are caused by eating food and drinking water contaminated by microorganisms. 1At the end of 999, the most serious food poisoning incident occurred in the United States because of eating food containing Listeria. According to the data of the US Centers for Disease Control, in Michigan, 14 people died from eating "hot dogs" and cooked meat contaminated by the bacteria, while in other 22 states, 97 people fell ill and 6 women miscarried. From the end of 2000 to the beginning of 2006, there were 5438+0 Listeria contamination incidents in France. Six people were victims of Listeria monocytogenes because they ate meat sauce and pig tongue processed and produced by French companies. 2002123 October, 165438+ Italy's "Pollio" cheese company recalled 6,600 cases of whey cheese and distributed them to the United States 18 states. In addition to the "O 65,438+057" E.coli pollution incident, there was also a Staphylococcus aureus pollution incident in blood-stained milk in Japan. On April 65, 2003, on April 38+08, a collective food poisoning incident occurred in the first primary school of Fruit Lake in Wuhan City, Hubei Province, China. Nearly 100 students in three classes of grade six in this school developed poisoning symptoms after eating dried tofu uniformly distributed by the school after school. The cause of the poisoning incident has been found out, which is a collective bacterial food poisoning caused by eating dried tofu with a serious amount of microorganisms exceeding the standard. Tested by the Health Supervision Bureau of Hubei Provincial Health Department, the total number of bacteria in dried bean curd cooked by Ace exceeded the standard 19 times.
In China, by the end of the second quarter of 2004, the Ministry of Health had received reports of 205 major food poisoning incidents, with 6,329 people poisoned and 156 people dead. From 2000 to 2002, the Institute of Nutrition and Food Safety of China Center for Disease Control and Prevention investigated raw meat, cooked meat, dairy products and aquatic products, and continuously monitored the pollution quality of pathogenic bacteria in vegetables. The results showed that microbial food poisoning still ranked first, accounting for 39.63%, chemical poisoning accounting for 38.56%, and unknown animal and plant food poisoning accounting for about 10%. Table 1 shows the food poisoning situation in China from 1990 to 1999.
Table 1 Food Poisoning in China1990-1999 [1]
The cause of disease
Number of poisoning cases
Composition (%)
Poisoning number
Microbial food poisoning
4 175
40.04
160599
Chemical food poisoning
2563
24.58
47033
Poisonous animal and plant poisoning
1753
16.8 1
2 1 124
other
696
6.67
1975 1
The reason is unknown
1 143
10.96
27244
The management mode of food safety emphasizes the whole process management from farmland to table, that is, the principle of prevention first, to reduce the food-borne harm caused by microorganisms. In the process of food processing, storage and sales, food raw materials are infected by microorganisms in the external environment, coupled with microbial pollution caused by incomplete sterilization and improper storage and transportation methods, which leads to food spoilage. The main reason that threatens consumers' health. Only by effectively controlling the potential microbial pollution in all aspects of food production can the food industry produce foods that reassure consumers. Controlling water activity is a key factor to prevent the growth of harmful microorganisms. In the United States, federal regulation 2 1 has clearly stipulated that water activity is an important index to test food safety. At the same time, the Good Manufacturing Practice (GMP) stipulated by the US Food and Drug Administration (FDA) clearly regards water activity as an important indicator to reflect food safety. In the HACCP monitoring system, it is clearly stipulated that "the growth of microbial pathogens can be controlled by limiting water activity." The United States stipulates that the water activity of stored food should not exceed 0.85, and Japan stipulates that the water activity of stored food should not exceed 0.90.
So what is the water activity of food? What is the significance of monitoring water activity to ensure food quality and safety?
As a thermodynamic concept, water activity is an energy state that describes water in food, and it has a strong correlation with Gibbs free energy of food system. It shows how closely water in food combines with other substances. Although both water content and water activity are used to describe the state of water, water activity is the most relevant factor to food quality and safety.
Strictly speaking, the ratio of fugacity of water to fugacity of pure water in food is called water activity (Aw).
Fugacity of water in food
F0-fugacity of pure water
The trend of water escaping can usually be approximately expressed by the vapor pressure of water. At low pressure or room temperature, the difference between f/f0 and P/P0 is very small (0.9 is the time for growth and reproduction; Followed by yeast, which needs Aw & gt0.87, followed by mold, which starts to reproduce when Aw is 0.8. In addition, microorganisms belonging to different species have different requirements for Aw.
Secondly, Aw value also affects the metabolic activity of microorganisms. Reducing Aw value can reduce the growth rate of microorganisms, and then the spoilage rate of food, the production of microbial toxins and the metabolic activity of microorganisms will also be reduced. It is worth noting that the water activity required to stop different metabolic processes is different. For example, the Aw value required for bacteria to form spores is higher than its growth value. Toxin production is the most relevant microbial metabolic activity to human health. When Aw is controlled in a certain range, it can effectively inhibit the production of some toxin-producing strains (such as the reproduction of Staphylococcus aureus and the formation of enterotoxin). Mold pollutes food very seriously. It is generally believed that the water activity required for the growth of toxin-producing molds is lower than that required for the formation of toxins. In addition, due to the production of metabolic water, the growing mold can increase the Aw value of the growing environment. Therefore, toxins are likely to exist in foods containing poisonous bacteria or molds. It can be seen that monitoring the water activity of food has important practical significance.
Thirdly, Aw value also affects the heat resistance of microorganisms. Heating is a common and effective method to inhibit or kill microorganisms in food. Different microorganisms and their spores have different heat tolerance. Among the factors that determine the heat resistance of bacteria, the physical properties, chemical composition and Aw value of hot solvents are very important. Generally speaking, the heat resistance of bacterial spores increases with the decrease of Aw value, and is the strongest in the range of Aw 0.2 ~ 0.4. Sometimes, in high concentration solution.
Therefore, the effect of thermal sterilization can be preliminarily judged by detecting the water activity of pre-sterilized food raw materials.
Fourthly, Aw value has obvious influence on the viability of microorganisms. Microorganisms that cannot grow will gradually die. Therefore, if the Aw value of food is lower than the minimum value of microbial growth, the number of microorganisms will gradually decrease. The study on the relationship between the survival of food toxic microorganisms such as Salmonella and Staphylococcus aureus and Aw proves that the number of bacterial spores will decrease in foods with low Aw value. Such food may even become sterile during storage. The survival of parasites in food is also affected by low Aw value, and these parasites can be killed during freezing or drying. When studying the survival of Trichinella spiralis in meat during drying, it was observed that these parasites would be inactivated when the Aw value in fermented sausages decreased to a certain value. To sum up, we can draw a conclusion that by choosing appropriate conditions (aw value, pH value, humidity, preservatives, etc. )
From the above discussion, it can be seen that water activity has a significant impact on microorganisms, and water activity is an important index in food quality control. Monitoring water activity in time in food field can effectively evaluate the safety and stability of food. Generally, foods with Aw value between 1.0-0.9 belong to high humidity foods, and AW 0.9-0.6 belongs to medium humidity foods. Aw0.6-0.0 belongs to low humidity food. High humidity food spoils because of bacteria, while moderate humidity food spoils mainly because of mold and yeast. In low-humidity foods, microorganisms generally do not grow, but the quality of low-humidity foods also depends on Aw. In addition, water activity can be used to predict the microbial safety and quality stability of foods with high and moderate humidity [9- 12].
Table 2 Water Activity and Microbial Growth in Food
Aw range
Microorganisms that cannot grow under the lower limit of Aw
food
1.00~0.95
Pseudomonas, Escherichia, Proteus, Shigella, Klebsiella, Bacillus, Wechsler and some yeasts.
Perishable fresh food, fruits, vegetables, meat, canned fish and dairy products, cooked sausages and bread. Foods containing about 40 %( W/W) sucrose or 7% sodium chloride.
0.95~0.9 1
Salmonella, Vibrio parahaemolyticus, Serratia, Lactobacillus, cocci, Rhodotorula, Rhodotorula and some molds.
Cheese, bacon and ham, some concentrated juice, foods containing 55 %( W/W) sucrose or 12% NaCl.
0.9 1~0.87
Multi-yeast, Micrococcus
Fermented sausages, cakes, dried cheese, margarine and foods containing 65% sucrose (W/W) or 15% sodium chloride.
0.87~0.80
Most molds, staphylococcus aureus, Bayer yeast, Dbaly yeast.
Most concentrated fruit juice, sweet frozen milk, chocolate candy, maple syrup, fruit juice candy
Pulp, flour, rice, beans with water content of 15~ 17%, fruit cake, ham and soft candy.
0.80~0.75
Most halophilic bacteria
Jam, Malailan, orange jam, marzipan, marshmallow.
0.75~0.65
Dry mold
Oatmeal with 10% moisture, candy bars, burdock (a soft milk candy), jelly, cotton candy, molasses, some dried fruits, nuts and sucrose.
0.65~0.60
Hypertonic yeast, some molds
15~20% dried fruit, some toffee and caramel, honey.
& lt0.5
No microorganisms can grow.
Water activity plays an extremely important role in predicting food safety, microbial growth and biochemical reaction rate. By measuring and controlling the water activity of food, we can achieve the following points: (1) predict which microorganisms are the potential sources of corruption and pollution; (2) Ensure the physical and chemical stability of food; (3) Minimizing non-enzymatic oxidation reaction and non-enzymatic oxidation of fat; (4) prolonging the activity of the enzyme; (5) Optimize the physical properties of food, such as texture and shelf life [13].
The role of water activity detection in meat quality control.
Water activity is an important barrier factor affecting meat preservation. As we all know, microorganisms can grow and reproduce on all kinds of meat and meat products. The pollution and reproduction of microorganisms will directly lead to the deterioration of meat, thus affecting the hygienic quality of meat and even causing food poisoning in serious cases. The relationship between microorganisms and meat preservation is self-evident. The normal growth and reproduction of microorganisms must meet three main conditions: (1) nutritional conditions, and meat and meat products are one of the best media for microbial growth and reproduction; (2) Temperature, generally speaking, high temperature leads to rapid growth and reproduction, and vice versa; (3) Appropriate amount of water (with a certain water activity). With the three, microorganisms can grow and reproduce well, otherwise the growth and reproduction of microorganisms will be affected. Among these three main conditions, water activity is closely related to microorganisms, because any microorganism in food (including meat and meat products) needs a minimum water activity value, below which microorganisms cannot grow and reproduce normally. In other words, the Aw value of meat products directly affects the species and quantity of microorganisms that may be contaminated by meat products, and then affects the anti-corrosion and fresh-keeping measures taken for meat products. Therefore, water activity has always been considered as an important barrier factor for the preservation of meat products. Under the same conditions, the Aw value is low and the shelf life of meat is long. The relationship between Aw value and meat shelf life is shown in the following figure:
Since the concept of water activity was introduced into the research field of food science, the theory of water activity has been widely used to guide production. At present, many measures in production practice are to inhibit the normal growth and reproduction of microorganisms by reducing the Aw value of meat products, such as drying method, freezing method and pickling method. [14]. Among meat products, the Aw of dried meat products, such as dried meat, preserved meat and floss meat, is mostly 0.60 ~ 0.60. Therefore, it is considered as a safe food with low water activity. Except for these dried meat products, the water activity of other meat products is usually higher than 0.75. These meat products occupy a large market share. Therefore, it is of great significance to detect water activity in time to control the quality change of meat products during storage. Xia Dayong et al. collected a bag of dried meat floss that was over the shelf life 10 month, and the detected water activity value was 0.45. Sensory examination: color. It is darker than normal yellow, the meat is not so fragrant and there is no corruption. It is not difficult to see that Aw value can better reflect the trend of internal quality change of dried products [15].
Function of water activity detection in quality control of aquatic products.
According to literature, the Aw of fresh aquatic products is generally 0.98-0.99, that of pickled products is 0.80-0.95, and that of dried products is 0.60-0.75. AW < 0.9, bacteria can't grow; Aw < 0.8, most molds can't grow; When Aw< was 0.75, the growth of most halophilic bacteria was inhibited; Aw < 0.6, the growth of mold was completely inhibited. Generally, the water activity of convenience foods such as grilled fish fillets and surimi products is required to be in the range of 0.70-0.75. In this water activity, it is difficult for bacteria to survive, and some drought-tolerant molds can grow. Long-term storage can be achieved as long as mildewproof measures are taken during processing, packaging and transportation. Because of the low water activity of microorganisms in food, food science and technology circles are exploring the possibility of controlling the Aw value of some foods according to predetermined requirements, so as to achieve preservation without sterilization. The drying of shrimp products is to reduce the water content and water activity (Aw) of shrimp meat to inhibit the proliferation of microorganisms, so as to achieve the purpose of long-term preservation. 0.69, the storage is safer, but the dried shrimp meat reaches aw.
Function of water activity detection in quality control of grain and oil products.
Pastry and other grain and oil products can't be eaten during storage because of the growth of microorganisms, which includes two aspects, namely mildew and corruption. The mildew of cake mainly refers to the proliferation of mold on the cake, and fluffy spots of various colors can be observed from the appearance, and some molds will produce toxins harmful to human body. There are many kinds of molds that pollute cakes, including Penicillium, Penicillium, Rhizopus, Aspergillus, White mold and so on. The cake is rotten. Mainly refers to the cake spoilage caused by the invasion and reproduction of bacteria, such as potato bacilli. The role of mold plays a major role in the deterioration of grain and oil products. There are many methods to control microorganisms, such as controlling raw material composition, active packaging materials, inflatable packaging, adding deoxidizer, selecting preservation environment, etc. However, the most effective method is to make the cake into a system that is not suitable for microbial growth. That is to say, adjust the moisture activity of the cake before adding antibacterial agent [16]. Hu et al. of China Agricultural University conducted experiments on the effects of pH, antimicrobial concentration and water activity on microbial growth in cream cake (pound cake) simulated culture medium. The results showed that the growth rate of microorganisms increased with the increase of water activity, and the growth of microorganisms was obviously inhibited by reducing the water activity of cakes (about 0.88).
The role of water activity monitoring in ice cream quality control
In ice cream paste, the water content is 60% ~ 70%, but the water activity is low. The more the total solid content of ice cream paste, the lower the water activity. Water activity affects the melting resistance, deformation resistance, softness or firmness of ice cream texture, and affects the number, particle size, structure, distribution position and orientation of ice crystals. To control the quality of ice cream, we must first control the water activity. Yang Xiang, Department of Food, Tianjin University of Commerce
In a word, monitoring water activity is of great significance to ensure food quality and safety.
After China's accession to the World Trade Organization, food import and export trade will be an important economic activity in China. However, it also poses a new challenge to food safety in China. Even the food produced and consumed in China is facing new challenges. With the acceleration of urbanization, people's demand for food transportation and processing has become greater, and agricultural production and food industry have been integrated. The way of food production and circulation has changed, the distance of food circulation is farther than before, and the transportation time is relatively long. Food from production to preservation, and then from circulation to consumption. This series of processes need effective and timely quality control. Effective preventive measures are the key to ensure food safety, whether from the perspective of producers or the government. Therefore, the food industry should attach great importance to food water activity while implementing HACCP system, because real-time detection of water activity is an effective process control means to ensure food quality and safety.
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Oligomycin sensitive comparison factor
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