Problem Area Handling and Storing Plant Products
Lesson Controlling Molds with Food Preservatives
Student Learning Objectives 1. Describe how different microbes cause food spoilage. 2. Describe methods used in controlling food contamination. 3. Explain the methods used in preventing the growth of microbes in food. 4. Explain methods for decreasing food contamination.
How do microbes lead to the spoilage of food? Food microbiology is the study of certain microbes and how they affect food. Microorganisms may synthesize compounds that alter the flavor, color, or odor of the food product. Fruits and vegetables, when they are harvested, are still living entities. They are capable of respiration and contain substances acted upon by enzymes. This enzymatic activity continues to occur even after harvest. Ripening and senescence (natural aging of plant products through biological processes) may occur after harvest, as well as deteriorative processes such as browning, softening, and oxidation of lipid material.
The natural fate of plant material after it dies is degradation by microbial action. Microbes are tiny organisms that can only be seen through a microscope. They depend on their environment to provide food. If the environment is favorable, the microbes will thrive and multiply. There are four important microbes in food science; some produce favorable results while others can have a very negative impact on foods.
A. Molds are a form of fungus which is a plant. Fungi are organisms that receive their nutrients from decaying organisms and can live almost anywhere where air and moisture are present. 1. Those commonly found on foods reproduce asexually. Asexual reproduction means that the offspring are formed by simple cell division or mitosis rather than sexual reproduction. The offspring are genetically identical to the parents. Molds produce spores which can be carried through the air. As the spores are deposited on substances with favorable characteristics for temperature, moisture, and nutrients a mold colony can develop. 2. Molds can be beneficial to food production; they are used to add flavor and color to cheese and in making soy sauces, citric acid, lactic acid, and many enzymes. Penicillin and other antibiotics are produced with the help of molds. Molds can also produce mycotoxins which are poisons and are a by-product of mold growth. They can be lethal. One example is aflatoxin, which can contaminate corn.
3. Molds can grow at extremely low or high temperatures and reproduce in acidic or basic solutions. a. Mesophiles are those molds that grow at room temperature (50° F–110° F). b. Psychrophiles are molds that grow at refrigerated temperatures (less than 50° F). c. Thermophiles are molds that grow at high temperatures (110° F–140° F). 4. Molds require oxygen in order to survive; they must live in an aerobic environment or one in which oxygen is present. Thus, one of the most effective methods of preventing mold growth is to store the food product under anaerobic conditions or in the absence of oxygen.
B. Yeasts are unicellular plants. This means they are single-celled. Yeasts are also members of the fungi family. They are a unique plant because they do not contain chlorophyll. Yeasts live mainly on sugars and starches or in high salt or sugar environments. Yeasts can be used to aid the fermentation process. In the case of making bread, beer, and yogurt, this is good. In the case of hot dog making, the presence of yeast leads to spoilage. Yeasts reproduce by budding. A bud or small knob forms on the parent cell. The bud grows and finally separates or buds from the parent producing an identical offspring.
C. Bacteria are the most adaptable of all microbes and they also are unicellular. The bacteria associated with food spoilage are heterotrophic. Heterotrophic or heterotrophs obtain their nutrients from other organisms versus an autotroph which can produce its own nutrients. Plants are autotrophs. Bacteria can use almost anything as a food source and come in a variety of shapes and sizes and are thus hard to control. Bacteria reproduce through a process called binary fission where the cell’s DNA is replicated and the cell simply splits into two identical cells.
Some bacteria under favorable conditions can reproduce every 20 minutes. One bacteria cell reproducing every 20 minutes would result in 17 million bacteria in only 8 hours. Bacteria can also be classified as psychrophilic, mosphilic or thermophilic. Bacteria will readily die at a temperature of 180°F. However, bacterial spores which are dormant can live indefinitely at this temper-ature. D. Viruses are among the smallest forms of life. They must be viewed with an electron microscope. All viruses are parasites. A parasite is a type of symbiosis or relationship where an organism lives off of another organism. In the case of a virus, it lives off the genetic material of another organism.
What are the methods used in food science to control contamination? Food contamination is the process of a food item becoming unfit for consumption due to contact with an unclean organism. Preventing contamination involves controlling four conditions. These conditions relate to both the ingredients in the food product and the environment where the product is processed. A. Controlling Air Movement—molds, yeasts, and bacteria travel through the air. If the movement of air is controlled, the chances for contamination are minimized. This is one reason why large food processing machines are covered with lids.
B. Segregation—every component in food preparation is a source of contamination. When foods are processed, each step is separated so that the contaminant does not travel from one step to the other. Segregation is routine in food processing plants. Most people are not as careful at home. The same cutting board may be used to cut different foods without being washed. C. Controlling Temperature—most food contaminants are mesophiles (grow at room temperature). For this reason, food is rarely kept at room temperature for extended lengths of time. Meat should be kept refrigerated before it is cooked. After it is cooked, it should not be left at room temperature for long. Foods should be kept below 40°F or above 140°F to prevent bacterial growth. D. Controlling Moisture Levels—most microbes thrive in high- moisture environments. High moisture environments are common at room temperature. This is why food spoils less quickly when heated or cooled.
How can the growth of microbes in food be controlled? Preventing the growth of microbes is secondary to preventing contamination. Effective prevention of contaminants makes the prevention of microbe growth easier. Microbe growth prevention is carried out using one of two methods. A. Death can be attained using heat or by cooling. Processing operations are carried out to halt the physiological deterioration and microbial action that occurs in fresh fruits and vegetables. 1. Heat is the most effective and least expensive method of killing the microbes. 2. Refrigeration destroys most of the microbes.
3. In freezing, extensive cell damage is created, which allows deteriorative enzymes to act rapidly upon thawing. Blanching (treating with water at 140 to 150° F) inactivates these enzymes and preserves the quality of the product when thawed. Rapid cooling after blanching prevents the loss of carbohydrates, vitamins, and minerals during the blanching process. Fruit that is frozen may also be treated with ascorbic acid to retard oxidative processes.
B. Inhibition is preventing the growth of the microbes. It involves altering the environment of the microbes so that they cannot thrive. The environment may be altered by reducing moisture, adding chemicals or changing the pH level of the food. 1. Food additives are substances added to foods during processing to retain or improve desirable qualities or characteristics. Processors can add sulfur dioxide, potassium sorbate, sodium propionate, sodium benzoate, or other additives which will inhibit the growth of unfavorable microbes. 2. Processing Methods can be used to control microbial growth. Foods can be canned (heated to high temperatures under pressure), frozen (destroying enzymes and microorganisms), or dried (to decrease moisture levels). The canning process not only inactivates enzymes but also kills spoilage bacteria. High acid foods such as fruits require less heating during canning, since high acidity tends to reduce spores of anaerobic bacteria.
How can consumers keep food safe? A. Wash your hands and any surface that food comes in contact with regularly. B. Cold foods should be kept at temperatures less than 40°F C. Hot foods should be kept above 140°F D. Cooked and raw food should always be kept separate. Keep utensils and dishes separate as well. E. Meats should never be thawed on the counter. Use the refrigerator or microwave to thaw them. F. Cooking should not be interrupted. G. Vegetables and fruits should be washed before consumption. H. Discard anything that looks suspicious I. As a general rule, foods should not be left out for more than 2 hours. J. Do not use foods that have passed their expiration date.
Review/Summary How do microbes lead to the spoilage of food? What are the methods used in food science to control contamination? How can the growth of microbes in food be controlled? How can consumers keep food safe?