Food BiotechnologyDr. Kamal E. M. ElkahloutFood Microbiology 2 Control of Microorganisms in Food
Definitions • Controlling access of microorganisms • Control By Physical Removal • Centrifugation • Filtration • Trimming • Washing • Control By Heat • Low-heat processing or pasteurization. • High-heat processing • Microwave Heating • Control By Low Temperature • Ice Chilling • Refrigeration • Freezing • CONTROL BY REDUCED Aw • Control by low pH and Organic acids • Control by Modified Atmospheric (O-R potential) • Control by Irradiation • Control by antimicrobial preservatives
Definitions • Sterilization: The process by which all the living cells, viable spores, viruses, and viroids are either destroyed or removed from an object or habitat. • Disinfection: Is the killing, inhibition or removal of microorganisms that may cause disease. Disinfectants are usually chemical agents, and are normally used only on inanimate objects. • Sanitization: Microbial population is reduced to levels that are considered safe by public health standards.
Antiseptics: Chemical agents applied to tissue to prevent infection by killing or inhibiting pathogen growth. • Bactericide: A disinfectant/ antiseptic against bacteria. Cidal: A suffix meaning that “the agent kills.” For example, a bacteriocidal agent kills bacteria. • Bacteriostatic: Do not kill, but prevent growth of bacteria. Static: A suffix that means “the agent inhibits growth.” For example, a fungistatic agent inhibits the growth of fungi, but doesn’t necessarily kill it.
Some microorganisms are desirable • for the production of bioprocessed foods • Many are undesirable due to their ability to cause food spoilage and food borne diseases • Several methods (individually or in combination) are used to achieve control
These are: • Controlling access of the microorganisms present in foods. • Control by physical removal. • Control by heat. • Control by low temperature. • Control by reduced Aw • Control by low pH and Organic acids • Control by Modified Atmospheric (O-R potential) • Control by Irradiation • Control by antimicrobial preservative
Controlling access of microorganisms (Cleaning and Sanitation)
Controlling access of microorganisms (Cleaning and Sanitation) • To minimize the access of microorganisms in foods: • the microbial quality of the environment to which a food is exposed (food contact surfaces) should be good. • The ingredients added to the food should be of good microbial quality. • Sanitation minimizes the access of microorganisms in food from various sources at all stages of food handling. • Proper sanitation helps to produce food that have a long shelf life.
Plant Design • When designing a food processing plant, an efficient sanitary program must be integrated in order to provide maximum protection against microbial contamination of foods. • This includes both the outside and the inside of the plant. - floor plan and approved materials used in construction. - adequate light, air ventilation, direction of air flow. - separation of processing areas of the raw and finished products. - sufficient space for movement and operations. - water supply and sewage disposal system, waste treatment facilities, drainage and surrounding environment.
Quality of Water, Ice, Brine and Curing Solution • Water is the most important element in food manufacturing operations. • Water is used as an ingredient in many foods • also used in some products after heat treatment. • Eg: ready-to-eat types, should not only be free from pathogens (like drinking water), but also be low (if not free) in spoilage bacteria, such as Pseudomonas spp.
Important for foods that are kept at low temperature for extended shelf life. • Eg., ice used for chilling unpackaged foods should also not contaminate a food with pathogenic and spoilage bacteria. • Brine and curing solutions used in products such as cured beef can be a source of contamination hence should be made fresh daily to be used for processing.
Quality of Air • Food processing operations, such as spray drying of nonfat dry milk, require large volumes of air that come into direct contact with the food. • Important to install air inlets to obtain dry air with least amount of dust and filtration of air.
Training of Personnel A processing plant should: • Have an active program to teach the plant personnel the importance of sanitation and personal hygiene in-order to ensure product safety and stability. • Also monitor the implementation of such program. • People with an illness and infection should be kept away from handling the food products.
Equipment • design of food processing equipment should protect a food from microbial contamination. • Protection is achieved if the equipment • does not contain dead spots where microorganisms harbor and grow and cannot be easily and readily cleaned in place or by disassembling.
Some of the equipments such as meat grinders, choppers, slicers and several types of conveyor systems not properly sanitized can be a source of contamination. • Equipment sanitizing is important for products that come in contact with equipment surfaces after treatment and before packaging.
Centrifugation • A process used to separate or concentrate materials suspended in a liquid medium. • The technique is based on the effect of gravity on particles in suspension. Two particles of different masses will settle in a tube at different rates in response to gravity. • Centrifugation - used in some liquid foods, such as milk, fruit juices and syrup,to remove suspended undesirable particles (dust, leukocytes and food particles).
Under high forces, as much as 90% of the microbial population can be removed. • Following centrifugation, a food will have fewer thermoduric microorganism (bacterial spores) that otherwise would have survived mild heat treatment (e.g. milk pasteurization).
Filtration • Filtration - used in some liquid foods, • such as soft drinks, • fruit juices, beer, • wine and water • to remove undesirable solids and microorganisms and to give a sparkling clear appearance.
As heating is avoided (or given only at minimum levels,) the natural flavor of the products and heat- sensitive nutrients (e.g. vitamin C in citrus juices) are retained to give the products natural characteristics. • Coarse filters are initially used to remove the large component, followed by ultra-filtration to remove small particles.
Filtration of air • used in food processing operations. • such as spray drying milk; to remove dust from air used for drying. • The process removes microorganisms with dust and they reduce the microbial level in food from source (air)
Trimming • Fruits and vegetables showing damage and spoilage are generally trimmed. • Areas heavily contaminated with microorganisms are removed. • Trimming of outside leaves in cabbage helps reduce microorganisms coming from soil.
Trimming is also used to remove visible mold growth from hard cheeses, fermented sausages, bread and some low pH products. • If a mold strain is a mycotoxin producer, trimming will not ensure removal of toxins from the remaining food. • Trimming is also used to remove fecal stain marks, unusual growths and abscesses or small infected areas from carcasses of food animals and birds. • Trimming allows complete removal of the causative microorganisms.
Washing • Washing equipment and work areas is discussed under cleaning and sanitation. • Fruit and vegetables are washed to reduce temperature (that helps to reduce metabolic rate of a produce and microbial growth) and remove soil. • Washing removes the microorganisms present, especially from the soil. It is also used for shell eggs to remove fecal materials and dirt.
The desirable effect of heat (fire) on the taste of foods of animal and plant origin, especially seeds, tubers and roots, was probably accidentally discovered by our ancestors . • They also possibly recognized that heated foods did not spoil as fast as raw foods.
The main objective (microbiological) of heating food is to destroy vegetative cells and spores of microorganisms that include molds, yeasts, bacteria and viruses. • Drastic heat treatment (sterilization) can be used to kill all the microorganisms, which is present in a food. • Most foods are heated to destroy – pathogenic and spoilage microorganisms
Antimicrobial Action of Heat • Depending upon the temperature and time of heating, microbial cells and spores can be sub-lethally injured or dead. • Death occurs from damages in vital functional and structural components.
Factors Affecting Heat killing of microbial cells • The effectiveness of heat in killing microbial cells and spores is dependent on factors: • related to the inherent nature of the foods • on both the nature of microorganisms and the nature of processing.
Nature of Food • Composition (amount of carbohydrates, proteins, lipids and solutes), • Aw (moisture), • pH, and anti-microbial content (natural or added) greatly influence microbial destruction • Microorganisms in liquid food and food containing small-sized particles suspended in a liquid are more susceptible to heat destruction than in a solid food or in a food with large chunks.
2. Nature of Microorganisms • Factors that influence microbial sensitivity to heat is inherent resistance, stage of growth, previous exposure to heat and initial load. • In general vegetative cells (moulds, yeasts and bacteria) are more sensitive than spores • thermoduric and thermophilic bacterial cells (important in foods) are destroyed in 5 to 10 minutes at 75 to 80°C
Yeast and most mould spores are destroyed at 65 to 70°C in a few minutes, • Spores of some moulds can survive as high as 90°C for 4 to 5 h. • Bacterial spores varies greatly • Heating at 80 to 85°C for few minutes does not kill. • But destroyed at 100°C in 30 min however some can withstand this • Destroyed at 121°C in 15min (sterilization Temp /Time)
Cells at exponential stage of growth are more susceptible to heat than the resting cells (stationary phase) • Cells previously exposed to low heat become relatively resistant to subsequent heat temperature. • The higher the initial microbial load in a food – the longer time at a given temperature it takes to reduce the population.
3. Nature of Process • Microbial destruction in food by heat ( inverse relationship). Higher the temperature, the shorter the period of time required for destroying the microorganisms provided other factors are kept constant. • As a food is heated by conduction (molecule-to-molecule energy transfer) and convection (movement of heated molecules), a liquid food is heated more rapidly than a solid food and a container with high conduction (metal) is better.
Food in a small container is heated more rapidly than in a large container • Heating a food at a given temperature for a specific time means that every particle of that food should be heated to the specified temperature and stay at that temperature for the specified time –”holding time”.
Methods using heat Low-heat processing or pasteurization. • temperature used is below 100oC. • Process aims to destroy all vegetative cells of pathogens and microorganism which cause food spoilage. • Pasteurization of milk has been used for a long time – heating at 62.8oC for 30 mins or 71.7 oC for 15 secs.
Methods using heat High-heat processing • Process involves heating food at or above 100oC. • Temperature and time of heating are selected on the basis of product characteristics and specific microorganisms to be destroyed. • Most products are given a commercially sterile (sterilization) treatment to destroy to destroy microorganism growing in a product under normal storage conditions.
High-heat treated products are either first packed in containers and then heated or heated first and then packed in sterile containers while still hot (hot pack). • Commercial sterility is also obtained by heating a food at very high temperatures for a short time (process called ultrahigh temperature (UHT) processing .
Methods using heat Microwave heating • Heating of foods by microwave (quite common at home). • Frozen foods can be thawed and heated rapidly in a few minutes depending upon the size of the product. • Microwave treatment is lethal to microorganisms and destruction is caused by high temperature. • If the food is not heated uniformly, some areas can remain cold and if food harbors pathogens, there is chance of their survival.
Effectiveness of low temperature, especially freezing in food preservation was probably recognized by our ancestors in the last Ice Age. • The major drawback of refrigerated goods is their relatively short shelf life. • But in recent years, several technological improvements have helped in increasing the shelf life.
Mechanisms of microbial control • Metabolic activities, enzymatic reactions and growth rates of microorganisms are maximum at optimum growth temperatures. • When temperature is lowered, microbial activities associated with growth slow down. • Rate of catalytic activity of enzymes decreases with reduced temperature. • As the temperature in a food drops to about – 2 oC, free water in the food starts freezing and forming ice crystals, hence Aw is also reduced.
Foods are stored at low temperature in different ways in order to extend their shelf life. • Many fresh fruits and vegetables are kept at temperatures between 10oC and 20oC or lower to reduce their metabolic rates. • Highly perishable products are generally stored at low temperature below 7 oC often in combination with other preservation methods.
Methods using heat Ice chilling • Usually used Retail stores where foods are kept over ice. • The surface is in contact with ice, temperature can reach between 0oC – 1oC. • Temperature fluctuation, duration of storage and cross contamination can cause microbiological problems – food borne pathogens.
Methods using heat Refrigeration • The temperature specification for refrigeration of foods has changed from time to time. • From 7oC, technological improvements have made it economical to have domestic refrigeration units at 4 to 5 oC. • For perishable products, ≤ 4.4oC is considered desirable refrigeration temperature.
Commercial food processors may use as low as 1 oC for refrigeration of perishable foods such as fresh meat and fish. • Refrigerated products are often combined with additional preservation methods with lowest temperature possible for long shelf life. • As the products are non-sterile, even a very low initial microorganism population is capable of growing under the storage conditions.
Methods using heat Freezing • Minimum temperature used in home freezers is – 20 oC, a temperature at which most of the free water in a food remains in a frozen state. • Dry ice ( -78oC) and liquid nitrogen (- 196oC) can also be used for instant rapid freezing, but not for food. • After freezing, the temperature of the food is maintained around -20oC to -30oC.
Microbial cells will die upon during frozen storage, but survivors can multiply in the frozen state. • Accidental thawing or slow thawing can facilitate growth of survivors. • Enzymes released by dead microbial cells can reduce the acceptance quality of food.