Control by Physical Removal and Heat - MIC 414 Lecture

Control by Physical Removal and Heat Dr. Sabbir Rahman Shuvo MIC 414, Lecture 8-9 1 2 Introduction  Microorganisms can be physically removed from solid and liquid foods by several methods.  In general, these methods can partially remove microorganisms from food, an...

Control by Physical Removal and Heat Dr. Sabbir Rahman Shuvo MIC 414, Lecture 8-9 1 2 Introduction  Microorganisms can be physically removed from solid and liquid foods by several methods.  In general, these methods can partially remove microorganisms from food, and by doing so they reduce the microbial level and help other antimicrobial steps that follow to become more effective.  They are generally used with raw foods before further processing.  Learning Objectives 3 Centrifugation Filtration Trimming Washing  Physical Methods  A. Centrifugation  Centrifugation is used in some liquid foods, such as milk, fruit juices, and syrups, to remove suspended undesirable particles (dust, leucocytes, and food particles).  The process consists of exposing the food in thin layers to a high centrifugal force. The heavier particles move outward and are separated from the lighter liquid mass.  Under high force, as much as 90% of the microbial population can be removed.  Following centrifugation, a food will have fewer thermoduric microorganisms (bacterial spores) that otherwise would have survived mild heat treatment (e.g., milk pasteurization) and increased the microbial load of the pasteurized product. 4  5 Physical Methods B. Filtration  Filtration is 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 flavors of the products and heat-sensitive nutrients (e.g., vitamin C in citrus juices) are retained to give the products natural characteristics.  The filtration process can also be used as a step in the production of concentrated juice with better flavor and higher vitamins.  Physical Methods 6  Many types of filtration systems are available.  In many filtration processes, coarse filters are used initially to remove large components.  This is followed by ultrafiltration. Ultrafiltration methods, depending on pore size of the filter materials (0.45 to 0.7 mm), are effective in removing yeasts, molds, and most bacterial cells and spores from liquid products.  Filtration of air is also used in some food-processing operations, such as spray drying of milk, to remove dust from air used for drying.  The process also removes some microorganisms with dust, and this reduces the microbial level in food from air.  Physical Methods 7 C. Trimming  Fruits and vegetables showing damage (greater chance of microbial contamination) and spoilage are generally trimmed. In this manner, areas heavily contaminated with microorganisms are removed.  Trimming the outside leaves in cabbage used for sauerkraut production also helps reduce microorganisms coming from soil.  Trimming is also practiced  To remove visible mold growth from hard cheeses, fermented sausages, bread, and some low-pH products. However, if a mold strain is a mycotoxin producer, trimming does not ensure removal of toxins from the remaining food.  Physical Methods 8 D. Washing  Fruits and vegetables are washed regularly to reduce temperature (which helps reduce the metabolic rate of a produce and microbial growth) and remove soil.  Washing also helps remove the microorganisms present, especially from the soil.  During the processing of chicken and turkey, the carcasses are exposed to water several times.  During de-feathering, they are exposed to hot water; following removal of the gut materials, they are given spray washings; and finally they are exposed to cold water in a chilling tank. Physical Methods  Although these treatments are expected to reduce microbial load, they can spread contamination of undesirable microorganisms, particularly enteric pathogens.  Carcasses of food animals, such as beef, pork, and lamb, are washed to remove hair, soil particles, and microorganisms.  Instead of hand washing, automated machine-washing at a high pressure is currently used to effectively remove undesirable materials and microorganisms from the carcass. 9 10 Physical Methods  In addition to high pressure, the effectiveness of hot water, steam, ozonated water, and water containing chlorine, acetic acid, propionic acid, lactic acid, tripolyphosphates, or bacteriocins (nisin and pediocin) of lactic acid bacteria has been studied for removing microorganisms.  Some of these agents also have a bactericidal property.  However, studies show that all the agents can reduce bacterial contamination to a certain level from the carcass surfaces, and a combination of two or more components may be better.  Problem with biofilm with washing 11  The suitability of the combinations as well as their concentrations and duration of application must be determined.  With time, microorganisms can form a biofilm on the carcass surface.  The nature of the biofilm varies with microbial species and strains.  Also, with time, the biofilm becomes more stable, and removing microorganisms by washing after the formation of a stable biofilm is relatively difficult.  This aspect needs to be considered while developing effective methods of carcass washing. 12 HEAT  Mechanisms of Antimicrobial Action of heat 13  Depending on the temperature and time of heating, microbial cells and spores can be heat-shocked, sub-lethally injured, or dead.  Heat-shocked cells acquire some resistance to subsequent heating and sub-lethally injured cells and spores retain the ability to repair and multiply.  Results of different studies have shown that following heat injury, bacterial cells show loss of permeability and increased sensitivity to some compounds to which they are normally resistant.  Sub-lethally injured cells seem to suffer injury in the cell membrane, cell wall, DNA (strand break), ribosomal RNA (degradation), and some important enzymes (denaturation).  Mechanisms of Antimicrobial Action of heat 14  Death occurs from damages in some vital functional and structural components.  Bacterial spores, following heating, were found to lose structural components from the spore coat, suffer damage to the structures that are destined to become membrane and wall, and develop an inability to use water for hydration during germination.  Death results from the inability of a spore either to germinate or to outgrow.  Mechanisms of Antimicrobial Action 15 of heat  Exposure of microbial cells to 45 to 50°C for a short time, which can occur while heating a large volume of a food, may induce production of heat shock proteins (stress proteins) by the cells.  In the presence of these proteins, the microbial cells can develop relatively greater resistance to subsequent heating at higher temperatures. 16 Influencing Factors  The effectiveness of heat in killing microbial cells and spores depends on many factors,  some of which are related to the inherent nature of the foods, and  others to both the nature of microorganisms and the nature of processing.  An understanding of these factors is important to develop and adopt an effective heat-processing procedure for a food. 17 Influencing factors 1. Nature of the food 2. Nature of Microorganisms 3. Nature of Process  Influencing Factors of heat 18 A. Nature of Food  Composition (amount of carbohydrates, proteins, lipids, and solutes), Aw (moisture), pH, and antimicrobial content (natural or added) greatly influence microbial destruction by heat in a food.  In general, carbohydrates, proteins, lipids, and solutes provide protection to microorganisms against heat.  Greater microbial resistance results with higher concentrations of these components. 19 Influencing Factors  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 a food with large chunks in liquid.  Microorganisms are more susceptible to heat damage in a food that has higher Aw or lower pH.  In low-pH foods, heating is more lethal to microorganisms in the presence of acetic, propionic, and lactic acids than phosphoric or citric acid.  Influencing Factors of Heat 20 B. Nature of Microorganisms  Factors that influence microbial sensitivity to heat include inherent resistance of species and strains, stage of growth, previous exposure to heat, and initial load.  In general, vegetative cells of molds, yeasts, and bacteria are more sensitive than spores.  Cells of molds, yeasts, and many bacteria (except thermoduric and thermophilic), as well as viruses, are destroyed within 10 min at 65ºC.  Most thermoduric and thermophilic bacterial cells important in foods are destroyed in 5 to 10 min at 75 to 80ºC.  Influencing Factors of Heat 21  Yeast and most mold spores are destroyed at 65 to 70ºC in a few minutes, but spores of some molds can survive at as high as 90ºC for 4 to 5 h.  Bacterial spores vary greatly in their sensitivity to heat.  Generally, heating at 80 to 85ºC for a few minutes does not kill them. Many are destroyed at 100ºC in 30 min,  but there are bacterial species whose spores are not destroyed even after boiling (100ºC) for 24 h.  All spores are destroyed at 121ºC in 15 min (sterilization temperature and time). Below this temperature (and time), spores of some bacterial species can survive; however, this depends on the initial number of spores and the nature of the suspending medium. 22 Influencing Factors  The higher the initial microbial load in a food, the longer the time it takes at a given temperature to reduce the population to a predetermined level.  This suggests the importance of lower initial microbial loads (through sanitation and controlling growth) in a food before heat treatment.  Influencing Factors of heat 23 C. Nature of Process  Microbial destruction in food by heat is expressed in terms of its exposure to a specific temperature for a period of time.  The higher the temperature, the shorter the period of time required to get the same amount of destruction when 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.  Also, food in a small container is heated more rapidly than in a large container.  A product can have a cold point at the center (in a solid food in a can) or near the end (in a liquid food in a can), which may 24 Mathematical Expressions  When a population of microbial cell suspension is heated at a specific temperature, the cells die at a constant rate.  This observation helps in expressing the microbial death rate due to heat as a function of time and temperature under a given condition.  These expressions are helpful to design a heat-treatment method for a food. 25 Mathematical Expressions A. Decimal Reduction Time (D Value)  The D value is the time in minutes during which the number of a specific microbial (cells or spores) population exposed to a specific temperature is reduced by 90% or 1 log.  It is expressed as DT = t min, where T is the temperature (ºC or ºF) and t is the time (min) for 1 log reduction of the microbial strain used.  Thus, it is a measure of heat sensitivity of microorganisms and varies with microbial species and strains, temperature used, and other variables, such as suspending media and age of the culture. 26 Mathematical Expressions  It can be determined by using the expression:  where x and y represent, respectively, microbial numbers before and after exposing at temperature T for t min. It also can be determined by plotting log10 survivors against time of exposure (min) for a specific temperature. 27 Mathematical Expressions B. Thermal Death Time (TDT), Z Value,  TDT is the time in log that is necessary to completely destroy a specific number of microbial cells or spores in a population at a specific temperature.  It indicates the relative sensitivity of a microorganism to different temperatures.  A TDT curve can be constructed either by plotting log time of complete destruction against temperature or by plotting log D values against temperature.  Z value  The slope of the curve is the Z value, which indicates the temperature (ºC or ºF) required to change the D value (or TDT) to transverse by 1 log. 28 29 Methods  On the basis of temperature and time of heating the food used to destroy microorganisms, the methods can be broadly divided as  low-heat processing and  high-heat processing.  Low-heat processing is used to destroy mainly the microorganisms relatively sensitive to heat and is not generally effective against thermoduric microorganisms.  In contrast, high-heat processing is used to destroy thermodurics, and especially the most heat-resistant spores of spoilage and pathogenic bacteria. 30 Methods A. Low-Heat Processing  The temperature used for low-heat processing or pasteurization is below 100ºC.  The objectives of pasteurization are to destroy all the vegetative cells of the pathogens and a large number (~90%) of associative (many of which can cause spoilage) microorganisms (yeasts, molds, bacteria, and viruses).  In certain foods, pasteurization also destroys some natural enzymes (e.g., phosphatase in milk).  Methods 31  The temperature and time are set to the lowest level to meet the microbiological objectives and to minimize thermal damage of the food, which otherwise could reduce the acceptance quality.  Depending on the temperature used, thermoduric cells of spoilage bacteria and spores of pathogenic and spoilage bacteria survive the treatment.  Thus, additional methods need to be used to control the growth of survivors.  Refrigeration, modified atmosphere packaging, incorporation of preservatives, reduction of Aw, and other techniques are used, when possible in combination, to prevent or retard the problem of microbial growth in low-heat- processed products.  Methods 32  Pasteurization of milk has been used for a long time. Two methods, heating at 145ºF (62°C) for 30 min or 161°F (71°C) for 15 sec, are used to destroy the most heat resistant Q fever pathogen, Coxiella burnetii.  The methods are also designated, respectively, as low temperature long time (LTLT) and high temperature short time (HTST) methods.  As indicated before, regulation requires every particle of milk to be heated at 62°C for 30 min or at 71°C for 15 sec (holding time).  Foods that are not uniformly heated to the specified temperature and time can be involved in foodborne diseases. Immediately after the holding time, the milk is cooled to 40ºF, packaged, and 33 Methods  In the production of some fermented foods, the raw materials are heated to a high temperature to destroy vegetative cells of pathogens and spoilage microorganisms, which include thermoduric bacterial cells.  Raw milk used for the production of buttermilk, acidophilus milk, and yogurt are given a 30- to 60-min heat treatment at 90ºC before adding starter-culture bacteria.  Heating helps the starter culture bacteria grow preferentially, in addition to improving the gelling properties of the milk proteins at low pH.  Methods 34 B. High-Heat-Processed Foods  The process involves heating foods at or above 100ºC for a desired period of time.  The temperature and time of heating are selected on the basis of product characteristics and the specific microorganisms to be destroyed.  Most products are given a commercially sterile treatment to destroy microorganisms growing in a product under normal storage conditions.  However, the products can have viable spores of thermophilic spoilage bacteria (e.g., Bacillus stearothermophilus, B. coagulans, C. thermosaccharolyticum, and Desulfotomaculum nigrificans).  As long as the products are stored at or below 30ºC, these spores will not germinate.  If the products are temperature abused to 40ºC and above even for a short time, the spores will germinate. 35 Methods  The time and temperature required for commercial sterility of a particular food are determined by actual pack inoculation studies.  Generally, C. sporogenes PA 3679 is used to simulate C. botulinum because this is a nonpathogenic strain, but the spores have the same heat resistance as C. botulinum Type A or B (both proteolytic and nonproteolytic).  For spoilage control studies, spores of B. stearothermophilus are used because spores of this species are most heat resistant. 36 UHT Methods  Commercial sterility is also obtained by heating a food at very high temperatures for a short time. This process is designated as ultrahigh temperature (UHT) processing.  Milk heated to 150°C for 2 to 3 s can be stored at room temperature (≤30°C) and the products generally have a 3- month shelf life.  However, if microbial heat-stable proteinases or lipases are present in the raw milk, the product can show spoilage. 37 Methods  In the UHT process, the milk is heated by injecting steam at high pressure for a rapid temperature increase. Following heat treatment in bulk, the milk is packed in small serving containers.  Microbial heat-sensitive toxins will be destroyed, but heat- stable toxins may remain active even after heating for commercial sterility.  Under special circumstances, foods are heated to destroy all microorganisms (cells and spores) and to achieve sterility.  Sterile foods are necessary for immunosuppressant individuals in order to avoid any complications from the 38 Methods C. Microwave Heating  Heating or cooking foods by the microwave at home is quite common in both developed and developing countries.  Frozen foods can be thawed and heated very rapidly, in a few minutes, depending on the size of a product.  However, the method has not been well accepted as a source of rapidly generated high heat for commercial operations.  Mechanism for microwave heating 39  In a microwave oven, the waves change their polarity very quickly.  Oppositely charged water molecules in a food rapidly move to align along the waves.  The movement of the water molecules generates frictional heat, causing the temperature of the food to rise very rapidly.  Depending on the exposure time and intensity of the wave, the temperature can be very high.  Microwave treatment is quite lethal to microorganisms and the destruction is caused by the high temperature.  Problem with microwave heating  At present, microwave- heated foods cannot be considered safe from pathogens.  Generally, when a food is heated in a microwave oven, it is not heated uniformly and some areas can remain cold.  If a food harbors pathogens, there are chances that they will survive in the cold spots. 40 41  https://www.youtube.com/watch?v=kp33ZprO0Ck 42  Heating has been used in food preparation and preservation long before civilization.  Following the recognition of microbial roles in food spoilage and foodborne diseases, precise heating techniques have been developed to destroy vegetative cells of yeasts, molds, bacteria, and viruses and spores of bacteria.  These include pasteurization, commercial sterilization, and UHT. Destruction of microorganisms by these methods is achieved by heat treatment of a food at a specific temperature for a time period. 43  Microbial destruction at high temperature results from structural and functional destabilization of cells and spores.  At a lower heating temperature, the cells and spores can be sub- lethally injured and stressed. Because they are not dead, these two phenomena have important implications in developing heating methods to destroy microorganisms in food.  Because extreme heat to kill all microorganisms cannot always be used, other methods are used mainly to prevent growth of microorganisms as well as to maintain acceptance qualities of food. 44 THE END 45 Model Questions  Discuss filtration, centrifugation, trimming and washing as a methods to remove microorganisms from samples.  What is the mechanism of antimicrobial actions of heat?  What are the factors that influences antimicrobial actions of heat?  Discuss decimal reduction time and thermal death time (TDT), Z Value  Briefly discuss low-heat processing and high-heat processing.  What is microwave heating?