P1: SFK/UKS BLBS102-c38 P2: SFK BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm 740 Printer Name: Yet to Come Part 7: Food Processing Table 38.10 Calculation of Process Time (B ) Using Stumbo Method 10 11 12 13 14 jh fh Process lethality (Fo ) Retort temperature (Tr ) Initial temperature (Ti ) Ih = Tr −Ti jh ∗ Ih Log(jch ∗ Ih ) Z value 250−Tr Fi = 10 z fh /U = fh /(Fo ∗ Fi ) jcc From Stumbo’s table for z = 18◦ F (18◦ F) (jcc = 1.6), obtain g value by interpolation fh /U g value 1.00 0.638 1.33 ? 2.00 2.34 Interpolated g value is 1.2 for fh /U value of 1.33 log g B = fh [log (jch Ih )−log g)] 0.96 12.6 min 245◦ F 175◦ F 70◦ F = 245−175 67.2 1.75 18◦ F 1.9 1.33 1.6 0.0792 21.68 two effects to get a better quality safe product This goal can be achieved by optimizing thermal process conditions In order to achieve proper process optimization, accurate determination of kinetic parameters of microorganisms, spoilage enzymes, and quality factors are crucial The need to optimize processing conditions arises when the kinetic behavior of the different components is considered because the rate of a chemical reaction generally doubles for a 10◦ C rise in temperature, whereas rates of bacterial destruction increase tenfold under similar conditions (Holdsworth 1985) Gathered kinetic parameters are used to develop reliable predictive models that will ensure to produce better quality products without compromising food safety There are two key issues that should be explored to produce better-quality safe products The first choice may focus on reducing thermal intensity levels, through reducing of the heating, and cooling times associated with the process through improving heat transfer property of processing condition and modification of geometry of packaging material The other option is to look for novel thermal or nonthermal food processing technologies that will ensure better quality products with relatively same degree of safety New preservation technologies are utilized because of their expected potential to inactivate microorganisms and spoilage enzymes with a very little damage on product quality High-Temperature Short-Time and Ultra-High Temperature Processing Table 38.11 Calculation of Process Lethality (F0 ) Using Stumbo Method 10 jh fh Operator process time (Pt ) CUT Retort temperature (Tr ) Initial temperature (Ti ) Ih = Tr −Ti jch ∗ Ih Log(jch ∗ Ih ) z value 11 12 12 13 14 15 Fi = 10 z B = Pt + 42% CUT B/fh log g = log(jch ∗ Ih )−B/fh g jcc From Stumbo’s table for z = 18◦ F (jcc = 0.8) Obtain fh /U value by interpolation fh /U g value 10.00 8.24 ? 9.55 15.00 10.16 Interpolated fh /U value is 13.41 for g value of 9.55 F0 = fh /[(fh /U)∗ Fi ] 16 250−Tr 1.9 36.2 42 10 255◦ F 160◦ F 95◦ F 180.5 2.26 18◦ F 0.53 46.2 1.28 0.98 9.55 0.8 5.1 As the names implies, high-temperature short-time (HTST) and ultra high temperature (UHT) processes use higher temperatures and shorter processing times than conventional thermal processes to improve quality of foods and beverages Because the products are exposed to high temperatures for short times, there is reduced degradation of quality factors of the products while causing a greater effect on destruction of food microorganisms The principle of HTST or UHT process is easily understood by comparing the heat resistance of nutrient components, microbial spores, and vegetative bacteria (Figure 38.9) The D value at reference temperature for vegetative bacteria are in fraction of seconds (z = 5◦ C); microbial spores, 0.2 minute (z = 10◦ C); and nutrient components, 100–150 minutes (z = 25−30◦ C) This variation gives an opportunity to optimize processing condition for better quality production of foods and beverages From destruction of vegetative bacteria point of view, area (A, B, and C) beneath the dotted line of the graph are not acceptable (Figure 38.9) In these sections, the vegetative bacteria are not yet destroyed The same is true for section D, E, and F, where the product is cooked but spore formers still survive the heat treatment All sections of G, H, and I above the bold line gives sterile product However, the question that comes here is that, to what extent the product is cooked with respect to the quality factors The top and bottom broken D lines represent 90% and 10% destruction of a nutrient component, respectively Particularly, section I of the graph gives a safe product with less than 10% destruction of nutrient This is the region corresponding to HTST or UHT treatment as indicated by the arrows P1: SFK/UKS BLBS102-c38 P2: SFK BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come 741 38 Thermal Processing Principles 90% destruction of nutrient Log time A D G 40% destruction of nutrient B E H C 10% destruction of nutrient F I Temperature Figure 38.9 A diagram of resistance of nutrient (- - -), microbial spores (–), vegetative bacteria (•••), and HTST heating principle For instance, in UHT sterilization process, milk is exposed for 2–5 seconds at high temperatures of 140–145◦ C to kill bacteria spores UHT treatment limits browning of the milk, development of cooked flavor, and denaturation of proteins In HTST pasteurization, milk is subjected to a temperature of 71.1◦ C for 15–20 seconds HTST or UHT approaches are not always beneficial for conduction heating food products that heat relatively slowly, exhibiting large temperature gradients between surface and center of the container However, they are best for liquid foods and liquids containing small particles that can heat rapidly while subjected to in-container thermal processing or in-heat exchangers, as an aseptic processing As the containers are agitated inside a retort, the contents of the containers are mixed uniformly; this eliminates cold spots and increase heat penetration rate Mixing largely is due to the movement of the headspace bubble during agitation, and to be effective, there must be sufficient headspace in cans Both small and large headspace may result in lower heat transfer, leading to underprocessing because of limitation of mixing of food components in cans In addition to headspace, fill of the container, solid to liquid ratio, consistency of the product, and the speed of agitation are the crucial factors to be standardized in agitating processing Unless and otherwise these conditions are properly optimized for a given product and processing condition, the required fast heat penetration may not be achieved Agitation Processing Some retorts agitate the cans during processing in order to increase the rate of heat penetration in cans As compared to static retorts, the process time may be reduced by 80% because the contents are heated up faster and more evenly Agitation processing is mainly groped into axial and end-over-end (EOE) type (Fig 38.10) EOE involves the containers being loaded vertically and a crate rotating around a central horizontal axis During axial rotation, cans are rotated individually in the horizontal plane Cans in vertical position Aseptic Processing The word “asepsis” implies the process of removing pathogenic microorganisms or protecting against infection by such organisms It can be defined as a state of control attained by using an aseptic work area and performing activities in a manner that precludes microbiological contamination of the exposed sterile product In food processing, aseptic processing involves a section that precludes microbiological contamination of the Cans in horizontal position Figure 38.10 End-over-end and axial agitation orientation of cans in retort cage P1: SFK/UKS BLBS102-c38 P2: SFK BLBS102-Simpson 742 March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come Part 7: Food Processing final sealed product During aseptic processing, the product is exposed to desired treatment temperature to eliminate food microorganisms and finally packed in sterile container in an aseptic environment This processing method is more efficient for liquid products and liquid products containing small particles Aseptic processing sterilizes food and beverages in a way that puts the least amount of thermal stress on a product, so nutrients and natural flavors, colors, and textures are maintained while sterility is ensured The aseptic process begins with sterilization of both the processing system and the filler Generally, this is done with hot water or saturated steam Food is then pumped into the aseptic processing system for sterilization The flow of food is controlled via a timing pump, so that no food flows too fast or too slow This process consists of heating, holding, cooling, and packing steps First, the product is heated and held for some time to attain desired degree of sterilization Once the food leaves the hold tube, it is sterile and subject to contamination if microorganisms are permitted to enter the system The best way to keep the product sterile is to keep it flowing and pressurized Heating step is followed by cooling of the product The next step is moving the product into an aseptic surge tank to hold the product just before packaging Meanwhile, the packaging material is sterilized from the other side of the aseptic environment Packaging material is sterilized by liquid hydrogen peroxide at a high temperature After the food and package are sterilized, the sterile package is filled with the food, closed, and sealed in a sterile chamber The direct and indirect heating processes used in aseptic packaging can affect the final taste of the food Indirect heating, the food comes into contact with either a metal plate or tube, which can give food, such as milk, a burnt flavor There are three types of indirect heating processes: plate heat exchangers, tubular heat exchangers, and scraped-surface heat exchangers The former two are mainly used for liquid foods and the latter one is for more viscous and particulate foods to prevent fouling due to high temperature effect All use a physical separation between the product and the heating medium and transfer heat through either a plate or tube to the product Direct heating uses steam injection or steam infusion and minimizes the burnt flavor of the product by letting the food come into direct contact with the heat source With steam injection, product and steam are pumped through the same chamber; while with steam infusion, the product is pumped through a steam-filled infusion chamber In all these heat exchangers, since the product is directly exposed to heating medium or subjected to thin profile mode of heating, it allows faster heat penetration within short period of time, which destroys microorganisms with limited effect on quality of the product That is why aseptic processing is considered as HTST processing in terms of optimization of quality with desired degree of safety However, for particulate foods, the rate of heat penetration in the center of the slowest heating point of the product should be determined to ensure desired degree of pasteurization/sterilization In particulate foods, first the heat is exchanged between the heating medium and liquid food, and then transferred to the particle In this case, it is difficult to measure the temperature of the moving particle The time–temperature profile in the particle can be estimated using mathematical models Based upon the data of the model used, the particle should be held at appropriate temperature for sufficient time to achieve the required sterilization value at the center Thin Profile and Retort Pouch Processing Quality optimization is mainly achieved through enhancing the heat transfer rate from the heating medium to the product Any resistance to rapid heat penetration slows down the heat penetration rate and exposed the product for longer heating time The nature of packaging material, thickness, and its overall thermal conductivity determine the heat transfer rate In conventional thermal processing, metal cans and glass containers are the main type of containers used However, the thickness of these containers limits fast transfer of heat between heating medium and product Modification of geometric configuration of the packaging materials from material thickness and geometry point of view is additional opportunity to optimize quality through improving heat transfer rate Foods in rigid polymer trays or flexible pouches heat more rapidly, owing to the thinner material and smaller cross-section of the container Much of the retort pouch development was conducted by the U.S Army Natick Research and Development Center for use in the Meal Ready-to-Eat (MRE), having relatively light weight as compared to conventional metal containers However, the thin thickness of the pouch and its flat shape allows fast heat penetration and contributes much on improvement of quality of foods as compared to conventional packaging materials The retort pouch is a flexible, heat sealable container that is thermally processed like a can and used to produce shelf stable, commercially sterile food products It is constructed of a 3-ply laminate composed of an outer layer of polyester film, a middle layer of aluminum foil, and an inner layer of polypropylene The layers are bonded together with a special adhesive The tri-laminate material provides seal integrity, toughness, puncture resistance, printability, and superior barrier properties for long shelf life It also withstands the rigors of thermal processing up to 135◦ C Retort pouches are filled with wet foods sealed and then heattreated in steam/hot water retort kettles to achieve commercial sterilization (for shelf-stable foods) or pasteurization (for refrigerated foods) Because processing time typically is faster in the pouch than in metal, glass, or rigid plastic containers, the product tends to end up with better quality and safety Pouches also offer lower shipping and storage costs (pouch material is lighter than cans, and pouches take up less storage space than cans); easier, safer handling (no can openers or thawing time required); and reduced product waste and reduced volumes of disposed packaging waste material as compared to cans Some of the disadvantages include a lack of physical durability and slow production rates due to slow filling and sealing rate as compared to cans or glass jars Furthermore, it needs an overpressure processing to protect the integrity of the packages during processing Pouches are more easily punctured; they can overwrap for safe distribution P1: SFK/UKS BLBS102-c38 P2: SFK BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come 38 Thermal Processing Principles Novel Thermal Food Processing Technologies Microwave and Radio Frequency Heating Microwave (MW; 300–300,000 MHz) and Radio Frequency (RF) waves (0.003–300 MHz) are a part of the electromagnetic spectrum MW and RF energy generates heat in dielectric materials such as foods through dipole rotation and/or ionic polarization MW ovens are now common household appliances Popular industrial applications of MW heating in food processing operations include tempering meat or fish blocks and precooking bacon or meat patties, while RF heating is commonly used in finishing drying of freshly baked products Such applications shorten processing times, reduce floor space, and improve product qualities compared to conventional methods Extensive research has been carried out over the past 50 years on MW and RF energy in pasteurization, sterilization, drying, rapid extraction, enhanced reaction kinetics, selective heating, disinfestations, etc., but with limited applications Technological challenges remain and further research is needed for those applications MW/RF sterilization applications demand more thorough and systematic studies as compared to other applications These studies will have far reaching impacts to the food industry and research communities Several commercial 2450 MHz MW sterilization systems produce shelf-stable packaged foods in Europe (e.g., Tops Foods, Olen, Belgium) and Japan (Otsuka Chemical Co., Osaka), but these systems are designed with multi-mode MW cavities Generally, MW/RF heating is a promising alternative to conventional methods of heat processing as it is regarded as a volumetric form of heating in which heat is generated within the product, which reduces cooking times and could potentially lead to a more uniform heating Reduction in processing time and uniform heating results in getting high-quality product in terms of its nutrient content, desired flavor, texture, color, and taste Ohmic Heating Ohmic heating (OH) is based on the passage of alternating electrical current through a food product that serves as an electrical resistance Because of the current passing through the food sample and its resistance to the flowing current, relatively rapid heating occurs OH has good energy efficiency since almost all of the electrical power supplied is transformed into heat Many factors affect the heating rate of foods undergoing OH: electrical conductivities of fluid and particles, the product formulation, specific heat, particle size, shape, and concentration as well as particle orientation in the electric field As compared to conventional heating technologies, internal heat generation by OH eliminates the problems associated with the heat conduction in food materials and then prevents the problems associated with overcooking Aseptic processing has been used commercially for long time for liquid foods, but for products containing particulates, the use of conventional heattransfer techniques leads to overprocessing of the liquid phase to ensure that the center of a particulate is sterilized This can result in destruction of flavors and nutrients, and mechanical damage to 743 the particulate However, OH-treated product is clearly superior in quality than those processed by conventional technologies This is mainly due to its ability to heat materials rapidly and uniformly, leading to a less-aggressive thermal treatment Hence, OH can be considered as a HTST aseptic process (Castro et al 2004) Novel Nonthermal Processing Technologies High-Pressure Processing High-pressure Processing (HPP) is an innovative technological concept that has a great potential for extending the shelf life of foods with minimal or no heat treatment It is a process aimed at controlling deteriorative changes such as microbial and enzymatic activity without subjecting the product to drastic thermal processing and mass (drying) transfer techniques such that the original quality is retained The application of hydrostatic pressure to food results in the instantaneous and uniform transmission of the pressure throughout the product independent of the product volume The treatment is unique in that the effects neither follow a concentration gradient nor change as a function of time A significant advantage is the possibility of operation at low or ambient temperatures so that the food is essentially raw Gelation, gelatinization, and texture modification can be achieved without the application of heat Apparently, HPP is a physical treatment and is not expected to cause extensive chemical changes in food system Once the desired pressure is reached, the pressure can be maintained without the need for further energy input Liquid foods can be pumped to treatment pressures, held, and then decompressed aseptically for filling as with other aseptic processes HPP is a novel technique for processing of foods and has attracted considerable attention in recent years It has been commercialized for a variety of acid and acidified food products For low-acid foods, it has been used only as a temporary measure of extending the shelf life under refrigerated storage conditions It has also been used for several other purposes, including control of some pathogens and viruses, for inducting functional changes, as well as improving nutritional and sensory quality of foods HPP has been used mainly for refrigerated and high-acid foods Pasteurization by HPP can be carried out at pressures in the range of 400–600 MPa at relatively moderate temperatures (20–50◦ C) Under these conditions, HPP can be effective in inactivating most vegetative pathogens and spoilage microorganisms, but the quality factors remain unaffected HPP for producing shelf-stable low-acid foods is still a topic of considerable controversy It has the potential to produce better quality foods than possible from the use of processing novelties such as MW, RF, or OH techniques in combination with aseptic processing This is because HPP allows the product temperature to increase very rapidly (due to adiabatic heating) from around 90–100◦ C to the sterilization zone (120–130◦ C; in about minutes) and bring it back almost instantaneously by depressurization The process can be formulated either as a pressure-assisted thermal processing A better understanding of the inactivation kinetics of pathogens or their surrogates under pressure-assisted P1: SFK/UKS BLBS102-c38 P2: SFK BLBS102-Simpson March 21, 2012 744 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come Part 7: Food Processing thermal process conditions is the key for the success of the HPP and for regulatory approval The critical factors in the HPP include pressure, time to achieve treatment pressure, time at pressure, depressurization time, treatment temperature (including adiabatic heating), product initial temperature, vessel temperature distribution at pressure, product pH, product composition, product water activity, packaging material integrity, and concurrent processing aids Although HP processing related research work has increased tremendously in the last decade, there is serious lack of information in this area to permit establishing a reliable process steam and allowing the air to escape through vent valves There is no overpressure during the sterilization phases of this process, since air is not permitted to enter the vessel at any time during any sterilization step However, there may be air-overpressure applied during the cooling steps to prevent container deformation This is because during cooling, the steam rapidly condenses in the retort and the food cools more slowly and pressure in the container remains high When the food temperature becomes below 100◦ C, overpressuring is stopped and the food is allowed to continuously cool up to 40◦ C Water Heating Retort (Immersion and Spray Modes) Pulsed Electric Field Pulsed electric field (PEF) processing is a nonthermal method of food preservation that uses short bursts of electricity for microbial inactivation and causes minimal or no detrimental effect on food-quality attributes PEF can be used for processing liquid and semi-liquid food products PEF processing involves treating foods placed between electrodes by high-voltage pulses in the order of 20–80 kV (usually for a couple of microseconds) The applied high-voltage results in an electric field that causes microbial inactivation A series of short, high-voltage pulses breaks the cell membranes of vegetative microorganisms in liquid media by expanding existing pores (electroporation) or creating new ones The membranes of PEF-treated cells become permeable to small molecules; permeation causes swelling and eventual rupture of the cell membrane The treatment is applied for less than one second, so there is little heating of the food, and it maintains its “fresh” appearance, shows little change in nutritional composition, and has a satisfactory shelf life (Castro et al 1993, Kozempel et al 1998) Since it preserves foods without using heat, foods treated this way retain their fresh aroma, taste, and appearance RETORT TYPES FOR COMMERCIAL APPLICATION Retorts generally are either batch or continuous types Their configuration may be either vertical or horizontal Horizontal retorts are commonly preferred due to their ease of loading and unloading facilities as compared to vertical retorts Batch Retorts Steam Heating Retort In this type of retort (static or rotary), saturated steam is used as a heating medium Latent heat is transferred from steam to the cans when saturated steam condensed at the surface of the cans The saturated steam process is the oldest method of in-container sterilization Since air is considered an insulating medium, saturating the retort vessel with steam is a requirement of the process If air enters in the retort, it forms insulation layer of the surface of the can and prevents the condensation of steam and causes underprocessing of the product It is inherent in the process that all air be evacuated from the retort by flooding the vessel with The water immersion process is the most widely accepted method of sterilizing product using an overpressure process The water immersion process is similar to a saturated steam process in that the product is totally isolated from any influence of cooling air The product is totally submerged in preheated water at preset temperature But it is different from saturated steam in that air can be introduced into the vessel during sterilization The water is preheated in different chamber with steam to desired temperature and pumped to retort chamber Overpressure is provided by introducing air (or steam) on top of the water In some instances, air is added to the steam (which then heats the air) The heated air agitates the water as it flows to the surface and serves to pressurize the process load Because it is an overpressure process, the machine can handle most, if not all, of the fragile containers For cooling, the water present in the retort passes through a heat exchanger, where it is gradually cooled by fresh water circulating in the service side The water spray process is also an overpressure process, like water immersion, except that the product is exposed to the influence of the overpressure air In this retort, low amount of water is stored in the bottom of the retort and circulated by a pump with high flow rate and sprayed on containers It is similar to the saturated steam process in that steam is the driving force for reaching the center of the load But the water spray process is different from saturated steam in that air can be introduced into the vessel during sterilization Overpressure is provided by introducing air (or steam) into the retort To overcome the insulating effects of the air, the spray nozzles vaporize the steam and mix the steam with the air The condensates are automatically evacuated by a drainer and can be returned to the boiler Steam-Air Heating Retort The steam-air process is an overpressure process, like water immersion, except that the product is exposed to the influence of the overpressure air Steam is directly injected inside the retort chamber and distributed and mixed uniformly with air with the help of a fan to prevent cold spots in the retort It is a “ventless” process, resulting in significant energy savings Furthermore, a precise control of the steam/air process results in a shortened cycle time for maximum production and consistent quality, and reduces the process time and optimal heat transfer characteristics, which allow foods to retain more of their natural qualities The opening of the steam inlet valve is automatically ... condenses in the retort and the food cools more slowly and pressure in the container remains high When the food temperature becomes below 100◦ C, overpressuring is stopped and the food is allowed to... exchangers, tubular heat exchangers, and scraped-surface heat exchangers The former two are mainly used for liquid foods and the latter one is for more viscous and particulate foods to prevent fouling due... impacts to the food industry and research communities Several commercial 2450 MHz MW sterilization systems produce shelf-stable packaged foods in Europe (e.g., Tops Foods, Olen, Belgium) and Japan