CHAPTER 28 FOOD PROCESSING INDUSTRY

Transcription

1 CHAPTER 28 FOOD PROCESSING INDUSTRY Because of its intimate connection with the public health, the food industry has had a long history of surveillance of its activities by local, state, and federal agencies. The U.S. Congress passed the original Food and Drug Act in That act, with subsequent legislation, controls not only the chemicals that are directly added to food salt, seasonings, and preservatives but also such chemicals as sizing in food wrap that may indirectly become food additives by contacting food. In addition to the close control of the FDA, additional surveillance is imposed on meat and poultry processing plants through the U.S. Department of Agriculture. No chemical can be brought into a meat or poultry processing plant unless approved by the USDA for its intended use, such as equipment cleaning or water treatment. Because of this close regulation, the choices of chemicals used in water or wastewater treatment may be more limited in the food industry than in other major water-consuming industries. There are many segments of the food processing industry, of which the major water-using categories include sugar cane and beet processing, beverage manufacturing, fruit and vegetable processing, meat and poultry, grain processing, fats and oils, and dairy products. The water consumptions of these segments are shown in Table Although there are wide variations in the process steps in each of these industry segments, there are a number of common unit operations. The distribution of water in the plant can be put into three categories: process water, cooling water, and boiler feed water. The percentage distribution varies considerably from a high of about 60% used for processing in the meat and poultry industry, to a low of only 15% in the sugar industry. However, 75% of the water used in the sugar industry is for cooling purposes (and later becomes process water), with only 25% being used in the meat and poultry industry. Most food processing plants generate steam for cooking or processing, and water used for boiler makeup ranges from about 6% of the total usage in fruit and vegetable processing, to about 15% in the fats and oils segment. Process water uses include: washing of raw materials and process equipment; conveying products from one process area to another; dissolving or extracting; and addition to the finished product. Cooling water may be used to operate refrigeration equipment, to condense steam from evaporators or turbines, or to cool process equipment such as compressors, cookers, and engine jackets. Steam may be generated for cooking, for heating evaporators, or for space heating. In some industries enough steam is required to justify installation of a turbine to extract power from the steam before it is sent to process (cogeneration). If the

2 TABLE 28.1 Water Usages in Food Processing* Industry segment Gross use Water flows, mgd intake Discharge Sugar Beverages Fruit and vegetables Meat and poultry Grain processing Fats and oils Dairy products * 1972 Census of Manufactures, U.S. Department of Commerce Publication MC72 (SR-4), Water Use in Manufacturing. steam can come into direct contact with food, there are strict limits on chemicals used for both steam and boiler water treatment, and on their maximum concentrations. Knowing the processing operations in a food plant is helpful to an understanding of water use; water may be used sequentially for several purposes. For example, in the sugar industry, which has a very high requirement for condenser cooling water because of the evaporation and concentration of syrups, this cooling water is used for washing cane brought in from the fields before it is discharged, and it is categorized as cooling water rather than process water. Because the problems of cooling water and boiler water treatment in the food industry are similar to other industries, this chapter will deal specifically with process water, with some consideration of its contamination and final treatment for disposal. THE SUGAR INDUSTRY There are many process steps in the sugar industry (some of which are similar to corn processing). As the largest food processing water user with a variety of process operations, the sugar processing industry offers a good example of water use in food processing. Sugar (sucrose), a chemical classified as a disaccharide with the formula Ci 2 H 22 O 11, is derived from two major crops, sugar cane and sugar beets. Cane is cultivated in tropical and semitropical climates (e.g., Puerto Rico, Florida), while beets are raised in temperate climates (Idaho, California). In the United States, about 7 million tons of sugar are produced annually compared to world production of 75 million tons each year. In the United States the major sugar cane producers are Louisiana and Florida, with Hawaii and Texas also important contributors. Beet sugar production is principally in the western and northwestern states. Cane exceeds beet sugar production. Because these crops spoil rapidly, they cannot be stored and the production of sugar is seasonal, with a mill operating a production campaign of about 120 days geared to the plant harvest. Sucrose, which constitutes up to 20% of the weight of the cane or beet, is readily degraded by bacterial action. The first step in the degradation is the production of invert sugars, fructose and glucose: C 12 H 22 O 11 + H 2 O - C 6 H 12 O 6 + C 6 H 12 O 6 m sucrose fructose ~" glucose ^ '

3 The second step is the production of lactic acid (C 3 H 6 O 4 ) under the conditions prevailing in beet sugar manufacture, or dextran (C 6 H 10 O 5 ) under the conditions common in the cane sugar mills. Since this degradation is primarily caused by bacteria, it is important to maintain control of microbial organisms throughout the mill to avoid loss of production of sucrose. Microbial activity also causes processing difficulties, such as filter blinding, slime formation, and odors. A basic flow diagram applying to cane and beet sugar processing is shown in Figure Water I ; I Wastewater > Washing -> r... I Solids to Extraction _"_ I recovery Juice ^ Lime I I Solids to process *> Purification STr*" I and disposal Steam I ~ I Condensate -* I Concentrate -.- -^^ ^ > Molasses -> i Steam _) Condensate -* Crystallization -^-, ^ I to recovery FIG f Sugar to market or further refining Flow sheet of sugar processing. As the crops arrive at the mill they contain soil and trash accumulated during the harvesting operation. In the case of cane sugar harvested by pushers, similar to bulldozers, the refuse may constitute as much as 10 to 25% of the weight of material delivered to the mill. This is not merely inert material; it represents a major source of bacterial inoculation since soil organisms are present with appreciable fecal matter from birds, rodents, and other small animals that live on the crop lands. Because of this, washing is a critical operation to the preparation of raw materials going to further extraction processes. On the other hand, washing should not be excessive, as this leads to loss of sucrose in the washwater. After washing, sucrose is extracted from the raw material. In cane sugar mills, this is usually done by crushing and milling the washed, cut cane stalks, producing a juice containing approximately 12 to 15% sucrose. In the beet sugar industry, the beets are sliced into long, narrow pieces (cossettes), and the sucrose extracted by washing with water in diffusers at about 16O 0 F (7O 0 C). There is growing interest in the use of diffusers in place of crushers and mills in cane processing to reduce maintenance costs and improve yield. The cane stalks are pressed after initial crushing and milling to reclaim as much sugar as possible, and the remaining solids (called bagasse) are usually

4 Syrup A Molasses B Molasses Weighing Tanks Tanks Tanks First Vapor to body heaters Second body Third body Condenser Fourth body Clanfiers Vacuum pan Vacuum pan Vacuum pan Scums Press juice (filtrate) Clarified juice OSoHdS x Rotary vacuum filter Mixer C Massecuite Crystallizers(B) Knives Syrup Centrifugal C Sugar magma for seed Mingler Mixed raw juice FIG Processflowchartof cane sugar mill. (Courtesy of Rio Grande Valley Sugar Growers, Inc.) Commercial sugar Final molasses

5 burned in boilers to generate steam. Bagasse may also be used as a raw material for such products as insulation board or acoustical tile. In the beet sugar industry, the beet pulp residue is quite high in protein, and it may be mixed with some of the plant production of molasses for cattle feed. As with most other natural products, there are a variety of chemicals other than sucrose in the cane and beets. These must be removed to maximize the yield of sugar, minimize the production of molasses, and reduce taste-, color-, and odor-producing impurities. Lime is used to precipitate these impurities, and the lime mud is removed by conventional solids/liquid separation devices. The mud is washed to reclaim as much sugar as possible, and it may then be (a) reburned to produce fresh lime, (b) returned to the fields for its fertilizer value (it often contains significant phosphate), or (c) sent to landfill. The purified juice must be concentrated to produce a thick syrup, the form of sugar often used by the beverage industry, or to produce a crystalline product. The juice is concentratd by evaporation. Since it contains calcium from the lime treatment, a common problem in the industry is the formation of scale in the pans (simple steam-jacketed evaporators) or in the multiple effect evaporators. Another common problem is foaming as the juices become concentrated during evaporation. The flow sheet of a cane sugar mill producing raw sugar is shown in Figure There is widespread use of steam throughout the plant, so the boiler house is a significant factor in economical production of sugar. Many sugar mills operate intermediate pressure boilers and produce electric power through turbo generators, taking extraction steam from the turbines for operation of pans, evaporators, and crystallizers, and in many cases taking part of the steam through the turbine to a condenser. Cooling water from both turbine and evaporator condensers may then be used as process water. Because the operation of a multiple effect evaporator produces more water as condensate than it consumes as steam, there is usually an excess of condensate available as boiler feed water. This condensate often presents special problems in that it is likely to be high in ammonia, and it may periodically contain sucrose or invert sugar. The introduction of sugar into the boiler quickly produces an acid condition, so careful monitoring of the condensate system for sugar content is important to the protection of the boiler system. The pollution control problems of the sugar mills are unusual because of the seasonal nature of the industry. The campaign is usually during the dry months, when streams cannot assimilate any excess organic loading. So the mills minimize discharge flows by recycling as completely as possible, and the wastewaters are treated biologically and impounded for solar evaporation or controlled discharge when stream flows return to acceptable rates. THE BEVERAGE INDUSTRY This segment of the food industry is another major consumer of water, some of which becomes part of the final product. The balance is used for washing of bottles and containers, cooling of compressors and refrigeration equipment, and makeup to boilers producing steam used for cooking, evaporation, heating of pasteurizers, and space heating. The water used in the product must, of course, be potable; in addition, there are standards within the industry related to the effect of water quality on the taste

6 of the finished beverage. In the soft drink industry, for example, it is common to lime soften the water for hardness and alkalinity reduction, since alkalinity destroys the flavor of acidic fruit extracts. In the lime softeners, breakpoint chlorination is also practiced. The finished water is filtered and then passed through activated carbon as a final precaution for the removal of chlorine and any residual tastes or odors. Most soft drink bottling plants have hot water boilers to provide the heat required for bottle and can washing. Breweries and distilleries, on the other hand, operate their own steam plants because steam is required for cooking and for the operation of evaporators. In many of these plants, steam passes through turbines to generate power, exhausting to lower pressures for process operations. The unit operations in these plants are quite similar to those found in the chemical industry in principle, but special designs enable the equipment to be readily cleaned to prevent microbial contamination of the product and to avoid risks to the public health. FIG A filter installation typical of the food and beverage industry practices. Note the use of special stainlesssteel piping and fittings. (Courtesy of Cross-Reynolds Engineering Company, Inc.) Special designs of piping and fittings, such as long-sweep pipe elbows, are used throughout the food industry because of this need for sanitation (Figure 28.3). Highly polished stainless-steel, monel, or chrome-plated steel eliminates scratches, nicks, and crevices which could offer a home for bacterial growths. The

7 careful cleaning of equipment after each use creates a special problem of pollution control in that spent chemical cleaners, especially those containing biocides, often interfere with the performance of pollution control equipment. Two water-using systems are unique to the food industry and require special attention to water quality: these are the bottle and container washing systems and the pasteurizer. In the bottle washing operation, both cleaning and sterilization are required, so detergents and biocides are applied to match the severity of the problem. If the bottle washer is handling returnable bottles, since there is no way of knowing what might have been in the bottles when in the hands of the public, it is important to use effective cleaning chemicals. These are quite alkaline. Because of this, it is beneficial to have zeolite-softened water for washing and rinsing as this reduces the demand for detergents and also greatly facilitates the drainage of the bottle after rinsing for spot-free surfaces. When strongly alkaline cleaners are used, these provide a biocidal effect that depends both on the length of time the chemical is in contact with the bottle, and the causticity of the cleaning solution. Even with this protection, however, chlorine is often applied to the final rinse water to ensure sterility. In the pasteurizing operation, as practiced in breweries, the bottled product is moved through the pasteurizer, passing first through a chilling zone to halt the growth of specific spoilage organisms. A controlled temperature water bath then slowly brings the beverage to approximately 16O 0 F (7O 0 C) and holds it for the time required to ensure that the entire contents of the bottle have been pasteurized. Usually two heating stages are used to prevent thermal shock and bottle breakage. The bottle is then moved into a chilling compartment before leaving the pasteurizer for packaging. It is useful to have zeolite-softened water for this operation also, to avoid spotting the bottles or cans. The temperature in the pasteurizing section is maintained by circulating hot water; and the chilling sections (also staged to avoid thermal shock) may be tied into a cooling tower and supplemented with a closed chilled-water system (Figure 28.4), although once-through cooling water is still widely used. In the event of bottle breakage, these water systems are inoculated with nutrients (the beverage), and microbial activity may quickly get out of hand. Biocides or chlorine-biodispersant treatments are used to keep this under control. For the most part, beverage industry wastewaters are handled by municipal sewage systems. This may require the plant to install equilization facilities to unify the composition and flow rate. In-house handling of strong wastes, such as chemical cleaners, may also be necessary to make the equalization program effective. A number of large distilleries provide their own independent waste treatment facilities, usually conventional biological treatment. FRUIT AND VEGETABLE PROCESSING Just as with fluming of sugar beets in the sugar industry, traditionally, water has been the media of in-plant conveyance of most fruits and vegetables in this industry segment. Not only has this choice been economical, but also it provides additional benefits in prewashing and cooling. However, because of the pollution load that results from fluming, new methods of conveyance (air, vacuum, and mechanical) are now becoming more common, so washing and rinsing, which may require as much as 50% of the total water used in process operations, is a separate

8 TEMPERATURE 0 F TIME-TEMPERATURE CURVE LEGEND SPRAY TEMPERATURE OFFAL TEMPERATURE BEER TEMPERATURE BEER OUT BEER IN FKOCESS TIME - MINUTES WATCH CHILUNO FUEHEAT HEATWM FASTEORIZWIO FRECOOL 2ND COOL 3RD COOL SERVICE WATER CHILLED WATER MFEED DISCHARGE FLOAT VALVE OVERFLOW FLOAT VALVE OVERFLOW OVERFLOW SERVICE WATER STEAM FIG Theflowof product and water through a pasteurizer for careful control of beverage temperatures. Flows are cascaded for energy recovery. (Courtesy of Barry- Wehmiller Company.) REFRIGERATED WATER

9 step. Grading and sizing are sometimes accomplished simultaneously with washing. After the fruits and vegetables are washed, peels are removed in a variety of ways. Steaming or soaking in caustic solutions are the most common, but air and mechanical peeling are also used. Dry caustic peeling of potatoes is gaining acceptance within the industry as a means of greatly reducing pollution loads. Water blanching is generally used for vegetable processing to remove air and to leach solubles before canning. Steam blanching of vegetables is usually used to destroy enzymes before freezing or dehydration. The blanching effluent stream contributes a signficant portion of the total pollution load in canning operations. In most canneries, the cans are filled with uncooked food and then passed through a steam exhaust box to eliminate air preceding the can sealing operation. The food is then cooked in the cans by direct contact of steam in retorts, which are pressure cookers in which cans are stacked in racks. After cooking, large volumes of water are required to cool the cans as they pass through cooling water canals. For continuous production, horizontal rotary cookers may be used in place of retorts and cooling canals, with steam cooking in the first cylindrical shell and water cooling in the second. Even larger continuous units are available to process up to 50,000 cans on a 2-h cycle including preheat, steaming, cascade cooling, and final cooling. A number of food products are cooked before they are bottled or canned, and many of these, such as catsup, require concentration through evaporation of water from the juice. So, there are many varieties of processing units used in the fruit and vegetable processing industry, including jacketed cookers and evaporators. All of this food processing equipment must be kept clean, usually by sanitizing with chlorinated water or detergents. Spray application Evaporation Grass and vegetative litter Slope 2-6% Perc /o//on Sheet flow Runoff collection FIG Spray distribution of food processing wastes to sloped grassland with collection of treated runoff. This system is very efficient in temperate climates. (Courtesy of U.S. Environmental Protection Agency.) Like most food industries, the fruit and vegetable processors are trying to adopt dry cleaning methods to reduce the pollutant loading on effluent water. This pollution loading varies enormously from one type of product to another. For example, in the processing of asparagus, the BOD and suspended solids are usually below 100 mg/l, whereas in the production of whole kernel corn, the BOD and suspended solids may be several thousand milligrams per liter. The solid wastes also vary considerably from one material to another.

10 Although many canneries are served by municipal sewage systems, many also operate their own waste treatment facilities. Those in farm areas where land is available have been successful in using spray irrigation as a means of disposal, often accomplishing as high as 99% BOD removal. One reason spray irrigation has been effective in the canning industry is because of the seasonal nature of its operations, permitting discharge during dry weather and avoiding the problems of freezing that would occur with a year-round operation of spray facilities in cold climates. A flow diagram of such a facility is shown in Figure Although activated sludge is an effective treatment process, strong waste from food processing may be fermented prior to final aerobic digestion to improve overall BOD reduction. This is especially useful in handling fruit processing wastes because anaerobic fermentation proceeds rapidly and greatly reduces the load on the aerobic polishing unit. MEA T AND POUL TRY A generation ago, Carl Sandburg called Chicago "hog butcher for the world"; today its great stockyards are almost empty. Economics and environmental control problems have moved the meat industry closer to the animals' grazing lands and feedlots. There, the packinghouses purchase live animals, weigh them, and keep them for several hours or overnight in "cow palaces," or holding pens, for processing. The diverse steps in meat processing are shown by the flow sheet in Figure Poultry processing is illustrated by Figure In the first step of meat processing, animals are stunned by electrical, mechanical, or sometimes chemical means before slaughter. Large packinghouses can slaughter over 300 cattle or almost 1000 hogs per hour, while giant poultry processors can kill almost 500 broilers per minute. Water is used sparingly on the kill floor; there are two separate drains so that the blood can be reclaimed for further processing without being diluted by cleanup water. Blood is either processed on-site or sold to Tenderers for processing. This consists of evaporating the water from the colloidal solids by "dry cooking" in a steam-jacketed vessel or by direct steam contact. The coagulated blood solids are then dewatered to about 57% moisture either by solids/liquid separation or further evaporation. The water from this process (serum water) carries a heavy pollutant load. Blood processing is a heavy contributor to air pollution also. The final product, blood meal, rich in amino acids, is sold as a nutrient to poultry and hog feeders. Cattle hides are removed from carcasses, and these "green hides" are usually cured on-site by salting and stacking for 14 days or by soaking in a brine solution, which takes about 18 h. The brine solution is filtered for recycle to remove horn and flesh particles, hair and other solids, and discarded after a few days use. After curing and sorting, the hides are sent out for tanning. Most hogs are processed without skinning; hair is removed by soaking in 14O 0 F water, usually containing a surfactant, and then processing through a dehairer, a device using rubber hair scrapers with copious sprays of hot water. A gas singer burns remaining hair from the carcass before further processing. Hair is usually further processed by a caustic soak followed by steam cooking to produce a high-protein animal feed supplement called hair meal.

11 Waste Solid Liquid Primary Secondary Products Animals Livestock pens Killing Blood processing Dried blood Hide removal Hog dehairing Hide processing Hog hair recovery Hides Hog hair Eviscerating, trimming Liver Heart Kidneys Viscera handling Tripe Cooling Inedible rendering Carcasses Byproducts Cutting, deboning Cut meat Edible rendering Lard Edible tallow Processing: Solid waste composting land fill Manure trap Grinding Curing Pickling Smoking Cooking Canning Meat products Secondary treatment Grease trap or flotation unit Final effluent Waste flow Process flow FIG Flowchart for a packinghouse. (EPA Contract No )

12 Potable water Empty coops Truck-borne coops a. Receiving area b. Killing station c. Blood recovery d. Scalding Blood t Defeathermg Feather flow away f. Feather recovery g. Whole bird washing Feathers h. Evisceration j. Offal recovery i. Final washing Offal k. Chilling I. Grading, weighing, packing Refrigerated delivery trucks m. Final waste water collection and control Sewer ^ ^ Product >» Potable water > By-product > Process water > Wastewater FIG Flowchart of poultry processing plant. (Courtesy of U.S. Environmental Protection Agency.) Large volumes of water are used as the cattle and hog carcasses are cut and processed. Viscera are removed and conveyed by water to edible and inedible product processing. Federally inspected and approved fat removed from the meat is rendered to make fine cosmetics and lard. This process of edible rendering is a "wet process." Live steam is injected into a pressure vessel and the fat is released to float on the

13 FLOTATION UNIT BAR SCREEN 8 RARSHALL FLUME LIFT STATION ANAEROBIC LAGOON ANAEROBIC LAGOON AEROBIC LAGOON AEROBIC LAGOON SECONDARY AEROBIC LAGOON EFFLUENT FIG Meat processing waste treatment, (a) Flow schematic; (b] aerial view. (From EPA 625 I , Water Treatment: Upgrading Meat Packing Facilities to Reduce Pollution, October 1973, U.S. Environmental Protection Agency, Technology Transfer.)

14 condensate (stick water). The stick water is high in BOD and extractables, and constitutes a high loading on the waste treatment plant. Most inedible rendering is accomplished dry. Ground offal is introduced into a steam-jacketed vessel, where water is evaporated under a slight vacuum. The cooked products are tallow and bone meal. In basic operations, poultry and hog processing are similar. Scalding tanks are used to enhance feather removal; and the production of feather meal is similar to hair meal. Large amounts of water are used throughout the process. Because of the high BOD of meat processing wastes, water pollution control is somewhat unusual in that anaerobic digestion is frequently used prior to aerobic digestion. A crust develops on the anaerobic lagoon which normally controls the odors often associated with anaerobic fermentation. Pretreatment by equalization, screening, and flotation is essential to good control of the operation. The digester loading is typically about 0.2 Ib BOD/day per cubic foot of digester volume. The wastewater is usually warm, about 80 to 10O 0 F (27 to 38 0 C), which is beneficial, as anaerobic digestion is greatly hindered at low temperatures. Detention may be 12 to 24 h. This combination of anaerobic-aerobic digestion followed by sedimentation usually results in 90% BOD reduction in the first stage and over 98% overall (Figure 28.8). Many packing plants discharging to city sewage plants are required to provide pretreatment (equalization, screening, grease removal) to reduce the load and equalize the composition of wastes to avoid upsetting the municipal plant operations. SUGGESTED READING Lund, H. F., (ed.): Industrial Pollution Control Handbook McGraw-Hill, New York, Shreve, R. N.: Chemical Process Industries, 3d ed., McGraw-Hill, New York, U.S. Environmental Protection Agency: Development Document for Beet Sugar, EPA-440/ b, March U.S. Environmental Protection Agency, Development Document for Cane Sugar Refining, EPA-440/ c, March U.S. Environmental Protection Agency: Development Document for Red Meat Processing, EPA-440/ a, March U.S. Environmental Protection Agency, Development Document for the Apple, Citrus and Potato Processing Segment, EPA-440/ a, March 1974.