POST-HARVEST HANDLING SYSTEMS:
MINIMALLY PROCESSED FRUITS AND VEGETABLES
“Minimally processed” horticultural products are prepared and handled to maintain their fresh nature while providing convenience to the user. Producing minimally processed products involves cleaning, washing, trimming, coring, slicing, shredding, and so on. Other terms used to refer to minimally processed products are “lightly processed,” “partially processed,” “freshprocessed,” and “preprepared.”
Minimally processed fruits and vegetables include peeled and sliced potatoes; shredded lettuce and cabbage; washed and trimmed spinach; chilled peach, mango, melon, and other fruit slices; vegetable snacks, such as carrot and celery sticks, and cauliflower and broccoli florets; packaged mixed salads; cleaned and diced onions; peeled and cored pineapple; fresh sauces; peeled citrus fruits; and microwaveable fresh vegetable trays.
Whereas most food processing techniques stabilize the products and lengthen their storage and shelf life, light processing of fruits and vegetables increases their perishability. Because of this and the need for increased sanitation, preparation and handling of these products require knowledge of food science and technology and postharvest physiology.
Growth in demand has led to increased marketing of fresh horticultural products in lightly processed form. An industry dedicated to this type of food processing has been established, and the National Association of Fresh Produce Processors was recently formed.
Minimal processing generally increases the rates of metabolic processes that cause deterioration of fresh products. The physical damage or wounding caused by preparation increases respiration and ethylene production within minutes, and associated increases occur in rates of other biochemical reactions responsible for changes in color (including browning), flavor, texture, and nutritional quality (such as vitamin loss). The greater the degree of processing, the greater the wounding response. Control of the wound response is the key to providing a processed product of good quality. The impact of bruising and wounding can be reduced by cooling the product before processing. Strict temperature control after processing is also critical in reducing wound-induced metabolic activity, as shown in the respiration data of intact and shredded cabbage stored at different temperatures. Other techniques that substantially reduce damage include use of sharp knives, maintenance of stringent sanitary conditions, and efficient washing and drying (removal of surface moisture) of the cut product.
Fruits and vegetables are ecological niches for a diverse and changing microflora, which usually does not include types pathogenic to humans. Intact fruits and vegetables are safe to eat partly because the surface peel is an effective physical and chemical barrier to most microorganisms. In addition, if the peel is damaged, the acidity of the pulp prevents the growth of organisms, other than the acidtolerant fungi and bacteria that are the spoilage organisms usually associated with decay. On vegetables, the microflora is dominated by soil organisms. The normal spoilage flora, including the bacteria Erwinia and Pseudomonas, usually have a competitive advantage over other organisms that could potentially be harmful to humans.
Changes in the environmental conditions surrounding a product can result in significant changes in the microflora. The risk of pathogenic bacteria may increase with film packaging (high relative humidity and low oxygen conditions), with packaging of products of low salt content and high cellular pH and with storage of packaged products at too high temperatures (>5°C or 41°F). Food pathogens such as Clostridium, Yersinia, and Listeria can potentially develop on minimally processed fruits and vegetables under such conditions.
With minimally processed products, the increase in cutdamaged surfaces and availability of cell nutrients provides conditions that increase the numbers and types of microbes that develop. Furthermore the increased handling of the products provides greater opportunity for contamination by pathogenic organisms.
Microbial growth on minimally processed products is controlled principally by good sanitation and temperature management. Sanitation of all equipment and use of chlorinated water are standard approaches. Low temperature during and after processing generally retards microbial growth but may select for psychrotropic organisms such as Pseudomonads. Moisture increases microbial growth, therefore removal of wash and cleaning water by centrifugation or other methods is critical. Low humidity reduces bacterial growth, although it also leads to drying (wilting and shriveling) of the product. Low oxygen and elevated carbon dioxide levels, often in conjunction with carbon monoxide, retard microbial growth. Plastic film packaging materials modify the humidity and atmosphere composition surrounding processed products and therefore may modify the microbial profile.
Minimal processing may occur in a “direct chain” of preparation and handling in which the product is processed, distributed, and then marketed or utilized. Many products are also handled in an “interrupted chain” in which the product may be stored before or after processing or may be processed to different degrees at different locations. Because of this variation in time and point of processing, it would be useful to be able to evaluate the quality of the raw material and predict the shelf life of the processed product.
Minimally processed products may be prepared at the source of production or at regional and local processors. Whether a product may be processed at source or locally depends on the perishability of the processed form relative to the intact form, and on the quality required for the designated use of the product. Processing has shifted from destination (local) to source processors as improvements m equipment, modified atmosphere packaging, and temperature management have become available.
In the past, processed lettuce operations often salvaged lettuce remaining in the fields after harvesting for fresh market. It is now recognized that first-cut lettuce should be used for maximum processed product quality. After trimming and coring, piece size may be reduced with rotating knives or by tearing into saladsize pieces. Damage to cells near cut surfaces influences the shelf life and quality of the product. For example. shredded lettuce cut by a sharp knife with a slicing motion has a storage life approximately twice that of lettuce cut with a chopping action. Shelf life of lettuce is less if a dull knife is used rather than a sharp knife.
Washing the cut product removes sugar and other nutrients at the cut surfaces that favor microbial growth and tissue discoloration. Because of differences in composition and release of nutrients with processing, some products such as cabbage are known as “dirty” products. It is desirable to maintain separate processing lines, or thoroughly clean the line before another product follows cabbage. Free moisture must be completely removed after washing. Centrifugation is generally used, although vibration screens and air blasts can also be used. The process should remove at least the same amount of moisture that the product retained during processing. It has been shown that removal of slightly more moisture (i.e., slight desiccation of the product) favors longer postprocessing life.
Packaging, Modified Atmospheres, and Handling
Polyvinylchloride (PVC), used primarily for overwrapping, and polypropylene (PP) and polyethylene (PE), used for bags, are the films most widely used for packaging minimally processed products. Multilayered films, often with ethylene vinyl acetate (EVA), can be manufactured with differing gas transmission rates. For lettuce processed at source, a 2.5 mil 8 percent EVA co-extruded PE bag has been used. Products are often packaged under partial vacuum or after flushing with different mixtures of gases (oxygen, carbon dioxide, carbon monoxide, and/or nitrogen). Vacuum packaging and gas flushing establish the modified atmosphere quickly and increase the shelf life and quality of processed products. For example, browning of cut lettuce occurs, before a beneficial atmosphere is established by the product’s respiration. For other products, such as fastrespiring broccoli florets, impermeable barrier films are used with permeable membrane “patches” to modify the atmosphere through the product’s respiration. It is not yet agreed what are the ideal films and atmospheres for minimally processed products. In addition to different atmosphere requirements for different products, the specifics of the handling chains must be taken into account, especially their time delays and temperature fluctuations.
The modified atmospheres that best maintain the quality and storage life of minimally processed products have an oxygen range of 2 to 8 percent and carbon dioxide concentrations of 5 to 15 percent.
Carbon monoxide concentrations of 5 to 10 percent under low oxygen (<5 percent) conditions retard browning and reduce microbial growth, lengthening shelf life in lettuce and other products. With some nonpermeable barrier-type PE films, an elevated oxygen level (25 to 50 percent) is used with carbon monoxide (3 to 10 percent) to maintain aerobic respiration during the handling period.
The following factors are known to be critical to maintaining quality and shelf life in minimally processed products: using the highest quality raw product, reducing mechanical damage before processing reducing piece size by tearing or by slicing with sharp knives, rinsing cut surfaces to remove released cellular nutrients and kill microorganisms, centrifugation to the point of complete water removal or even slight desiccation, packaging under a slight vacuum with some addition of CO to retard discoloration, and maintaining product temperature at 1° to 2°C (34° to 36°F) during storage and handling. Temperature maintenance is currently recognized as the most deficient factor.
Other techniques such as irradiation, chemical preservation (dips in ascorbic acid, calcium chloride, and/or citric acid), modification of pH, and reduction of water activity (with sugars/salts) may also control deterioration of processed products, mainly by controlling microbial growth.
Quality of Minimally Processed Products
The nature of the demand for minimally processed products requires that they be visually acceptable and appealing. The products must have a fresh appearance, be of consistent quality throughout the package, and be reasonably free of defects. Field defects such as tipburn on lettuce can reduce the quality of the processed product because the brown tissue is distributed throughout the packaged product.
In mixed salads, the quality of the total product is only as good as that of the most perishable component. This also applies to cleaned and washed spinach and other products where differences in leaf age or physical damage to leaves may yield a product of nonuniform perishability.
Quality assurance programs, long regarded as essential in the processed food industry, are difficult to apply to horticultural crops and the corresponding minimally processed products. Fresh horticultural products have not yet been subjected to the same sanitation, labeling, and shelf life requirements as other processed foods.