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MINIMAL PROCESSING There is an increase in fresh fruit and vegetable consumption around the world that is mainly motivated by the recommendations made by different organizations, such as the World Health Organization, the Food and Agricultural Organization, the US Department of Agriculture (USDA), and the European Food and Safety Authority, etc., because of their healthy properties. Fruits and vegetables are important sources of a wide range of vital micronutrients, phytochemicals (e.g., anthocyanins and other phenolic compounds), and fiber of great importance from the human nutritional point of view. Minimally processed fresh fruit and vegetables (MPFVs) are commonly defined as “any fruit and vegetable that has been subjected to different processing steps (e.g., peeling, trimming, cutting, washing, disinfection, rinsing, etc.) to obtain a fully edible product while providing convenience and functionality to consumers and ensuring food safety” OR “Fresh-cut fruits & vegetables that have been trimmed, peeled and cut into a fully usable form, which are subsequently packed to offer consumers convenience while maintaining the freshness”. It is also called partial / fresh / light processing or pre prepared products. These commodities contain exclusively natural ingredients, and are bagged or pre-packed in polymeric films able to generate optimal modified atmosphere packaging (MAP) conditions, and they are kept under chilling until consumption. MPFVs have similar characteristics to the whole original fruit or vegetable, and they usually need no further processing before use, offering advantages for consumers because, in addition to convenience, they have high quality and they produce little waste at a reasonable price. Demand for minimally processed fruits and vegetables has increased due to: busy life style, increased purchasing power and health conscious trends. In addition consumers are increasingly demanding convenient, ready to use, ready to eat fruits and vegetable products with a fresh like quality and containing only natural ingredients. Minimally processing of raw fruits and vegetables has two purposes: a) It is important to keep the produce fresh, yet supply it in a convenient form without losing its nutritional quality. b) The product should have a shelf life sufficient to make its distribution feasible to its intended consumers. In an ideal case, minimal processing can be seen as “invisible” processing. However, although conventional food processing methods extend the shelf life of fruits and vegetables, the minimal processing to which fruits and vegetables are submitted renders the products highly perishable, requiring chilled storage to ensure a reasonable shelf life. The preparation of MPFVs causes damage to plant tissue in which the natural protective layers are eliminated, promoting many physical and physiological disorders that accelerate produce decay, reduce shelf life compared with the intact fruits and vegetables, and provide an easy entry for microbial pathogens and chemical contaminants. The microbiological, sensory and nutritional shelf life of MP fruits or vegetables should be at least 4 - 7 days but preferably even longer up to 21 days depending on the market. The loss of ascorbic acid and carotenes is the main limiting factor of nutritional quality.

The deterioration of MPFVs occurs mainly due to further physiological aging, biochemical changes, and microbial spoilage, which result from changes in respiration, ethylene (C2H4) emission, transpiration, and enzymatic activity of the living tissues after processing. Many of the compositional changes influence the MPFVs’ color, texture, flavor, and nutritive values, leading to an irreversible loss of quality. The main spoilage changes affecting MPFV are discoloration, off-flavors, softening (loss of crispness or juiciness), and water loss. Therefore, sensory quality can never improve during further storage; instead, the quality can only be retained or its deterioration can be slowed down. To slow down deterioration, storage temperature is the single most important factor affecting the spoilage of MPFVs. Factors and processing operations that affect the quality of minimally processed plant foods The quality and safety of MPFVs depend on many factors, including those also affecting the quality and safety of intact fruit and vegetables (e.g., GAPs and GHPs) and specific factors such as handling procedures; the quality of process wash water; processing technologies; packaging methods and materials; and transportation, processing, storage, distribution, and retail sale temperatures. The most common factors influencing the quality, shelf life and safety of MPFVs are summarized in Figure and table below. Sp o i l a g e a t h o g e n s

Me c h n a i c a l d a ma g e

Bi o c h e mi c a l Ch a n g es Su r f a c eb r o wn i n g So f t e n i ng L o s so f t e x t u r e

Mi c r o b i a l g r wo t h

De h y d r a t i o n De c a y

Ph y s i o l o gi c a l a g i n g

n i n g: a t i o nr a t e y l e n ep r o d u c t i o n L o s so f a p pe a r a n c e

Figure 1: Factors that affect MPFVs’ decay and shelf life Table 1: Summary of the factors affecting total quality of the MPR fruits and vegetables Parameters Affects Pre-harvest agricultural practices Composition, phytonutrients Growing crops practices Size, shape, scars, pesticide residues Harvest practices Maturity, brix /acid, color Postharvest handling Mechanical damage, biochemical changes Processing Microbial growth, browning, weight loss Packaging, handling, and storage Spoilage, weight, loss, softening, surface drying Physiological changes from harvest Senescence, changes in respiration rates, ethylene

to fork (respiration rates of F & V O2/CO2)

production, yellowing, loss of water, size changes, loss of phytonutrients, shelf-life

Minimal Processing Technologies In minimally processed foods, storage, processing, packaging, and distribution are accomplished in highly integrated systems. Each step must be considered in conjunction with the other steps. Different minimal processing methods can be applied at the various steps in the food distribution chain, from storage of agricultural products to packaging/processing of the ready-to-eat product. In this introduction and overview, different minimal processing technologies and methods will be presented following the food distribution chain. Detailed accounts of these methods can be found in the literature. A list of the methods reviewed here is given in Table 2. Table 2 Application of minimal processing methods

Modified Atmosphere Storage By modifying the composition and sometimes the overall pressure of the atmosphere in the storage environment, quality-and safety-degrading biological reactions can be slowed down or inhibited. For storage of fruits and vegetables, increased carbon dioxide concentration (up to 10%) and reduced oxygen concentration (3–5%) will retard respiration and prolong the shelf-life. Relative humidity is also an important factor, as is control of ethylene emission from the respiring fruits and vegetables. A reduced pressure during storage is also used, often

in combination with modified CO2 and O2 concentration levels. This method is often called hypobaric storage, using pressures of some hundreds of mmHg. Postharvest Treatments In order to add convenience to vegetables and other agricultural products, centralized cleaning, peeling, and cutting is common. The resulting products are often less stable after the treatment, due to enzymatic activity of cut cell walls and bacteriological contamination from the handling. Various postharvest treatment methods are employed to add biological stability and extend shelf-life. Chlorinated cleaning water is used. Soaking in solutions of reducing agents such as ascorbic acid or sulfite or preservatives such as sorbate or benzoate are used. Also, divalent ions, Ca2+, are used to strengthen the texture. In all these treatments low temperature and good processing hygiene are essential to achieve the desired shelf-life, as treatments such as cutting instead reduce the shelf-life. Clean-Room Technologies The objective in clean-room technology is to eliminate microbiological contamination from humans or from the environment. The handling of the food is automated as much as possible. Equipment and the processing environment are pre-sterilized before production. Air curtain and a positive air pressure are maintained in the processing line using sterile-filtered air. Production personnel use extensively protective clothing to reduce air- and human-borne microorganisms. Clean-room technologies are primarily developed for fresh prepared food and for dairy products. Clean-room technology is often an expensive method, which often means that the application is limited to high-value-added products and to a limited part of the production line. Also the hygiene classes used in the food industry are higher than in other industries using clean-room technology. Protective Microbiological Treatment It is well known that many microorganisms produce antimicrobial agents. Some lactic acidproducing bacteria produce bacteriocins that are efficient in stopping the growth of, in particular, Gram-positive spoilage bacteria. By adding selected lactic acid producing strains to the surface of foods, controlled growth can create an antimicrobial condition at the food surface. The application of these methods is still in development. Combination Methods: Hurdle Technology Reducing the levels of salt, sugar, or acid in foods in order to improve consumers’ acceptance often means increased perishability of the food. To attain sufficient shelf-life, a combination of preservation methods is often applied. The ‘hurdle concept’ is a simplified illustration of the principle for combination processing. By fine-tuning a preservation system with a number of methods, more knowledge is built into the product – an important step in product development. Non-thermal Processing High-pressure processing: By applying pressures in the range of some thousand atmospheres on biological material, some enzymes and vegetative microorganisms can be inactivated. Cell membranes are broken. Very high pressures in combination with elevated temperatures are

needed for the inactivation of bacterial spores. High-pressure treatment also changes the texture of food, e.g., coagulate proteins and swell starches. The method is commercialized in Japan and Europe for treatment to prolong the shelf-life of low-pH fruit products. In Spain, ham products are treated, with a resulting prolonged shelf-life. In the USA, avocado paste is treated at very high pressure. Extensive research and development is now taking place to investigate the possibilities and limitations of the method. Irradiation: Very extensive research into the method in the 1950s and 1960s, particularly the wholesomeness of the irradiated products, demonstrated that the method is an efficient and safe preservation method. In spite of this, the commercial use has been very limited, due to consumer scepticism coupled with legislative limitation. Use of the method is gradually increasing in Europe and the USA, with applications for fresh fruits, poultry, and spices. The method is expensive, with typical processing costs of 0.1–0.2 Euro/kg product. Of course, this limits the application to high-value products. High-electric-field pulses: When cells are subjected to electrical pulses with field strength of 15–35 kV/cm, cell membranes are broken due to an uneven distribution of electrical changes on both sides of the cell membranes. The broken cell walls cause inactivation of microorganisms. The method is more efficient the larger the cell, e.g., for yeast. Thus, the most interesting future application is for products where yeast growth is limiting the shelflife, e.g., fruit products and other drinks. A number of active research programs are found in Europe and the USA, but the method is not commercial yet. Thermosonication: By combining ultrasound treatment at 20–40 kHz with heat treatment at moderate temperatures, the inactivation of microorganisms can be strongly enhanced. It will thus enable pasteurization of drinks at lower temperatures with less thermally induced quality changes. Thermosonication is also efficient for enhancing enzyme inactivation. The method is not used commercially in the food industry. Thermal Processing Mild heating methods: There is much interest in mild heat treatment methods, which avoid excessive temperatures, resulting in thermally induced quality losses. Often a reduction of only a few degrees can have a dramatic influence on liquid losses of meat or fish. Yet the heat treatment needs to give microbiological safety to the product. With the help of modern process optimization and control methods, mild heat treatments that combine these objectives have been developed. Among these, most interest is found in direct heating methods such as microwave heating that can be used to raise the temperature quickly and shorten the processing time. For industrial applications, the microwave equipment is designed by computerized methods to control the heating uniformity, by controlling the overheating of the edges and corners of the foods. The direct electric heating methods are typically two to five times more expensive than traditional heating methods. Thus their use is limited to processes with benefits in terms of better production yield or product quality. Sous-vide cooking: In sous-vide cooking, fresh food is vacuum (sous-vide)-packed, under hygienic conditions. The packed products are cooked at fairly low temperatures in water to internal temperatures determined by culinary objectives. The long cooking times give some

tenderization effects. Excessive temperatures are avoided, allowing for high moisture retention and juiciness. As the microbiological safety of the sous-vide cooking can give rise to some concerns, depending on the processing temperature, the shelflife is often limited to 6–20 days at temperatures of +3°C. Packaging Modified-atmosphere packaging: The methods described above for controlled-atmosphere storage are also applied for individually packed food products. Fresh meat and fish, prepared foods, and baked foods are packed in modified atmosphere, with high concentrations of CO2. Permeability of the packaging material to CO2 is important to control. Carbon dioxide is bacteriostatic and fungistatic. Increased CO 2 concentration will thus inhibit the growth of microorganisms as long as a sufficient concentration of dissolved CO 2 is maintained in the surface of the food. Refrigerated storage is required for CO 2 to be effective. A wide range of fresh and prepared foods are distributed and stored in the atmosphere with high CO2 concentration (50–10%). Often O2 concentration is reduced or essentially nil, except for fresh meat, where it is held at 20% oxygen in order to maintain the red meat color. For vegetables, where the respiration is continuing and the CO 2 concentration would increase to levels that adversely affect the quality, micro-perforations in the packaging film allow the respired CO2 gradually to escape from the package. Table 3 Packaging material for vegetables Vegetable Peeled potato Grated carrot Sliced turnip Grated turnip Sliced beetroot Grated beetroot Shredded Chinese cabbage Shredded white cabbage Shredded onion Shredded leek

Packaging material and thickness both whole and sliced PE-LD, 50 m (also PA/PE, 70–100 m or comparable) PP-O, 40 m, microholed PP-O, PE/EVA/PP-O, 30–40 m PE-LD, 50 m PE/EVA/PP-O, 40 m PE-LD, 50 m (also PA/PE, 70–100 m or comparable) PP-O, 40 m, microholed PP-O, PE/EVA/PP-O, 30–40 m PP-O, 40 m, PE/EVA/PP-O, 30–40 m PP-O, 40 m, PE/EVA/PP-O, 30–40 m PP-O, 40 m (also PA/PE, 70–100 m or comparable) PE-LD, 50 m, PP-O 40 m (also PA/PE, 70–100 m or comparable)

Active packaging: This term covers packaging methods and agents that actively influence the shelflife of the food during storage. The best-known example is the oxygen absorbers or scavengers, which reduce the head space and permeating oxygen levels. The scavengers come as small sachets or tablets to be introduced into the package. It is predominantly Fe 2+ ions that are used. The reduced level of oxygen in the head space prevents development of rancidity, and is also effective in reducing growth of certain types of microorganisms, e.g., molds.

Other types of active packaging systems are ethanol vapor generators. The ethanol absorbed on silicon dioxide powder and contained in paper sachets prevents the growth of molds. They are mainly used for bakery products. Edible Coatings The rapid development of biodegradable films for food packaging has helped to strengthen the development of edible coating applied directly on food. Coatings are made from films of proteins, starches, or waxes. The coating will protect against oxygen, aroma components, and moisture to the product, reducing the requirements on packaging. Most films are sensitive to moisture, which limits their application to dry, frozen, and semimoist foods. Processing line Minimal processing operations involve the application of several unit operations (Figure 2) that can provide opportunities for cross-contamination, in which a small lot of contaminated product may be responsible for the contamination of a large lot

Figure: Unit operations that can provide opportunities for cross-contamination. The general unit operations and the maximum recommended temperatures for each processing step in the production line of fresh-cut leafy vegetables are shown in Figure 3. However, the most significant steps of the processing chain (washing, cutting, disinfection, and packaging) that significantly affect the final quality of the products match up in both production lines. It should be taken into account that once the raw material is within the industry, all accepted products must be received in a low-temperature controlled area, or quickly moved into a cold room at the appropriate temperature, or moved directly to the processing room. If a plant product exhibits signs of chemical or physical contamination or other defects, then interventions should focus on the use of equipment for the grading, trimming, and selection of raw materials to eliminate damaged, spoiled, or potentially hazardous product. Key requirements in the minimal processing of fruits and vegetables 1. Raw material of good quality (correct cv. variety, correct cultivation, harvesting and storage conditions) 2. Strict hygiene and good manufacturing practices, HACCP 3. Low temperatures during workingMinimal Processing of Fruits and Vegetables: An Overview 4. Careful cleaning and/or washing before and after peeling 5. Water of good quality (sensory, microbiology, pH) used in washing

6. Mild additives in washing for disinfection or browning prevention 7. Gentle spin drying after washing 8. Gentle cutting/slicing/shredding 9. Correct packaging materials and packaging methods 10. Correct temperature and humidity during distribution and retailing

Figure 3 General unit operations in a processing plant of MPFVs and the maximum recommended temperatures for each processing step Minimal Processing in the Future The minimal processing technologies represent a means of meeting the well-established, long-term trends in consumer demands for convenience, variety, and fresh-like quality. Such technology will allow the food industry the possibility of producing highquality, high-valueadded products to meet future consumer demands. The technologies require varying degrees of capital investment. But, most importantly, the technologies require investment into product and process know-how, not only of the minimal processing technology itself, but also of the integrated chain of food distribution, from agricultural production to the consumer. An important future area is the understanding of the antimicrobial effects of enzymes and other biochemical agents. In addition, non-thermal processing methods need to be further investigated in terms of the mechanisms of preservation as well as engineering aspects of the in...