CHAPTER 5. PROCEDURE FOR VEGETABLES PRESERVED BY COMBINED METHODS

5.1 Preliminary operations

Vegetables are subjected to several preliminary operations before processing and after harvesting. As a result of peeling, grating and shredding, produce will change from a relatively stable product with a shelf life of several weeks or months to a perishable one with a shelf life as short as 1-3 days at chill temperatures. The major preliminary operations include:

Washing: Root vegetables are washed first to remove all field dirt and to allow inspection.

Inspection: Vegetables are inspected for quality to comply with consumer demands.

Selection: Vegetables are selected and graded on a basis of firmness, cleanness, size, weight, colour, shape, maturity, mechanical damage, foreign matter, disease, and insects. This operation can be done manually, or by employing a variety of separation machines to separate and discard unfit produce.

Subsequent Operations:

Peeling, cutting and shredding: Some vegetables such as potatoes and carrots require peeling. Ideal peeling is done very gently, by hand with a sharp knife. It has been reported that hand peeling of carrots increases the respiration rate over that of unpeeled carrots, by approximately 15%, whereas abrasion peeling almost doubles the respiration rate compared to hand peeled carrots (Ahvenainen, 1996). Carborundum abrasion peeled potatoes must be treated with a browning inhibitor, whereas washing is enough for hand peeled potatoes (Alzamora et al., 2000). These authors proposed the following guidelines for prepeeled and sliced potatoes:

Second washing and drying: Commonly, a second washing is needed after peeling and/or cutting (Alzamora et al., 2000; Ahvenainen, 1996). For instance, Chinese cabbage and white cabbage should be washed after shredding, whereas carrots should be washed before grating (Alzamora et al., 2000). Washing after peeling and cutting removes microbes and tissue fluid, thus reducing microbial growth and enzymatic oxidation during subsequent storage. Washing fruits and vegetables in flowing or carbonated water is more preferable than dipping the product into a tank of water. The microbiological and sensory quality of the washing water must be good and its temperature low, preferably < 5°C. The recommended quantity of water used is 5-10 L/kg for produce before peeling/cutting and 3 L/kg after peeling/cutting (Alzamora et al., 2000).

Preservatives can be used in the washing water to reduce microbial load and to retard enzymatic activity, thus improving the shelf life and sensory quality of produce. The recommended dosage for chemical preservatives in washing water is 100-200 mg/L of chlorine or citric acid (Alzamora et al., 2000). These levels are effective in the washing water before, after, or during cutting to extend the shelf life. However, when chlorine is used, vegetable materials require a subsequent rinse to reduce the chlorine concentration to the level of drinking water and to improve the sensory shelf life. The effectiveness of chlorine should be improved by using low pH, high temperature, pure water, and correct contact time (Alzamora et al., 2000). The optimum contact time for chlorine is 12-13 s, if the chlorine concentration is 70 mg/L (Ahvenainen, 1996). According to Ahvenainen (1996), chlorine compounds are effective in inactivating microorganisms in solutions and on equipment, and in reducing the aerobic microorganism count (i.e., in some leafy vegetables such as lettuce, but not necessarily in root vegetables). Chlorine compounds are not very effective at inhibiting the growth of Listeria monocytogenes in shredded lettuce or Chinese cabbage.

Another disadvantage of chlorine is that some food constituents may react with chlorine to form potentially toxic reactive products. Thus, the safety of chlorine use for food or water treatment has been questioned, and future regulatory restrictions may require the development of alternatives. Some proposed alternatives are chlorine dioxide, ozone (O₃), trisodium phosphate, and hydrogen peroxide (Alzamora et al., 2000). The use of hydrogen peroxide (H₂O₂) as an alternative to chlorine for disinfecting freshly cut fruits and vegetables shows some promise. H₂O₂ vapour treatment appears to reduce the microbial population on freshly cut vegetables such as cucumbers, green bell peppers, and zucchini. Moreover, H₂O₂ vapour treatment extends the shelf life of vegetable products without leaving significant residues or causing loss of quality. However, more research is needed to optimize H₂O₂ treatments with regard to efficacy in delaying the growth of spoilage bacteria in a wide variety of vegetable products.

According to Alzamora et al. (2000), the following processing guidelines for shredded Chinese cabbage and white cabbage should be followed:

Waxing: During washing, fresh vegetables (also fruits) lose part of their outer layer of wax, which protects against humidity loss. As a result, waxing is re-established after washing with an artificial layer of wax that has adequate thickness and consistency to improve appearance and to reduce the loss of water.

Classification: The main objective of this operation is to attain a uniform product for the market. Fresh vegetables are classified by size, weight, or degree of maturity. Classification by size can be done manually in small packing houses with trained personnel. In mechanized packing houses, operations are carried out with perforated belts, divergent belts or cylinders, and sieving. Sorting by mass is usually done electronically but some manually operated machines can classify different weights by a tipping mechanism. Classification by degree of maturity can be done using colour charts or by optical methods.

Labelling: Commercial fresh vegetables (as well as fruits) can be labelled individually with automatic adhesive stickers to identify the product brand, farmer, or retailer. This is extremely important when exporting produce to other countries.

Premarketing Operations:

Packaging: This operation is one of the most critical in the marketing of vegetables, and involves putting a number of required units in the appropriate package according to weight. Generally, for exporting fresh fruits, corrugated fibreboard boxes of variable capacity are employed.

The most common packaging technique for prepared raw vegetables and fruits is modified atmosphere packaging (MAP). The basic principle of MAP is that a modified atmosphere can be created passively by using specified permeable packaging materials or by actively using a specified gas mixture in combination with such materials.

Storage: The packaged fresh product can be stored at ambient or refrigeration temperature until it is shipped to the overseas market.

5.2 Combined optional treatments

5.2.1 Irradiation

Most studies conducted on irradiation of vegetables (also on fruits) have been targeted to alter ripening and to control post-harvest pathogens and disinfectants. Several countries are exploring alternative methods suitable for the control of human pathogens in fruits and vegetables, and ionization radiation could be one such alternative. It has been demonstrated in literature that application of ionizing radiation (irradiation of foods) is an effective technology for controlling spoilage microorganisms and for increasing the shelf life of strawberries, lettuce, sweet onions, and carrots. Although extensive studies exist on the control of pathogens in meat and poultry products with irradiation, very few studies exist on the value of ionizing radiation in eliminating food borne pathogens in fruit juice, fruits and vegetables, such as lettuce and sprouts (including the seeds to grow sprouts) (Thayer and Rajkowski, 1999). Pathogens in these foods when eaten raw have not generally been controlled in most parts of the world, although this could often be accomplished by combined methods, such as controlled atmosphere packaging (MAP) and ionizing radiation. Thayer and Rajkowski (1999) presented a review on the different dosages of ionizing radiation applied to vegetables to control spoilage and pathogens. In general, most vegetables can withstand irradiation dosages up to a maximum of 2.25 kGy; higher doses can, however, interfere with the organoleptic properties of food products. Combining irradiation with temperature control and gaseous environment, along with adequate processing conditions, is one of the most effective approaches to vegetable preservation.

The moisture content in foods and the surrounding environment during treatment influence the sensitivity of microorganisms to irradiation. For example, high environmental relative humidity and high water content in foods reduce the effectiveness of irradiation; therefore, control of these parameters during irradiation treatments could extend the shelf life and quality of irradiated vegetables. Recently, disinfections of vegetables with chlorinated water have been replaced with irradiation treatments. Treatment of shredded carrots with irradiation at 2 kGy inhibited the growth of aerobic and lactic acid bacteria, in which case sensory analysis panellists preferred the irradiated vegetables (Thayer and Rajkowski, 1999).

5.2.2 Refrigeration

Refrigeration of vegetables can halt the growth of certain pathogen and spoilage microorganisms but will not eliminate them. (The reduction of temperature increases the lag time and decreases the growth of microorganisms). It is generally recognized that maintaining foods at 5°C is sufficient to prevent the growth of most common food-borne pathogens. However, some emerging psychrotrophic pathogens such as Listeria monocytogenes, Yersinia enterocolitica, Clostridum botulinum types A and E, Aeromonas hydrophyla, (enterotoxigenic), and E. coli are able to multiply slowly in refrigerated foods. Therefore, refrigeration cannot be solely relied upon to maintain the safety of high moisture foods (HM). Considering the increased popularity of MPR foods, this issue has great significance, because refrigeration may be the only hurdle in the preservation of such products. And since psychrotrophic pathogens might eventually prevail, additional factors in the preservation system are needed for safety assurance.

In the conventional refrigeration storage environment, three important factors must be controlled: temperature, relative humidity, and air movement.

Temperature: The system should always be able to meet the demands placed upon it and controlled automatically by the use of thermocouples, pressure valves, etc.

Relative humidity (RH) should be kept high in a refrigerated storage room by controlling the refrigerant temperature. High RH prevents water loss affects texture, freshness, colour appearance and overall quality of food products.

Air movement in the refrigeration environment must be sufficient to remove respiration heat, gases, and the heat penetrating through the door, junctions, and structure of the refrigeration room. However, excessive air movement can cause food dehydration. Air circulation must be uniform throughout the room. Packages must be correctly stacked to achieve good air circulation. Optimum temperatures, RH levels, and expected shelf life of stored horticultural products are described in Table 5.1.

5.2.3 Modified atmospheres

A modified atmosphere (MA) implies removal of, or the addition of gases, resulting in an atmospheric composition different from the one normally existing in air. For example, the N₂ and CO₂ levels may be higher, and the O₂ levels lower than those found in a normal gaseous atmosphere (78% N₂, 21% O₂, and 0.03% CO₂). In this type of storage, the CO₂ and O₂ levels are not controlled under specified conditions.

Appropriate use of MA can supplement refrigerated storage for some products, which could translate into considerable reduction in post-harvest loss. Major benefits can be obtained from its use:

• Reduction in senescence associated with biochemical changes, such as reduction in respiration rate and ethylene production, softening, and compositional changes in fresh produce.

• Decreased sensitivity of fruit to ethylene action at levels of O2 and CO2 below 8% and 1%, respectively.

• Can relieve some physiological disorders such as cooling damage in a variety of products.

• Can have a direct or indirect effect on post-harvest pathogens and insect control.

Some disadvantages of MA include:

• Initiation of physiological damage, such as black spots in potatoes.

• Irregular maturation of certain fruits, such as bananas and tomatoes (O2 < 2% and CO2 > 5%).

• Abnormal development of flavours and odours at low O2 concentrations (anaerobic conditions).

• Increase susceptibility to diseases.

• Stimulation of sprouting and delay of epidermis development in roots and tubers (e.g., potatoes).

Table 5.1. Optimum refrigeration temperature, relative humidity, and shelf life for horticultural products.

(From: Flores Gutierrez, A.A, 2000)

5.2.4 Pickling

Vegetables can be macerated in a brine solution for pickling, which preserves the product for a long time. The high concentration of salts in the brine inhibits the growth of microorganisms that decompose and change the flavour, colour, and texture of vegetables. Vegetables can be maintained under maceration with a salt concentration of 6 to 10% during the first ten days of the pickling process. Then, the salt concentration is gradually increased to 16% for six weeks.

Under these conditions, vegetables can be kept in barrels for long periods until final processing. This involves washing the vegetables with water to release large amounts of salt (as much as possible), and packaging the product into glass jars with 5% vinegar and 3% salt. An alternative method is to precook vegetables at 80 to 90°C for 2 to 10 min. Then, packaging is performed using a blend of 3% salt, 6% vinegar, and 5% sucrose.

5.2.5 Fermentation

Vegetables can also be preserved by a fermentation process. During fermentation of raw vegetables, lactic acid bacteria develop, transforming the natural sugars present and the added sugar into acid. In general, a low salt concentration of 3-5% is used to prevent the growth of spoilage bacteria, while lactic acid bacteria are under development. The characteristic flavour and texture of fermented vegetables is produced by the action of lactic acid bacteria. Vegetables must be kept submersed in the liquid to prevent contact with air, which can cause decomposition, due to action of yeasts and moulds. During the fermentation process (2 to 3 weeks), the salt becomes diluted due to water drained from the vegetables, therefore salt must be frequently added to maintain the concentration at 3 to 5%. The pickled vegetables are washed with water and packaged into glass jars containing a solution of 3% salt and 5% vinegar. Vegetables can be pasteurized (or the liquid heated) and packages hot filled.

A typical formulation and application:

Lactic acid fermentation occurs when small amounts of salt are used. This allows bacteria to convert the sugar in vegetables to lactic acid; the acid mixed with salt inhibits the growth of other microorganisms that would cause major damage. This type of fermentation is used to prepare sauerkraut or sour cabbage, and in pickling cucumbers (pickles). Because salting is very softening, -both vegetables and salt are edible, thus preserving nearly all of the nutrients.

A typical application is in the preparation of sauerkraut:

1. Select good, mature cabbages; remove external leaves; wash remaining heads well.

2. With a sharp knife cut the heads into four sections, removing the hearts. Slice two and a half kilos of cabbage into fine strips approximately 2 to 3 cm long.

3. Put above cabbage in pot or plastic container and mix well, adding two tablespoons of salt. Let stand for 15 minutes or more, while preparing another batch of cabbage. The quantity of salt added must be in accordance with the amount of cabbage used for proper fermentation. While the cabbage is in repose, the salt works to reduce the lot size, extract the juice, and soften the cabbage. This will prevent breakage of strips during packaging.

4. The cabbage is packed into clean wide-mouth 4 L glass or plastic jars.

5. Eliminate air bubbles from the cabbage by pressing hard with hand. This allows juices to penetrate the tissues and holes formed between strips. Soft pressing is recommended to avoid breaking the finer strips.

6. Place plastic bag full of water on top of the cabbage to prevent air from penetrating the container and the cabbage. Close the jars tightly. After approximately 24 hours of fermentation, the juices should have completely covered the cabbage. Otherwise, add a brine solution composed of 25 g of salt per L of water until all cabbage strips are covered. The presence of bubbles is an indication that fermentation is in progress. This process lasts from 5 to 6 weeks or until the bubbles disappear from the solution, after which the fermented cabbage is heated in a pot until boiling.

7. Pack cabbage into sterile jars and cover with hot juice, leaving a head space of 2.5 cm below the jar's rim.

8. Place lids on each jar and sterilize the jars in a boiling water bath for 15 and 20 minutes for 0.5 L and 1 L jars, respectively.

5.3 Packaging methods

5.3.1 Plastic containers and bags

The plastic containers used to handle fresh and processed vegetables are tough, easy to handle due to light weight, can be reused, and facilitate the stacking of produce into piles without damaging the product. Initially, the cost of plastic containers and bags is high, but if protected from sun and extreme conditions, they can last for years. In developed countries various plastics are used at all stages of post-harvest packaging and processed fruits and vegetables. Polyvinylchloride (PVC) is used primarily for overwrapping, while polypropylene (PP) and polyethylene (PE), for bags, are the films most widely used for packaging minimally processed products.

Plastic bags are suitable for handling small amounts of vegetable products, and used at supermarkets and retail stores in developed and developing countries.

5.3.2 Vacuum packaging

Vacuum packaging extends the shelf life of vegetables for long periods. This technique relies on withdrawing air from the package with a suctioning machine. Removal of air retards the development of enzymatic reactions and bacterial spoilage. 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 can be established by the product's respiration. In addition to vacuum packing the specifics of handling must be taken into account, especially the time delays and temperature fluctuations.

5.3.3 Modified atmosphere packaging

For some products, such as fast respiring broccoli florets, impermeable barrier films with permeable membrane "patches" to modify the atmosphere through the product's respiration are used. It is not yet agreed as to which films and atmospheres are preferred for minimally processed products.

Modified atmosphere packaging (MAP), can be created passively by using proper permeable packaging materials, or by actively using a specified gas mixture, together with permeable packaging materials. The purpose of this procedure is to create an optimal gas balance inside the package, where the respiration activity of the product is as low as possible; on the other hand, the oxygen concentration is not detrimental to the product. In general, the objective is to have a gas composition of 2-5% CO₂, 2-5% O₂, and the rest nitrogen. One limitation in the design of control atmosphere packaging is in finding good permeable material that will match the respiration rate of the produce; only a few choices are available in the market. Most films do not result in optimal O₂ and CO₂ atmospheres in products with high respiration rates. This problem can be tackled by making micro holes of defined sizes and quantity in the material to prevent anaerobiosis. Another alternative is to combine ethylene vinyl acetate with oriented polypropylene and low-density polyethylene at a specified thickness. These materials have significantly higher permeability than the polyethylene or oriented polypropylene used in the packaging of salads, gas permeability should however be even higher. These materials have good heat-sealing properties and are commercially available.

High O₂ MAP treatment has been found to be particularly effective at inhibiting enzymatic browning, preventing anaerobic fermentation reactions, and inhibiting aerobic and anaerobic bacterial growth. The modified atmospheres that best maintain the quality and storage life of minimally processed products have been found to have an oxygen range of 2 to 8 percent and carbon dioxide concentration of 5 to 15 percent (Cantwell, 2001).

5.4 Transport, storage, and use of vegetables preserved by combined methods

5.4.1 Open vs. refrigerated vehicles

After harvest, open vehicles (trucks, tractors, trains, boats, ships, etc.) are used to transport the product to the packing houses and retail markets. These vehicles are not equipped with refrigeration units and thus the produce decays faster, compared to that in refrigerated transport. If the produce is treated with chemicals or additives after harvest, it can withstand longer distances in open vehicles, without noticeable damage, especially in cases where produce is consumed or processed upon reaching its final destination. Refrigerated vehicles (trucks, trains, ships, airplanes, etc.) contain installed refrigeration units with sufficiently low temperatures to maintain vegetables in a fresh-like state. These types of vehicles are hermetically sealed with insulation material inside the walls of the cave or container, which maintains the cooled product at maximum quality. Vegetables must be classified in order to separate those susceptible to cold temperatures (carrots, potatoes, bananas) and those that are not (tomatoes, peppers, eggplants, cucumbers, etc.). This eliminates the possibility of product damage when cooling at low temperatures during transport. Refrigeration temperatures can vary from 0°C (32°F) to 13°C (55.4°F) and RH from 70 to 95%. Maintaining a high RH in the refrigerated container is very important, as it prevents water loss and degradation in product appearance. This can be accomplished through strict control of the temperature. There is usually little or no environmental humidity control available during transport and marketing. Thus, the packaging must be designed to provide a partial barrier against movement of water vapour from the product. Plastic liners designed with small perforations to allow some gas exchange are one option.

5.4.2 Unloading

Unloading vegetables and fruits from vehicles is a very delicate operation and can be done by hand with a box tipper or with the aid of a forklift. Generally, vegetables and fruits are stacked on pallets to ease the unloading process and to prevent damage to the product. Exported crops arrive at the unloading port in bulk containers are unloaded directly into the storage container with the aid of conveyor belts connected from the vehicle to the container.

At village level a range of head packages and barrows are used to transfer crops from the field. Cushioned surfaces must still be used, however, to protect the crop when unloading.

5.4.3 Storage temperature vs. shelf life

As described in Table 5.1, recommended storage temperatures for vegetables can range from 0°C (32°F) to 13°C (55.4°F) with relative humidity between 70 and 95%; under these conditions shelf life can range from days to months. Controlled or modified atmosphere packaging techniques assist in maintaining adequate temperature control and relative humidity for refrigerated products. These systems can be used during transportation of fresh produce for short or prolonged storage periods. During pre-cooling of some vegetables, high levels of O₂ are utilized for shelf life extension. Recently, injection of CO2 (%) gas into controlled or modified atmosphere systems to control pathogens was carried out.

5.4.4 Repackaging considerations

Repackaging of vegetables is common when the product has been packaged in large containers, such as sacks, boxes, plastics containers, etc. The repackaging process is often carried out using small trays covered with transparent plastic film, which gives the product an appearance more appealing to consumers. Supermarkets and retail stores display packaged vegetables either on refrigerated shelves or under ambient conditions. Some retail stores and market places use open packages so that consumers can also handle the goods.

5.4.5 Optimal utilization of the final product

Optimal utilization of the final product can vary according to consumer demand. In some cases, the demand is for fresher products. Thus, the optimal utilization of fresh vegetables should be for direct consumption, with perhaps very small quantities remaining for processing or industrial uses. The latter usually occurs with seasonal crops when an abundance of fresh produce floods the market. The produce must meet official regulations concerning product safety and quality whether it is marketed as fresh or processed. Vegetables must be free of foreign matters, chemicals, and microbes that constitute a risk to human health. Therefore, good manufacturing practices (GMP) should be followed during handling, transport, and processing of vegetables for human consumption.

In developing countries, preservation of commodities represents a big problem for small farm crops because of the lack of adequate infrastructure to store harvested products. A novel alternative is to use combined methods technology for preserving large quantities of vegetables without using sophisticated equipment. To implement this preservation technology for stable vegetable products with high moisture content (HMVP), the following considerations should be addressed:

• Technology must be easy to use and be located near production centres.

• Technology must be cheap: does not require the use of sophisticated equipment or machinery, or the use of refrigeration or freezer storage.

• Resulting product must be of high quality: safe and tasty.

• Product must retain fresh-like characteristics.

• Product shelf life should be more than 30 days without refrigeration.

• Product may be commercialized as a final product or kept in storage containers for use as raw material in other processes.

Example of HMVP vegetable preparation:

The procedure used to obtain stable vegetable products was discussed in section 4.3 in the previous chapter and is illustrated in Figure 5.1. In this case, lettuce is selected according to size, weight and degree of maturity. Classification of lettuce is done according to microbiological quality, colour appearance and texture. External leaves are removed by hand and cutting of heads and removal of hearts are accomplished with a sharp knife. Lettuce is sliced into strips approximately 2 to 3 cm long, after which washing of slices is performed with chlorinated water (100-200 g/L) and drying by centrifugation. The finished product is packed into polyethylene-zipped bags and stored at 4-7°C for 2-4 weeks.

Figure 5.1 Schematic flow diagram to prepare lettuce salad.

5.5 Quality control

5.5.1 Recommended microbial tests

Control tests for microbial invasion in fresh vegetables must be assayed to analyze the growth of spoilage and pathogenic microorganisms. Total aerobic, psychrophile, and coliform bacteria counts are performed in standard plate count agar (SPC) and red violet bilis agar (VRBA). A series of dilutions are made in sterile 0.1% peptone and then pour plated onto SPC and VRB agars; plates for total aerobic and coliform bacteria are incubated at 35-37°C ± 2°C (95-98.6°F ±2°F) for 24/48 hours, and psychrophile bacteria at 7°C ± 2°C (45± 2°F ± 2°F) for 7 days, respectively. However, major contaminants and spoilage organisms in fresh vegetables and fruits or by-products, are moulds and yeasts. These organisms are counted by using potato dextrose agar (PDA) and poured plates, and are incubated at room temperature for 5 to 7 days.

5.5.2 Nutritional changes

Minimally processed vegetables retain nutritional and fresh-like properties because heat is not a major detrimental factor during processing. When using controlled or modified atmosphere packaging in combination with refrigerated storage, prolonged shelf life of vegetable products and retention of vitamins is favoured compared to thermally treated vegetables (e.g., canned vegetables), in which high amounts of nutrients are lost due to severe temperature treatment.

5.5.3 Changes in sensory attributes and acceptability

Since minimally processed vegetables resemble fresh produce, changes in sensory attributes and acceptability are minimized during processing. Thus, flavour, texture, and appearance are retained. Traditional food preservation processes involving high temperature treatments, freezing or dehydration produce an adverse effect, however, on the texture, flavour and aroma of processed food products.

The following factors are critical in maintaining the quality and shelf life of 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 to kill microorganisms; centrifugation to the point of complete water removal or even slight desiccation; packaging under a slight vacuum with some addition of CO₂ to retard discoloration; and maintaining product temperature from 1° to 2°C (34° to 36°F) during storage and handling. Temperature maintenance is currently recognized as the most deficient factor in the cool chain (Cantwell, 2001).

Other undesirable sensorial changes are a result of enzymatic activity in raw vegetable products. Two groups of enzymes are responsible for these changes:

Oxidative enzymes such as (Polyphenoloxidase, PPO, and peroxidase) in unprocessed vegetable and fruit products cause browning or other changes in colour. Changes in taste and flavour are caused by lipid oxidation due to the action of the enzyme lipoxygenase.

Hydrolytic enzymes cause softening of vegetable and fruit products (i.e., pectinecterase, and cellulase enzymes); and sweetening of vegetables and fruits by hydrolysis of the starch (amylases). Activity of such enzymes can be prevented by application of thermal treatment, but since the products are minimally processed, the use of heat is not a true option. One has to use other barriers to prevent changes in colour such as anti-browning agents (i.e., ascorbic acid) and anti-oxidizing agents (sulphites) as well as calcium salts to enhance texture firmness of vegetable tissues.