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«Fresh-Cut Vegetables M. Margaret Barth, Hong Zhuang, and Mikal E. Saltveit Barth and Zhuang are with Redi-Cut Foods, Inc., Franklin Park, IL; ...»

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Fresh-Cut Vegetables

M. Margaret Barth, Hong Zhuang, and Mikal E. Saltveit

Barth and Zhuang are with Redi-Cut Foods, Inc., Franklin Park, IL; Saltveit is with Mann Lab,

Department of Vegetable Crops, University of California, Davis, CA.

Introduction

Wounds inflicted during the preparation of fresh-cut vegetables promote many physical and

physiological changes that hasten loss of product quality (Brecht 1995, Saltveit 1997). Foremost

among these are the removal of the protective epidermal layer and exposure of internal cells.

These changes not only facilitate water loss but also provide easy entry for microbial pathogens and chemical contaminants. Packaging or application of edible films can lessen water loss by maintaining a high RH near the cut surface and providing a physical barrier that protects the product from contamination.

Water loss and collapse of injured cells at the cut surface can alter the appearance of the freshcut product. As the cut surface loses water, adhering cellular debris may impart a white blush to the surface that masks varietal color; for example, white blush on “baby” carrots decreases the intensity of the underlying orange color. Differential dehydration of exposed cortex and vascular tissue may produce an uneven surface, as with formation of vascular strands projecting a few millimeters from the cut end of celery petioles. Consumers associate both of these surface changes with the loss of freshness.

Physiological processes devoted to wound repair can be beneficial (such as curing potatoes) or detrimental to quality retention. Physiological changes following wounding include increased respiration and ethylene production, promotion of the ripening of climacteric fruit vegetables (such as melons and tomatoes), and enhanced synthesis and accumulation of phenolic compounds that contribute to tissue browning. These changes are managed by the use of low temperatures, creation of reduced O 2 or elevated CO2 atmospheres, and application of inhibitors of specific chemical reactions or metabolic pathways.

Wound-enhanced respiration and ethylene production can deplete carbohydrate storage reserves and stimulate tissue softening associated with fruit ripening (melons, squash, tomatoes) or chlorophyll loss associated with leaf or tissue senescence (spinach, broccoli). In many tissues, wounding produces a signal that induces the increased synthesis and accumulation of phenolic compounds. Tissue browning can result from the oxidation or polymerization of accumulated phenolic compounds. Lignification and toughening of tissue are others way in which the metabolism of phenolic compounds can reduce quality.

Sanitation, implementation of Good Agricultural Programs (GAPs) in the field, use of Good Manufacturing Programs (GMPs) and Hazard Analysis and Critical Control Point (HACCP) monitoring plans during all steps of processing, and proper temperature control, are all needed to ensure that an initial low microbial load on fresh-cut vegetables is maintained during marketing.

The current industry standard washing and disinfection procedures used on whole produce are rather ineffective with fresh-cut vegetables once they have been contaminated with microbial pathogens. Overall microbial load relative to spoilage organisms usually remain below levels of concern as long as visual quality remains acceptable. However, postharvest treatments that maintain visual quality under abusive temperatures may allow microbial loads to reach dangerous levels before quality is reduced below acceptable levels.

Many vegetables are sensitive to nonfreezing temperatures below about 10 °C (50 °F) and suffer physiological damage if held at these chilling temperatures beyond a specific period. These vegetables include jicama, pepper, sweet potato, tomato, and zucchini. In contrast to the whole commodity, which should not be chilled, the increased susceptibility of fresh-cut products to spoilage often means that the product has a longer market life at 0 °C (32 °F) than at higher, nonchilling temperatures. Development of chilling injury symptoms is less pronounced in riper tissue and takes some time to develop. Since most fresh-cut vegetables are consumed soon after purchase and fruit vegetables are fully ripe when processed, chilling injury symptoms usually do not develop to a significant extent because the rate of deterioration due to chilling is slower than the rate of spoilage that is inhibited by low temperatures. The two most important fresh-cut commodities, lettuce and carrots, are not chilling sensitive and should be stored as close to 0 °C (32 °F) as possible without freezing: 1 to 3 °C (34 to 38 °F) in a commercial setting.

The imposed physical damage done to the product during preparation and its increased vulnerability to deleterious internal and external changes requires that fresh-cut vegetables be handled with a greater degree of care than the whole product. Storage requirements for intact produce may be inadequate to handle increased physiological activity and susceptibility to water loss encountered with fresh-cut vegetables.

The highest quality that is available (usually USDA No. 1) and that is compatible with an economic return should be used for processing. Although poor quality pieces of fresh-cut product can be discarded to upgrade quality (an example is culling discolored pieces of cut lettuce leaves or broccoli florets), the associated cost usually adversely affects quality and yields and greatly outweighs the differential cost of using higher quality raw material.





The remainder of this chapter provides information on the commercial packaging and storage of a number of fresh-cut vegetables.

Beets, Red (Grated, Cubed, Whole Peeled) Fresh-cut beets should be stored at 1 to 3 °C (34 to 38 °F) before and after processing.

Respiration is slightly reduced during storage in 5% O2 and 5% CO2 at 5 °C (41 °F).

–  –  –

To get mL CO2 kg-1 h-1, divide the mg kg-1 h-1 rate by 2.0 at 0 °C (32 °F), 1.9 at 10 °C (50 °F), and 1.8 at 20 °C (68 °F). To calculate heat production, multiply mg kg-1 h-1 by 220 to get BTU ton-1 day-1 or by 61 to get kcal tonne-1 day-1.

Broccoli (Florets) Fresh-cut broccoli florets should be tight, firm, turgid, and dark green without blooming buds.

There should be no sulfur odor or discoloration along the stems and cut ends. The core temperature of raw material should be 1.5 °C (35 °F). Raw and processed material should be stored at 1 to 3 °C (34 to 38 °F) to ensure quality and reduce potential for freezing of product during handling, distribution, and storage. Yellowing is a common problem caused by either chlorophyll loss or blooming of the buds. The cut surface and damaged floret stems can turn black during storage. Development of off odors can be a major concern when MAP is used.

Temperature abuse promotes soft rot and mold growth.

Whole broccoli heads are hand-cut into florets that are between 2.5 cm (1 in) and 5 cm (2 in) long. They are washed in water containing up to 200 µL L-1 total chlorine to wash residual material from the florets as well as reduce aerobic plate counts. Repeated daily washing did not maintain lower microbial counts for 2 days.

The benefit of CA (5% O2 and 4% CO2) may be marginal for storage up to 14 days at 0 to 5 °C (32 to 41 °F) compared to air. Lowering O2 to 0.25% or increasing CO2 to 10% at 0 to 5 °C (32 to 41 °F) reduces respiration by about 50%. Use of appropriate polymeric film in MAP maintained green color at 0 to 5 °C (32 to 41 °F) for more than 21 days (Cabezas and Richardson 1997). Severe off odors and discoloration at cut ends can develop during MAP storage at 10% CO2 and 2.5% or less O 2 (Makhlouf et al. 1989). Perforated and microperforated polymeric packages reduce off odor (Izumi et al. 1996a). Reducing ethylene below 1 to 10 µL L -1 did not significantly reduce color loss at 1 °C (34 °F), but did have an effect at higher temperatures.

No food-borne disease outbreak has been reported associated with fresh-cut broccoli, although the aerobe population is usually high (100,000 cfu g-1 fresh weight). Florets stored in 5% O2 and 8% CO2 at 8 °C (46 °F) have lower aerobic plate counts, total coliform plate counts, and yeast/mold plate counts compared to storage in air.

–  –  –

Cabbage, Green and Red (Shredded, Diced) Fresh-cut green cabbage should be light green with a moderately pungent flavor and no sulfur aroma. Raw and processed material should have typical cabbage flavor with no off notes and should be stored at 1 to 3 °C (34 to 38 °F) to ensure quality and reduce potential for freezing of product during handling, distribution, and storage. Fresh-cut cabbage includes diced and shredded product with cut size varying from 0.63 cm (¼ in) to 0.95 cm (3/8 in). Heads are trimmed to remove wrapper leaves and cored, cut, and washed using chlorinated water (100 µL L-1 of total chlorine) for about 1 min before being spun-dried and packaged. For coleslaw, cut cabbage (shredded or diced) can be preblended with carrots or plain-packaged.

A CA of 5 to 7.5% O2 and 15% CO2 is recommended (Hiroaki et al. 1993). Lowering O2 below 5% caused rapid proliferation of fermentative bacteria and off odors within 6 days at 5°C (41 °F). The fermentation induction point of coleslaw mixes in low density polyethylene bags varied with the cabbage to carrot ratio and with temperature. At 5 °C (41 °F), a 70:30 mix went anaerobic at 1.8% O2, while 3% O2 was the limit for aerobic respiration at 10°C (50 °F).

Off odors forming in cut cabbage bags with low OTR film (3,000 mL/m2 atm-1 d-1) and discoloration of cabbage leaf packed in high OTR film (12,000 mL/m2 atm-1 d-1) bags are the major quality deterioration during storage (Pirovani et al. 1997). At 11 °C (52 °F), Listeria spp.

grew faster in fresh-cut cabbage packages which have an atmosphere of 1.8% O2 and 20% CO2 than in air (Omary et al. 1993).

–  –  –

To get mL CO2 kg-1 h-1, divide the mg kg-1 h-1 rate by 2.0 at 0 °C (32 °F), 1.9 at 10 °C (50 °F), and 1.8 at 20 °C (68 °F). To calculate heat production, multiply mg kg-1 h-1 by 220 to get BTU ton-1 day-1 or by 61 to get kcal tonne-1 day-1.

Cabbage, Chinese (Sliced, Sticks, Shredded) Fresh-cut Chinese cabbage should be handled like cabbage and stored at 1 to 3 °C (34 to 38 °F).

The recommended CA is 5% O2 and 5% CO2 ; benefits are moderate compared to air storage at 0 °C (32 °F).

Carrots (Diced, Shredded, Sticks, Peeled, Grated, Sliced, Cubed) Fresh-cut carrots are orange without a white brush or slimy surface. Raw and processed product should be kept at 1 to 3 °C (34 to 38 °F) to ensure quality and reduce potential for freezing of product during handling, distribution, and storage. Most quality loss in cut carrots results from formation of either white blush or off odor and slimy surface generated by bacteria (Carlin et al.

1989).

Fresh-cut carrots include whole peeled (baby), sticks, sliced, shredded, grated, and diced. Whole carrots are washed with water to remove undesirable field material. The stems and tips are excised and the trimmed carrots are peeled, cut, and washed in 100 µL L-1 NaOCl for less than 1 min. Washed carrot cuts are centrifuged to remove excess water and packaged in plastic bags.

Dehydration of surface debris on cut and peeled carrots imparts a whitish translucent appearance to the surface (Tatsumi et al. 1991, Cismeros-Zevallo et al. 1995), which is undesirable because consumers associate it with the loss of freshness (Bolin and Huxsoll 1991). Applying an edible coating (Howard and Dewi 1995, 1996, Li and Barth 1998), such as sodium caseinate-stearic acid (Avena-Bustillos et al. 1994), or heating and raising the pH (Bolin and Huxsoll 1991) may be helpful in reducing white blush. Treatments that modify the water-retaining capacity of the cut surface also prevent white blush development (Cisneros-Zevallos et al. 1997).

Fresh-cut carrots derive slight benefit from 2 to 5% O2 and 15 to 20% CO2 atmospheres (Izumi et al. 1996b). Lower O2 or increased CO2 levels promoted slimy appearance, increased lactic acid bacteria growth, and accelerated microbial decay and excessive alcohol production. Grated carrots retain good quality for up to 10 days at 2 to 10 °C (36 to 52 °F) in MAP with high O2 permeability films of 10,000 to 20,000 mL m2 atm-1 d-1 at 25°C (Carlin et al. 1990). Low permeability films of 950 mL m2 atm-1 day-1 resulted in low O2 damage.

The quality and headspace composition of sliced carrots (0.5 cm) treated with 1% ascorbic acid before packing in MAP was unaffected during 14 days at 4 °C (39 °F) (Galetti et al. 1997).

Growth of aerobic mesophilic bacteria on sticks was suppressed by a 0.5% O2 and 10% CO2 atmosphere at both 0 and 5 °C (32 and 41 °F); but total microbial count on slices and shreds was unaffected.

The mean microbial population after 9 days storage was much lower (1,300 cfu g-1) for irradiated shredded carrots (0.5 kGy) than for non-irradiated, chlorinated controls (87,000 cfu g-1) (Hagenmaier and Baker 1998). Ethanol and O2 content of the headspace were not affected.

–  –  –

To get mL CO2 kg-1 h-1, divide the mg kg-1 h-1 rate by 2.0 at 0 °C (32 °F), 1.9 at 10 °C (50 °F), and 1.8 at 20 °C (68 °F). To calculate heat production, multiply mg kg-1 h-1 by 220 to get BTU ton-1 day-1 or by 61 to get kcal tonne-1 day-1.



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