1、37.1CHAPTER 37VEGETABLESProduct Selection and Quality Maintenance 37.1In-Transit Preservation. 37.2Preservation in Destination Facilities 37.3Refrigerated Storage Considerations 37.5Storage of Various Vegetables 37.6OSSES (shrinkage) in marketing fresh vegetables (harvesting,Lhandling, packing, stor
2、ing, and retailing) are caused, in part, byoverly high temperatures during handling, storage, and transport,which increase ripening, decay, and the loss of edible quality andnutrient values. Some cases may involve freezing or chilling injuryfrom overly low temperatures. Other serious losses are caus
3、ed bymechanical injury from careless or rough handling and by shrinkageor wilting because of moisture loss. Shrinkage can be reduced sub-stantially by following recommended handling, cooling, transport,and storage practices. Improved packaging, refrigerated transport,and awareness of refrigerations
4、role in maintaining quality through-out marketing have made it possible to transport vegetables in field-fresh condition to distant markets.This chapter covers postharvest handling, cooling, packaging,in-transit preservation, and storage at destination locations for freshvegetables. It also gives st
5、orage requirements for specific vegeta-bles, including potential product deterioration due to improper han-dling and storage conditions. Vegetable precooling is covered inChapter 28, and vegetable processing and freezing in Chapter 40.Chapter 21 also provides storage requirements for many types ofve
6、getables.PRODUCT SELECTION AND QUALITY MAINTENANCEThe principal hazards to quality retention during marketingincludeMetabolic changes (composition, texture, color) associated withrespiration, ripening, and senescence (aging) Moisture loss with resultant wilting and shrivelingBruising and other mecha
7、nical injuriesParasitic diseasesPhysiological disordersFreezing and chilling injuryFlavor and nutritional changesGrowth (sprouting, rooting)Ethylene-caused injuryFresh vegetables are living tissues and have a continuing need forO2for respiration. During respiration, stored food such as sugar isconve
8、rted to heat energy, and the product loses quality and foodvalue. In maintaining commodity temperatures during storage ortransportation, some of the refrigeration load can be attributed torespiration. For example, a 20,000 lb load of asparagus cooled to39F can produce enough heat of respiration duri
9、ng a cross-countrytrip to melt 7900 lb of ice.Vegetables that respire the fastest often have greater handlingproblems because they are the most perishable. Variations arecaused by the type of plant part involved. For example, root cropssuch as carrots and radishes have lower respiration rates than f
10、ruitvegetables (cucumber, pepper) and sprouts (asparagus). Refrig-eration is the best method of slowing respiration and other lifeprocesses. Chapters 19 and 21 give more information on the respi-ration rates of many vegetables.Vegetables are usually covered with natural populations ofmicroorganisms,
11、 which will cause decay under the right conditions.Deterioration from decay is probably the greatest source of spoilageduring marketing. When mechanical injuries break the skin of theproduce, decay organisms enter. If it is then exposed to warm (espe-cially warm, humid) conditions, infection usually
12、 increases. Ade-quate refrigeration is the best method of controlling decay becauselow temperatures control growth of most microorganisms.Many color changes associated with ripening and aging can bedelayed by refrigeration. For example, broccoli may show yellow-ing in 1 day on a nonrefrigerated coun
13、ter, but remain green at least3 to 4 days in a refrigerated display.Refrigeration can retard deterioration caused by chemical andbiological reactions. Freshly harvested asparagus will lose 50% of itsvitamin C content in 1 day at 68F, whereas it takes 4 days at 50For 12 days at 32F to lose this amoun
14、t (Lipton 1968). Recommendedconditions for long-term storage are listed in Table 1 of Chapter 21.Loss of moisture with consequent wilting and shriveling is one ofthe obvious ways to lose freshness. Transpiration is the loss ofwater vapor from living tissues. Moisture losses of 3 to 6% areenough to c
15、ause a marked loss of quality for many kinds of vegeta-bles. A few commodities may lose 10% or more in moisture and stillbe marketable, although some trimming may be necessary, such asfor stored cabbage. For more on transpiration, see Chapter 19.Postharvest HandlingAfter harvest, most highly perisha
16、ble vegetables should beremoved from the field as rapidly as possible and refrigerated, orthey should be graded and packaged for marketing. Because agingand deterioration continue after harvest, marketable life dependsgreatly on temperature and care in physical handling.The effects of rough handling
17、 are cumulative. Several smallbruises on a tomato can produce an off-flavor. Bruising also stimu-lates the ripening rate of products such as tomatoes and therebyshortens potential storage and shelf life. Mechanical damage in-creases moisture loss; skinned potatoes may lose 3 to 4 times asmuch weight
18、 as nonskinned ones.Use care in stacking bulk bins in storage, to maintain proper ven-tilation and refrigeration of the product. Bins should not be so deepthat excessive weight damages product near the bottom.Quality maintenance is further aided byHarvesting at optimum maturity or qualityHandling ca
19、refully to avoid mechanical injuryHandling rapidly to minimize deteriorationProviding protective containers and packagingUsing preservative chemical, heat, or modified-atmosphere treatmentsEnforcing good plant sanitation procedures while handlingPrecooling to remove field heatThe preparation of this
20、 chapter is assigned to TC 10.9, Refrigeration Appli-cation for Foods and Beverages.37.2 2010 ASHRAE HandbookRefrigerationProviding high relative humidity to minimize moisture lossProviding proper refrigeration throughout marketingCoolingRapid cooling of a commodity after harvest, before or after pa
21、ck-aging or before it is stored or moved in transit, reduces deteriorationof the more perishable vegetables. The faster field heat is removedafter harvest, the longer produce can be maintained in good market-able condition. Cooling slows natural deterioration, including agingand ripening; slows grow
22、th of decay organisms (and thereby thedevelopment of rot); and reduces wilting, because water lossesoccur much more slowly at low temperatures than at high tempera-tures. After cooling, produce should be refrigerated continuously atrecommended temperatures. If warming is allowed, much of thebenefit
23、of prompt precooling may be lost.Types of cooling include hydrocooling, vacuum cooling, aircooling, and cooling with contact ice and top ice, which are dis-cussed in detail in Chapter 28. The choice of cooling method de-pends on factors such as refrigeration sources and costs, volume ofproduct shipp
24、ed, and compatibility with the product.Protective Packaging and WaxingVegetables for transit and destination storage should be packedin containers with adequate stacking strength and durability toprotect against crushing under high humidity. Bulging cratesshould be stacked on their sides or stripped
25、 between layers to keepweight off the commodity. Many vegetables are stored or shippedin corrugated fiberboard containers, but fiberboard weakening bymoisture absorption at high storage humidities is frequently a seri-ous problem.Fiberboard strength has improved, and its rate of moistureabsorption s
26、lowed. Special fiberboard treatments allow use of somecartons in hydrocooling and with package and top ice. Cartons maybe strengthened for stacking using dividers, wooden corner posts,and full telescoping covers.Produce is often consumer-packaged at production locationsusing many types of trays, wra
27、ps, and film bags, which may presentspecial transit and handling problems when master containers forthese packages lack stacking strength.Desiccation often can be minimized by using moisture-retentiveplastic packaging materials. Polyethylene film box liners, palletcovers, and tarpaulins may be helpf
28、ul in reducing moisture loss.Plastic films, if sealed or tightly tied, may restrict transfer of CO2,O2, and water vapor, leading to harmful concentrations of theserespiratory gases; films also restrict heat transfer, which retards therate of cooling (Hardenburg 1971).Waxes are applied to rutabagas,
29、cucumbers, mature green toma-toes, and cantaloupes, and to a lesser extent to peppers, turnips,sweet potatoes, and certain other crops. With products such ascucumbers and root crops, waxing reduces moisture loss and shriv-eling. With some products, an improved glossy appearance is themain advantage.
30、 Thin wax coatings may give little if any protectionagainst moisture loss; coatings that are too heavy may increasedecay and breakdown. Waxing alone does not control decay, butwaxing combined with fungicides may be beneficial. Waxing is notrecommended for potatoes either before or after storage (Har
31、den-burg et al. 1959).IN-TRANSIT PRESERVATIONGood equipment is available to transport perishable commodi-ties to market under refrigeration by rail, trucks, piggyback trailers,and containers. High relative humidity (about 95% rh) is desirablefor most vegetables to prevent moisture loss and wilting.
32、Many veg-etables benefit from 95 to 100% rh. Humidity in both iced andmechanically refrigerated cars and trailers is usually high.Cooling Vehicle and ProductVehicles used to ship vegetables that require low transit temper-atures should have their interiors cooled before loading to preventproduce loa
33、ded near container walls from warming under hot ambi-ent conditions or cooling too much under cold ambient conditions.In temperate climates with low humidity, the container should becooled to the carrying temperature. If loading from an open dock ina humid environment, the container should be cooled
34、 to the dewpoint of the outside air. Temperatures below the dew point causewater to condense on walls, which may damage fiberboard pack-ages. In all cases, refrigeration should be turned off when containerdoors are open, to prevent moisture from condensing on the evapo-rator coils.Generally, vegetab
35、les that require a low temperature during ship-ping should be cooled before they are loaded into transport vehicles.Cooling produce in tightly loaded refrigerator cars or containers isslow, and the portion of the load exposed to the cold air dischargemay be frozen when the interior of the load is st
36、ill warm. Highwaytrailers do not have adequate airflow to remove field heat from per-ishable produce.Packaging, Loading, and HandlingIn this section, the term “boxes” is used for corrugated containersand “containers” is used for marine containers. Boxes must protectthe commodity, allow heat exchange
37、 as necessary, and serve as anappropriate merchandising unit with sufficient strength to withstandnormal handling. Freight container tariffs describe approved con-tainers and loading procedures.Boxes should be loaded to take advantage of their maximumstrength and to allow adequate stripping or use o
38、f spacers to hold theload in alignment. Proper vertical alignment of containers is essen-tial to obtain their maximum stacking strength. Previously, whenboxes were hand-stacked, stacks were spaced to provide channelsfor air circulation. Air channels are no longer specified with the useof pallet load
39、s.When different types of boxes are used in the same load, stacksshould be separated so that one type will not damage another. If sep-aration of stacks is impossible, boxes made of lighter material, suchas fiberboard, should always be loaded on top of heavier wood boxes.Providing Refrigeration and A
40、ir CirculationSafe transit temperatures for various vegetables are given inTable 1. For safety, the suggested thermostat settings for cool-season vegetables are usually 2 to 4F above the freezing point. Perishables are often shipped in loads with other commodities.When this occurs, the loads should
41、be set up so that different typesof produce have compatible temperature ranges and ethylene sensi-tivities. Use Table 2, which groups common fruits and vegetables bytemperature range and ethylene sensitivity, to select compatible pro-duce. Produce in the same column can be safely held at the sametem
42、perature range. Mixing produce from different temperatureranges can compromise produce quality, especially with longertransit times. The greater the difference in recommended tempera-tures, the greater the potential for quality loss.Dry vegetables (row 1 of Table 2) should not be mixed with anyother
43、 produce in the table, and should be held in a 50 to 70% rh envi-ronment to prevent decay. Most of the vegetables in the lowest tem-perature range (32 to 36F) are sensitive to moisture loss and shouldbe held at more than 90% rh or packaged to minimize water loss.Other vegetables and fruit should be
44、held at 85 to 95% rh.Ethylene-sensitive vegetables (row 2) should not be mixed withethylene-producing fruits (row 5). If, for some reason, they must bemixed, damage may be reduced by using a fresh-air exchange rate of45 cfm (Thompson et al. 2000) and/or ethylene scrubbers. In somecases, a controlled
45、 atmosphere (CA) will allow ethylene-sensitiveproduce to be shipped with ethylene-producing produce, but theVegetables 37.3acceptable produce combinations and atmosphere prescriptions arenot well documented. Produce that neither is sensitive to norproduces ethylene (rows 3 and 4) can be mixed with p
46、roduce aboveor below them in the same temperature column.Some produce can exchange odors with other items. See thenotes at the bottom of Table 2 for precautions.Certain produce have a short postharvest life and are not suitedfor container shipments, especially if held at nonoptimal tempera-tures. Fo
47、r example, asparagus has a maximum postharvest life of 3weeks at 36F. If shipped at 32F in a load of broccoli, it is subjectto chilling injury after only 10 days. Modified-atmosphere (MA)packaging can sometimes increase produce life and allow shippingto destinations that require several weeks transp
48、ort time. If a CAenvironment is used, it should, at a minimum, not reduce the post-harvest life of any of the mixed commodities.Table 2 also applies to fruits covered in Chapter 35.With the many kinds of refrigeration, heating, and ventilatingservices available, the shipper has only to specify the d
49、esired trans-port temperature. Generally, the shipper or the receiver is responsi-ble for selecting the protective service for the commodity in transit.Protective services are described in detail in USDA (2000).Protection from ColdIn winter, vegetables must be protected from freezing. Refriger-ated transport vehicles, equipped to handle the full range of both freshand frozen commodities, are also designed to provide heat for cold-weather protection. Heat is supplied by electric heating elements orby reverse-cycle operation of the refrigerating unit (hot gas from thecompressor is
