ASHRAE REFRIGERATION SI CH 37-2010 VEGETABLES《蔬菜》.pdf

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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 QUALITYMAINTENANCEThe 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 mechani

7、cal injuriesParasitic diseasesPhysiological disordersFreezing and chilling injuryFlavor and nutritional changesGrowth (sprouting, rooting)Ethylene-caused injuryFreshvegetablesarelivingtissuesandhaveacontinuingneedforO2for respiration. During respiration, stored food such as sugar isconverted to heat

8、 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 10 Mg load of asparagus cooled to 4Ccan produce enough heat of respiration during a cross-countr

9、y tripto melt 4 Mg 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 fruitvegetables (cucu

10、mber, 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, which will cause de

11、cay 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 increases. Ade-quat

12、e 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 counter, but remain gree

13、n at least3 to 4 days in a refrigerated display.Refrigeration can retard deterioration caused by chemical andbiological reactions. Freshly harvested asparagus will lose 50% ofitsvitamin C content in 1 day at 20C, whereas it takes 4 days at 10Cor 12 days at 0C to lose this amount (Lipton 1968). Recom

14、mendedconditions for long-term storage are listed in Table 1 of Chapter 21.Loss of moisturewithconsequentwilting and shrivelingis one ofthe obvious ways to lose freshness. Transpiration is the loss ofwater vapor from living tissues. Moisture losses of 3 to 6% areenough to cause a marked loss of qual

15、ity 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 perishable vegetables should bere

16、moved 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 are cumulative. Several s

17、mallbruises 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 mass as nonskinned ones.Use care

18、 in stacking bulk bins in storage, to maintain proper ven-tilation and refrigeration of the product. Bins should not be so deepthat excessive mass damages product near the bottom.Quality maintenance is further aided byHarvesting at optimum maturity or qualityHandling carefully to avoid mechanical in

19、juryHandling rapidly to minimize deteriorationProviding protective containers and packagingUsing preservative chemical, heat, or modified-atmospheretreatmentsEnforcing good plant sanitation procedures while handlingPrecooling to remove field heatThe preparation of this chapter is assigned to TC 10.9

20、, Refrigeration Appli-cation for Foods and Beverages.37.2 2010 ASHRAE HandbookRefrigeration (SI)Providing high relative humidity to minimize moisture lossProviding proper refrigeration throughout marketingCoolingRapid cooling of a commodity after harvest, before or after pack-aging or before it is s

21、tored 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 growth of decay organisms (and

22、 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 of prompt precooling may b

23、e 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 shipped, and compatibility with

24、 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 between layers to keepwei

25、ght 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 slowed. Special fiberboard

26、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, wraps, and film bags, which m

27、ay 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 helpful in reducing moisture lo

28、ss.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, cucumbers, mature green to

29、ma-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. Thin wax coatings may giv

30、e 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 (Harden-burg et al. 1959).IN-T

31、RANSIT 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. Many veg-etables benefit f

32、rom 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 loaded near container walls f

33、rom 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 to the dewpoint of the ou

34、tside 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,vegetablesthatrequirealow temperat

35、ureduringship-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 still warm. Highwaytrailers do not

36、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 as necessary, and serve as anapp

37、ropriate 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 of spacers to hold theload in alig

38、nment. 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 loads.When different types of boxes a

39、re 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, suchasfiberboard,shouldalwaysbeloadedontopofheavierwoodboxes.Providing Refrigeration and Air CirculationSafe transit temperatures for

40、various vegetables are given inTable 1. For safety, the suggested thermostat settings for cool-season vegetables are usually 1 to 2 K above the freezing point.Perishables are often shipped in loads with other commodities.When this occurs, the loads should be set up so that different typesof produce

41、have compatible temperature ranges and ethylene sensi-tivities.UseTable2,whichgroupscommonfruitsandvegetablesbytemperaturerangeandethylenesensitivity,toselectcompatiblepro-duce. Produce in the same column can be safely held at the sametemperature range. Mixing produce from different temperaturerange

42、s 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 produce in the table, and should be held in a 50 to 70% rh en

43、vi-ronment to prevent decay. Most of the vegetables in the lowest tem-peraturerange(0to2C)aresensitivetomoisturelossandshouldbeheld at more than 90% rh or packaged to minimize water loss. Othervegetables and fruit should be held at 85 to 95% rh.Ethylene-sensitive vegetables (row 2) should not be mix

44、ed withethylene-producing fruits (row 5). If, for some reason, they must bemixed,damagemaybereducedbyusingafresh-airexchangerateof20 L/s (Thompson et al. 2000) and/or ethylene scrubbers. In somecases, a controlled atmosphere (CA) will allow ethylene-sensitiveproduce to be shipped with ethylene-produ

45、cing 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 produce aboveor below them in the same temperature column.Some produce can exchange odor

46、s 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. For example, asparagus has a maximum postharvest life of 3weeks at 2.5C. If shipped at 0C

47、 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 transport time. If a CAenvironment is used, it should, at a minimum, not reduce the post-harv

48、est 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 desired trans-port temperature. Generally, the shipper or the receiver is responsi-ble f

49、or 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-atedtransportvehicles,equippedtohandlethefullrangeofbothfreshand 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 circulated in the cooling coils). The change from cool-ing to heating is done automatically by thermos