Effect of Cooling Delays on Fruit and Vegetable Quality

February 2001 Perishables Handling Quarterly Issue No. 105 Page 1

Effect of Cooling Delays on Fruit and Vegetable QualityJim Thompson, Department of Biological & Agricultural Engineering, UC Davis; Marita Cantwell, Dept. of

Vegetable Crops, UC Davis, Mary Lu Arpaia, Dept. of Botany and Plant Science, UC Riverside; Adel Kader, Dept.of Pomology, UC Davis; Carlos Crisosto, Dept. of Pomology, UC Davis; Joe Smilanick, USDA-ARS, Fresno

Californias electricity interruptions will most likely lastfor one to two hours and occur from noon to 6 PM.Power interruptions will not stop picking and fieldpacking operations but they will halt packinghouse andrefrigeration equipment during a critical time of day.Because quality begins to drop for most perishablesright after harvest, the common recommendation is tobegin cooling as soon as possible after harvest. Shouldpacking and cooling operations invest in expensivestandby generation to power their cooling operationsduring an outage or is it possible to increase the intervalbetween harvest and cooling and not cause noticeabledamage to produce? The answer is not simple anddepends on the commodity, weather conditions, andother handling decisions.

Cooling delays cause reduced product quality forthree main reasons: 1) allowing respiration and associ-ated normal metabolism to continue at high rates,consuming sugars, acids, vitamins, and other constitu-ents, 2) fostering water loss, and 3) increasing decaydevelopment. Delays may also allow increased suscep-tibility to ethylene damage, but ethylene concentrationsare usually low near cooling facilities and ethylene doesnot usually cause as much damage as the other threefactors. Delays can also cause undesirable productcurvature and growth, but this is a problem with only afew commodities such as asparagus and green onions.

RespirationFresh produce consumes photosynthates that were

stored in the product before harvest. Consumption ratedepends on the respiratory activity of a particularcommodity and its temperature. Commodities such asapples, cabbage, citrus, potatoes and table grapes havelow respiration rates compared with avocados, mush-rooms, asparagus and sweet corn. At temperaturesabove 70 to 75F (21 - 24C), respiration is especiallyhigh. For example at 90F (32C), asparagus respiresthree times faster than at 60F (16C). Temperaturesabove 90F (32C) may cause ripening disorders in

commodities such as tomatoes and most tree fruits.Exposure to high temperatures and to direct sunlight cancause sunburn and sun scald injury. Cooling productsfrom high summer field temperatures to room tempera-ture, 70 to 75F (21 - 24C), significantly slowsrespiration.

Increased respiration caused by a few hours ofcooling delay rarely causes a noticeable effect onexternal product quality. The consumer may detectslightly poorer eating quality and appearance and theproduct may have lost some nutritional value, but it isoften very salable. Several tests with cantaloupe melonshave shown that cooling delays up to 12 hours, evenwith afternoon harvested product, cause no consistentloss of visual appearance or soluble solids. Howeverdelays do cause measurable effects on quality and manyproduce items may not be suitable for long-distancetransport or long-term storage if exposed to significantcooling delays. For example, sweet corn begins losingsweetness immediately after harvest and the rate of lossincreases with increasing temperature. If cooling isdelayed sweet corn should be sold quickly.

A few commodities such as peaches, nectarines,plums, Bartlett pears, and tomatoes are intentionallyheld at 60 to 75F (16- 24C) after packing to pro-mote ripening. If kept at a relative humidity above85%, these commodities will actually improve in qualityin a controlled ripening process. But they must becooled after ripening has been started to slow furtherripening during transport and handling. Lots prone todecay may experience increased decay losses because ofripening conditions. Although this can be considered anadvantage in some situations because decayed productcan be removed before shipping.

Water lossShriveling and the loss of fresh, glossy appearance

are two of the most noticeable effects of cooling delays,particularly for commodities that lose water quickly and

During the last year packinghouse operators andfarmers have seen significant increases in the cost ofnatural gas, propane and diesel. The cost of these fuelshas more than doubled and electricity prices are also

expected to rise before the season starts. Managers arelooking for economical ways to cut their energy use.The last feature article outlines ways of reducing energyconsumption in packing operations.

Jim Thompson


show visible symptoms at low levels of water loss, likemost leafy vegetables. Air temperatures 70-75F (21-24C) have a particularly great effect on rate of waterloss. For example, Thompson seedless grapes showvisible symptoms of water loss (stem shriveling) at 3%or less. To prevent the consumer from seeing shrivel,moisture loss in the field should be kept less than 1%.At 90F, a 1% loss can occur in about 2 hours, but at70F cooling can be delayed up to 12 hours.

Moisture loss is slowed by holding produce inplastic liners. Liners should be vented to preventtemperature rise caused by product respiration anddamaging levels of carbon dioxide. Solid bin coversalso act as a moisture barrier. Some produce items cantolerate water contact, and spraying them with waterslows product moisture loss and can even rehydrateslightly wilted produce. However water remaining onthe surface tends to increase decay development andwater should be clean and sanitized. Many leafyvegetables can completely recover from less than a fewpercent water loss by contact with hydrocooler water.If hydrocooling is not available, many vegetables can besprayed with water before forced air or vacuum coolingto regain some water and reduce loss during subsequentcooling.

Water loss is not always a detriment. Some prod-ucts, like carrots, need some turgidity loss to reducemechanical damage in handling and fresh-cut process-ing. Slight water loss in green onions reduces curvatureproblems.

DecayCooling delays tend to increase decay losses,

although decay becomes apparent many days after thecooling delay. Damage can be minimized by applyingdecay control treatments within a reasonable time afterharvest. For example table grapes should fumigatedwith sulfur dioxide within 12 hours after harvest. Theprocess can be done with warm fruit and cooling delaysmay cause cold storage operators to set up facilities forfumigation separate from their forced air cooler.Another option for dealing with cooling delays ingrapes to field-pack them in plastic liners and a sulfurdioxide generator pad. The liner will slow water lossand the pad will control Botrytis decay.

Free water together with high humidity speedsdecay development in tree fruits and berries. Forexample at room temperatures, only four hours ofcontact with free water allows brown rot to penetrate

fruit tissues. Decay-prone fruits and vegetables shouldbe protected from prolonged water contact and veryhigh humidity during cooling delays.

Recommendations for minimizing damage caused bycooling delays:

1. Use the following tables as a guide for acceptabletime between harvest and the start of cooling. It isbased on typical ambient conditions in Californiaduring normal harvest periods. Colder than normalair temperatures may allow longer delays in cooling.If the delay time added to the typical time betweenharvest and the beginning of cooling is greater thanthe allowable delay, then backup generation orrefrigeration may be needed.

2. Protect produce from temperatures above 70 -75F (21 - 24C). Start harvest as early as possiblein the morning. On days that are predicted to beespecially warm, harvesting may need to be haltedwhen air temperatures exceed about 85F (29C).

Underground vegetables are at a temperature equalto the surrounding soil temperature, which isusually much lower than daytime air temperature.Shaded fruit temperature is usually within a fewdegrees of air temperature, but product exposed tosunlight can be 7 to 11F (4 - 6C) warmer than airtemperature. Shade produce after harvest toprevent excess temperature rise and sunburn andsun scald damage. Use shaded receiving areas.Even placing an empty picking box on top of afilled box protects product from temperature gain.

Portable generators and lights allow harvest to startat 3 or 4 AM and finish before the heat of theafternoon. Night harvest has been used to a limitedextent in California with melons and tables grapes.Mechanically harvested crops are usually welladapted to night harvest. Night harvest has theadded benefit of allowing the commodity to begin oreven finish cooling before the likely times of powerinterruption.

An exception to this is citrus fruits and some leafyvegetables. High turgidity early in the morningcauses the fruit to be susceptible to mechanicaldamage which releases toxic oil from the fruits oilglands causing spotting of the ripened fruit. This isa problem mostly in the cooler months of the year.Leafy vegetables may be too turgid in the earlymorning and susceptible to stem cracking andbacterial decay.


3. Get as much product as possible to the cooler beforethe period of expected electrical interruptions. Trucksshould make trips back to the cooler on a scheduledbasis not just when they are full. If outages typicallybegin at 2 PM, all trucks should leave the field by 11AM to noon so that the maximum amount of productcan be in the cooler for at least some time before thecooler shuts down.

4. Protect product from moisture loss by using ventedplastic liners, bin covers, or plastic containers. Someproducts like carrots can be sprinkled with water toreduce moisture loss during temporary holding atwarm temperatures.

5. Begin decay control procedures within 8 to 12 hoursafter harvest. Field and packinghouse decay controlprocedures must be carefully applied with productexposed to cooling delays because of its increasedpotential for decay development.

6. Product destined for long-term storage or long distancetransport should not be subjected to cooling delays.

7. Product subjected to long cooling delays should bemarketed quickly.

Special Thanks: Bob Elliot and Chuck Orman of SunkistGrowers provided us with a helpful review of handlingrequirements for citrus.

Product Allowable Disadvantage of Advantage of Commentsdelay (hours) cooling delay cooling delay

VegetablesArtichoke 8 Water loss NoneAsparagus 4 Increased toughness; None

Reduced shelf-lifeBroccoli 4 Water & firmness None

loss, reducedshelf-life

Cauliflower 8 Water loss NoneCarrot 8 Water loss, Reduced cracking if Carrots reabsorb

loss of crispness carrots are cold and water duringturgid hydrocooling

Cucumber 8 Water & chlorophyll Noneloss

Dry garlic 16 None Curing & dryingDry onion 16 None Curing & dryingGreen beans 8 Water loss NoneGreen Onion 8 Texture loss Water loss during

cooling delay mayreduce curvature

Mushroom 4 Water loss, decay NonePotato 16 Weight loss if low Cure harvest wounds

relative humidity w/ high RHPeppers 16 Water loss; loss of Reduce mechanical

firmness, increased injury if peppersdecay cold and turgid

Sweetpotato 16 Weight loss if low Cure harvest woundsrelative humidity w/ high RH

Spinach 4 Water loss None Can be rehydratedSummer squash 8 Water loss NoneSweet corn 4 Sugar loss NoneTomato 16 Increased decay; Color development Apply fungicide within 8 hr.,

color development higher pulp temp. can causedecay from dump tank waterabsorption

Leafy Green 4 8 Loss of crispness, Product is subject to Greens reabsorbvegetables water loss mechanical injury if water during

cold & turgid hydrocooling

Table 1. Allowable maximum cooling delay between harvest and the start of initial cooling for vegetables. The delay timesare only estimates. Actual field temperatures and product quality at harvest may modify the actual allowable cooling delay.


Table 2. Allowable maximum cooling delay between harvest and the start of initial cooling for fruits and melons.The delay times are only estimates. Actual field temperatures and product quality at harvest may modify the actualallowable cooling delay.

Product Allowable Disadvantage of Advantage of Commentsdelay (hours) cooling delay cooling delay

Fruits and MelonsApple 24 Texture loss & 24 48 hr delay

shorter storage life delays scalddevelopment in Granny Smith

Apricot 4 Softening & decay NoneAvocado 12 Premature ripening None Less delay is

with high fruit maturity allowable at high temp.& with high fruit maturity

Cantaloupe 8 Water loss NoneGrape 4 , 8 hrs at Shriveling, stem None Treat with SO2

Documents

Effect of Cooling Delays on Fruit and Vegetable Quality