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Post Harvest Handling for Best Crop Quality

What causes loss of quality and freshness?

  1. improper temperature management
  2. drying (wilting, shriveling)
  3. mechanical injury (abrasion, puncture, bruising, vibration)
    • wounds cause higher metabolism rate,
    • more water lost through broken cuticle
  4. attacks by bacteria or fungi
    • esp. after entry through a mechanically caused wound,
    • also higher metabolism releases sugars: substrates for microbial growth

Plants breathe like humans do, respiring day and night, continuously giving off water as they release energy for growth and metabolism. In respiration, plants use oxygen to break down an energy source such as carbohydrates, proteins, and fats into CO2 and water. In the field, this water is replaced by water taken up through roots. Harvest cuts off the link to roots, and the plant instantly loses water.

Produce breathes like people do, and like a person, it breathes harder and loses more water on a hot day than on a cool day. The higher the respiration rate, the faster you lose water.

Respiration leads to:

  • drying out
  • less food value of the crop (since energy is being released continually)
  • less sweetness (since CHO is broken down)
  • less dry weight

Harvested vegetables give off respiration heat (vital heat) and contain field heat.

A higher respiration rate = a more perishable vegetable. For ex., asparagus and peas are very high, leaf lettuce is in the middle, and potatoes and celery are very low. Temp. influences respiration: for every 18°F increase in temp the rate of decay increases 2x-3x. Cooler temperatures slow down respiration.

Two types of temperature injury:
 (cell sap not frozen) and freezing injury. Some vegetables are chilling sensitive. Chilling injury is cumulative: severity depends on temp. and length of exposure. The best temperature to keep vegetables is just above their freezing point (32°F), or just above their chilling point for chilling sensitive plants (41-46°F, moderately sensitive; 55-59°F highly chilling sensitive). 40-41°F is ok for most, except tomato & eggplant. Soluble solids make vegetables freeze at lower temps than water.

4 keys to maintain crop quality post harvest:

  1. avoid mechanical injury (from impact, puncture, compression, vibration, abrasion)
  2. promptly and thoroughly cool your crop
  3. maintain the crops’ optimum storage temp
  4. avoid water lost

Three take-home messages:

  1. Successful postharvest handling cannot be taught by following a set of rules because there are so many continually changing variables. You need to experiment within your own system to learn what works best for you, and train yourself to notice parameters such as temperature, humidity, wind, crop temperature, ventilation, and rough handling.
  2. Good postharvest handling can ensure your competitive edge (both from the market perspective and by preventing stress and strain to your body).
  3. List of resources is attached to this handout.

A. At harvest:
Harvest when outside temperatures are cool, such as early in the morning before sun warms the crop. Produce is coolest just before daybreak. If you can’t harvest in the early morning, do it in the evening, or when it’s cloudy.

  • Less radiant heat so less field heat.
  • Slower metabolism so less vital heat.

Prevent wounds, abrasions, bruising, or punctures at harvest. Be gentle.
Watch out for:

  • Puncture wounds from tips of knives or sharp fingernails, or sharp edges of containers.
  • Bruising from dropping the produce or dropping something on the produce. Onions will show internal bruising damage if dropped 30 cm.
  • Bruising from compression (don’t overfill boxes).
  • Bruising from vibration (loosely packed crop moves during transport and hits each other, or walls of container, or walls of cart or truck, and suffers abrasion).

Throw out culls to prevent disease from spreading. Pathogens can contaminate produce through wounds.

Some plants need curing first, to heal wounds to their outer skin (garlic, potatoes).

B. On the way to the packing shed:
Shade the load to prevent sunburn and overheating. Give the produce a smooth ride, not bumpy.

C. In the packing shed:
Get the crop into a cooler as quickly as possible (30 minutes to a few hours). Optimum temperature is the most important factorto maintain quality.

Pre-cool many of your crops, with either a forced air cooler inside your walk-in or a dunk tank outside. A continuously flowing cold water bath conducts heat away. Do not hydro-cool eggplant, garlic, or onion. 75-100 ppm chlorine in the wash water will lower the number of pathogens (human or plant) floating around, but note that most organic certifiers allow chlorine only at the same level allowed by the Federal Safe Water Act for municipal water (4ppm). Check with your certifier about using hydrogen peroxide instead, in a 0.3-3% concentration. Amount varies with crop.

D. Packaging:

  • Packaging reduces air movement and therefore rate of cooling also, so there is a trade-off between close packing for higher relative humidity and too close packing which restricts air circulation.
  • No rough surfaces, sharp edges, or nails which could puncture or wound.
  • Prevent water loss by covering stacks of produce with tarps, or packing it into bags of either paper or plastic. Microperforated plastics have tiny pores to let condensation out.
  • Don’t underfill boxes and don’t overfill, but hold items tight within the box. Pack to immobilize, cushion, and avoid impacts. Can use paper wraps, dividers, or cushioning pads.
  • The package, not the produce, should bear the stacking load.

E. In the cooler:
Temperature: Keep vegetables just above their freezing point (32°F), or just above their chilling point for chilling sensitive plants(41-46°F, mod. sensitive; 55-59°F highly sensitive). Refer to tables (Knotts, Kader, or Bartsch) for specific recommendations for each vegetable. 40-41°F is ok for most.

Vegetables which are highly chilling sensitive are eggplant and tomato. Keep these ~ 70°F and tell customers not to put tomatoes in the fridge. Some moderately chilling sensitive vegetables are beans, peas, cucumbers, peppers, summer squash, pumpkins, some melons, and potatoes.

Relative humidity: At a given temp. and rate of air movement, the rate of water loss depends on relative humidity (RH). Most vegetables want 90-98% except for dry onions and pumpkins (70-75%).




Be sure to measure relative humidity, and maintain or add moisture: Put a humidity gauge and a thermometer in the cooler, away from the door and not under the refrigeration unit.

  • put a vapor barrier on the warm side of the cooler wall and install good insulation
  • regulate air movement and ventilation in relation to the produce load in the cooler
  • wet the floor and sprinkle produce with water
  • use waxed boxes
  • place a damp cloth on top of produce in the box. This also helps cool by evaporation.
  • use plastic bag liners in containers, or poly films for packaging, or use a 5% vented liner
  • keep the refrigeration coils within ~2°F of air temp
  • add crushed ice to crops which are not chilling sensitive and tolerate water (carrot, sweet corn, cantaloupes, lettuce, spinach, broccoli, green onions)

Ethylene (C2H4) is a hormone produced by plant metabolism. Some plants give off a lot (apples, other fruit), and most ripen faster if exposed to it. Ethylene also enters the environment from combustion engine exhaust and non-electric heaters. Some ethylene sensitive vegetables are: leafy greens, carrots, cucumber, legumes, eggplant, watermelon, potato and sweet potato. Good air circulation helps.

F. Loading the cooler:
Stack boxes with air space between pallets and room walls for good air circulation.

G. Storage life:
Usually, the lower the storage temp (down to just above their freezing point) the longer vegetables keep. Refer to Knotts or Kader for tables on longevity in storage. Best way is to know your own product and check it yourself. Would you want to buy it and eat it?

H. Individual Crops as Examples:
Lettuce: High surface to volume ratio, high respiration rate. Wilting is main cause of damage and Vit. C loss. Leafy green, so stay away from ethylene (decreases shelf life and causes browning). 
 Tender skin, easily damaged. Harvest by pulling up at the natural stem/stalk break point. Chilling sensitive: below 50°F you see lack of color development, decreased flavor, increased decay, water soaked appearance. If dunked in water to wash, then water temp should be warmer than tomato so that tomato does not take up water and pathogens from the water. 
 Physical or mechanical damage is main cause of loss. Carrots sprout under improper storage conditions. Sensitive to ethylene. Carrots can be top iced.

(Source – http://bse.wisc.edu/HFHP/tipsheets_html/postharvest.htm)

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