|What causes loss of quality and freshness?
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:
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:
|4 keys to maintain crop quality post harvest:
Three take-home messages:
A. At harvest:
Prevent wounds, abrasions, bruising, or punctures at harvest. Be gentle.
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:
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.
E. In the cooler:
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.
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:
G. Storage life:
H. Individual Crops as Examples:
(Source – http://bse.wisc.edu/HFHP/tipsheets_html/postharvest.htm)