Post Harvest Handling for Best Crop Quality Wisconsin School for Beginning Market Gardeners, 2001
Astrid Newenhouse, Ph.D.
Healthy Farmers, Healthy Profits Project, Biological Systems Engineering.,
UW-Madison
460 Henry Mall, Madison WI 53706
Written with input from Cindy Tong, Ph.D., Univ. of MN Dept. of Horticulture
What causes loss of quality and freshness?
- improper temperature management
- drying (wilting, shriveling)
- mechanical injury (abrasion, puncture, bruising, vibration)
• wounds cause higher metabolism rate,
• more water lost through broken cuticle - 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:
chilling (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:
- avoid mechanical injury (from impact, puncture, compression, vibration, abrasion)
- promptly and thoroughly cool your crop
- maintain the crops' optimum storage temp
- avoid water lost
Three take-home messages:
- 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.
- Good postharvest handling can ensure your competitive edge (both from the market perspective and by preventing stress and strain to your body).
- 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).
