Main Menu

Everyone Welcome - Open 7:30am - 9:30pm daily

Postharvest Horticulture

Her voice and expression projected considerable concern. “Here is a new statistic for your life,” said ABC News’ Diane Sawyer. In America, she continued, “every average home, every household, throws out $190 worth of food every month.” (ABC Evening News, 1/28/13.)

What followed was a segment on food waste in the home, reporting off studies by the Natural Resources Defense Council (NRDC) and the U.S. Dept of Agriculture (USDA) that quantified the alarming amount of food loss in this country. As you can guess, the numbers are not pretty. The USDA estimates that in America we waste about 40% of all edible food. In the home, this can range from 17% of purchased dairy products to 35% of purchased fruits and vegetables. The USDA estimates that two-thirds of food loss at the consumer level is due to spoilage; the other third is the result of purchasing more than you or the dog can eat, and plate waste. The dollar cost is simply astronomical (billions and billions) and, as pointed out by the NRDC, “In addition to the wasting of water, energy, chemicals, and global warming pollution that goes into producing, packaging, and transporting discarded food, nearly all of the food waste ends up in landfills where it decomposes and releases methane, a heat-trapping greenhouse gas that is 21 times more potent than carbon dioxide.”I personally never considered this guilt-inducing connection between the moldy orange in my refrigerator and global warming, but there it is.

The purpose of this article is to help you “pare down” food loss in your household, specifically the wonderful fresh produce you buy or grow yourself. It will begin with a brief overview of harvest standards, and the biology of harvested produce. Next, we’ll look at how the science of postharvest horticulture, generally defined as the processing, preservation and storage of food and other horticultural crops, e.g. cut flowers and herbs, has facilitated the modern grocery produce department, and provides guidelines for the optimal handling of products from field to table. Lastly, I’ll bring you behind the scenes in the Co-op Produce department to show you how we strive to bring you fresh organic produce throughout the year.

But first, let’s agree, for the purpose of this article, on the pesky words vegetables and fruits. Savory or sweet? Dinner or dessert? Green or brightly colored? From a tree or from the earth? Whatever system or label you choose to use is fine, but botanically, pretty much everything you buy in the produce aisle that isn’t a fruit, is a vegetable. Fruits are the ripened, usually fleshy ovaries of a flowering plant that contains the seeds. (So-called seedless fruit are still fruits.) A tomato is a fruit, so are cucumbers and zucchini. Vegetables are non-seed bearing plant parts including roots (carrot), shoots (asparagus), and immature flowers (broccoli.) Botanists sub-classify fruits and vegetative plants, but those labels are not in common use nor are they important for our purposes. Where necessary, I’ll make the vegetable or fruit distinction. Plants that have been harvested (a.k.a. postharvest) I’ll refer to simply as produce.

Ripeness is Relative
Your idea of “harvest-ready” homegrown or farm stand produce may be quite different from that harvested at a commercial farm for distant markets. You can pick a red, soft, juicy tomato from your garden or market and it will be perfect for your table that night. But the commercial market has to be concerned with delivering a product that will be salable after harvesting, cleaning and packing, storage, shipping (possibly thousands of miles), more storage, distribution, unpacking and retail display. This may mean harvesting a product before it is fully ripe, or mature, and always dictates how the produce is handled from the time it leaves the field.

In the commercial world, produce is harvested according to measurable levels of horticultural maturity and quality indices that take into account the rigors of global markets and consumer expectations. For example, tomatoes can be harvested at six different maturity levels depending on internal fruit development; pineapples and grapes will be harvested when the sugar content reaches a minimum level affording acceptable taste and texture. Additional standards, which may include a numbered grade, are also applied to each piece picked. These usually describe external appearance attributes such as size, uniformity, color, and shape; using the previous examples, tomatoes are graded on color, shape and firmness, pineapples by skin color, uniformity of shape, and size. Crop-specific maturity indices, grades, and size standards have been determined by university and other crop scientists, and further developed and published for industry use by national governments, some state governments, and producer/commodity organizations. A compilation of U.S. standards for scores of commodities is in the USDA Handbook 66, The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks (2004). Also useful are the commercial produce fact sheets, with helpful photos, published by the University of California at Davis Postharvest Technology Center, available online, in 4 languages, at

At the end of the day, produce standards and grades allow wholesale and retail buyers to trade successfully in a commodity they cannot see, touch or taste. The wholesale buyer in Chicago who purchases U.S. Grade 1, medium, maturity stage 3 tomatoes from a Mexican or Midwest farmer knows what should be coming in the box, and handles and resells it accordingly.

storing produce chart

Life Goes On
When produce is harvested it doesn’t immediately “stop living.” At the cellular level it’s still alive. Three metabolic processes continue that affect postharvest quality: respiration, transpiration and hormone production.

Respiration is a chemical process occurring in all plant cells that releases stored energy. Whenthe plant is intact, this energy, in the form of sugars and carbohydrates, feeds cellular growth; the plant restores depleted energy via photosynthesis, water and nutrient uptake. Postharvest, the produce continues to respire, but the food withdrawn from plant cells is no longer restored, resulting in weight loss and other chemical changes that affect food quality. Respiration also produces heat that is sent off into the air. In a field or greenhouse full of living plants the heat can be blown away, but in a bin, box or building the heat builds up and can quickly degrade harvested products. Later we’ll see how the unavoidable results of postharvest respiration are handled.

Transpiration is the loss of water through minute openings in plants’ skin. A plant in the ground is moving a constant unbroken stream of water from the roots out through all of its parts in a process called transpiration. The rate of transpiration, before and after harvest, is influenced by temperature and humidity. Postharvest, the still transpiring plant parts cannot replace lost water, and wilting and shriveling result. Manipulating temperature and humidity are among the tactics used in the industry to mitigate against water, and thus quality, loss.

Like other living things, plants produce a variety of hormones that act as gatekeepers of growth and development. One of these, ethylene, is a multi-tasker that influences all aspects of a plant’s lifecycle. I’ve come to view it as the yin and yang of plant hormones: it regulates seed germination, root growth and flower formation, but it also triggers flower and leaf drop, fruit ripening, and eventually decay. It ushers in tissue death but also provides protection against pathogens and stress. Poetry aside, understanding the role of, and managing, ethylene effects on produce is crucial in both commercial postharvest operations and in your home refrigerator.

Ethylene is a small hydrocarbon gas, chemical formula C2H4. As mentioned, plants produce ethylene naturally (biosynthesis), but it also occurs as a result of combustion, like in smoke and fumes from fires and vehicle exhaust, and can easily be made in synthetic form. You can’t see or smell it. Some fruit will produce ethylene as ripening begins and continue to produce it after harvest. Apples and pears are examples of fruit that produce ethylene postharvest. (Fruits such as cherries and blueberries do not produce much ethylene and it doesn’t influence their ripening after picking.) Plants produce more ethylene in response to age, disease and wounding. Ethylene can increase undesirable postharvest respiration.

Thus, “one bad apple spoils the lot” rings true: a bruised, overripe apple produces more ethylene and hastens ripening and deterioration of its neighbors in the barrel. To continue the yin-yang metaphor, here are some good and bad effects ethylene has on produce:

Beneficial Effects        

  • Promotes color development in fruit

  • Stimulates postharvest ripening of some fruits, including changes in taste, texture and/or color

  • Promotes de-greening of citrus

  • Promotes ripening and yellowing of bananas

  • Promotes flowering of pineapples

Detrimental Effects

  • Accelerates tissue decay

  • Causes excessive softening of fruits

  • Stimulates chlorophyll loss (e.g. yellowing)

  • Stimulates sprouting of potato

  • Promotes tissue browning or other discoloration

  • Promotes leaf and flower drop on intact and cut flowers

  • Induces formation of off-flavors in some produce

Dents, Diseases and Disorders
Rough handling, fungus, bacteria and disorders caused by improper storage also reduce produce quality and shelf life.

It is said the ancient Egyptians gashed figs in order to stimulate ripening; they didn’t know that the wounding stimulated ethylene production that in turn hastened ripening. Nowadays, however, postharvest bruising and skin breaks are avoided because of the ethylene problem, the increased susceptibility of damaged surfaces to infection by molds and bacteria, and reduced consumer appeal. Gentle handling of produce from harvest to retail display is aided by cultivar selection (some varieties are tougher to bruise), worker training, special handling equipment and highly engineered packaging, including product-specific inserts, wraps and boxes.

Microorganisms that cause postharvest decay are usually present on the plant at harvest. Careful handling to prevent wounding, and washing, prevents some infections. But the main weapon against disease is temperature management: rapid cooling after picking, and continuous storage at the lowest safe temperature and humidity for the product. Less frequently, chemicals are used before or after harvest to lessen infections. (Special rules apply that limit the use of agricultural chemicals on organic produce.)

A lesser-known postharvest problem is “chilling injury,” caused by low, nonfreezing temperature exposure of “chilling sensitive” produce. The injury incurred is dependent on time and temperature and varies with the crop. The lower the temperature below the injury threshold, the quicker and more severe the injury. The maturity of the crop can also affect sensitivity; for example, ripe avocados, tomatoes, and stone fruits are more tolerant of damaging low temperatures than when they are unripe. Thus, for some crops there is a fine line between an acceptable postharvest keeping temperature that protects freshness, and one that will retard shelf life.

The term, chilling injury, may not sound terribly familiar, but most of us have had the experience (at least once) of destroying bananas by putting them in the refrigerator; the blackening of the skin and squishy texture are the result of chilling injury. Chilling sensitive crops include those of tropical and subtropical origin like avocado, banana, papaya, mango, tomato and citrus, and summer-ripening warm season crops, including zucchini, cucumber, eggplant, peppers, basil, honeydew and watermelon. Potatoes, sweet potatoes and pumpkins are also chilling sensitive. Symptoms of this disorder are quite variable, and include: rapid decay, failure to ripen properly or uniformly, pitting of the surface, discoloration (russetting on the surface or darkening of the flesh), microbial infections, off-flavors, and watery consistency. Chilling injury is particularly troublesome because symptoms may not appear until days after the injury has occurred. Produce that looks healthy on receipt at the store may develop symptoms on display, or in your home, that are the result of chilling before it left the farm or during transit. Fortunately, the industry is aware of this problem and does take steps to minimize chilling injury in susceptible crops.

Putting It All Together
Scientists and agricultural technologists, over many decades of effort, have identified and quantifiedthe biological processes that occur, postharvest, in hundreds of agricultural products. With this information, and in concert with ever-improving transportation and refrigeration technology, they’ve developed and continue to develop systems for protecting the quality, shelf life, and safety of fresh produce. Plant breeders too have been selecting and breeding food and ornamental crops with what the seed catalogs call “keeping qualities;” their contribution to our food system must be noted, though I won’t take it up again here. Below are some ways the postharvest problems identified above are addressed in the industry.

Respiration and transpiration: Postharvest scientists have measured the respiration rates of hundreds of crops, and classified them as having very low to extremely high respiration rates, which correlates generally with theduration of shelf and/or storage life.

Cooling is necessary to remove postharvest field heat, and to continually remove the heat of respiration. Rapid cooling immediately after harvest is critically important for maintaining the quality of high respiration crops, in whom a day of shelf life can be lost after as little as an hour at too high postharvest temperatures.

Temperature is the most important factor that affects the deterioration of produce. Modern science has established crop-specific optimal temperatures for produce transport and storage, time guidelines for reaching temperature goals for sensitive crops, and the best cooling methods for different crops, e.g., only some crops can be safely cooled with ice. Agricultural technology has devised cooling equipment for on- and off-farm use, and sophisticated refrigerated storage and transportation. Increasingly accurate, affordable and easy-to-use thermometers and sensors regulate and/or record crop temperature from field to market, helping everyone maintain produce at the right temperature.

Like respiration, postharvest water loss is crop-specific and measurable. Leafy green vegetables such as lettuce lose water quickly, and in higher amounts, compared with thick-skinned crops like potatoes. Temperature, air movement and relative humidity in the atmosphere around the product affect water loss.

Postharvest science has also determined optimal postharvest humidity levels for different crops. Methods used to achieve and maintain these levels include cooling and refrigerated storage options; special transport and retail packaging such as waxed and plastic-lined boxes and engineered “breathable” plastic bags, boxes and wraps, and an old standard, putting ice on top of produce in shipping containers, a.k.a. “top icing.” At the retail level humidity is maintained with refrigeration, hand and automated misting systems, icing, and lidded displays. In the home, some modern fridges have separate humidity controls for produce drawers.

Ethylene: Lists, lists and more lists! Science has identified which crops produce ethylene (C2H4) postharvest and how much, what crops are, or are not, ethylene sensitive, how crops should be segregated to avoid one causing ethylene damage to another, and how the air around crops can be modified to reduce damage from ethylene, or create controlled ripening. Chemical treatments have also been discovered that control crops’ ethylene responses.

This topic could be a whole article in itself, but here is a brief account of some industry practices:

To reduce damage from ethylene:

  • Cooling and ventilation systems reduce the amount of ethylene around stored produce

  • Special filters chemically remove, or “scrub” ethylene from storage coolers and packaging

  • Ethylene-sensitive produce is separated from ethylene-producing products (Home refrigerators have separate fruit and vegetable drawers for this reason.)

  • Large operations may also modify the oxygen and carbon dioxide levels in storage rooms and transport containers, a technique is called Controlled Atmosphere (CA) or Modified Atmosphere storage. For example, apples and pears are kept for months in CA storage. Gas-regulating Modified Atmosphere Packaging, in the form of bags and wraps, extends this technology right to store shelves.

To enhance ethylene effects (note: ethylene is approved for use on organic crops):

  • Carefully administered ethylene treatments are commercially used to ripen fruits such as bananas, avocados, mangoes, tomatoes, and kiwifruits. “Ripening rooms” have been in use for many years, but the latest in experimental technique seeks to extend the ripening technology to crops in transit, bringing ripe products to markets even faster.

  • Many citrus varieties harvested ripe are too green for consumers’ tastes. Ethylene treatments turn the green peels to the brighter colorswe expect.

Meanwhile, Back at the Store
I recently fielded a phone call from a customer asking if I knew how much produce we throw away in a week. She had been reading some disturbing food waste numbers in a book and was curious about the Co-op picture. My answer was, I don’t know exactly in terms of weight, but it’s not very much. I am confident that the quality of our produce, and our low loss numbers, can be attributed directly to the combined skill and knowledge of our growers and distributors, and our buying and in-store handling methods.

Early every day we’re open, the produce buyer is chilling in the cooler and dodging the misters to take inventory. They count everything in back stock and on the display shelves, and what will come in with the day’s deliveries. Then, using the store’s actual daily sales numbers, plus what’s on hand, and figuring in things like specials, holidays, sporting events, and weather that could affect near-term sales, they arrive at the numbers needed for the day’s order. We place orders with our suppliers every day, and, depending on the product and supplier, the order will be fulfilled in one to three days. (An exception to this process is made for some produce that we preorder on contract, mainly from local growers, but also from out-of-state organic growers whose exceptional produce we want to reserve.) Of course we want our displays to look bountiful and provide you with choices, but we buy based on sales, not to overfill displays and be left with shabby product we have to cull several days out. We display and store produce, to the best of our ability, at industry-recommended temperatures. We also have limited space at both stores for display and back stock so we sell through fresh items quickly!

We source our produce from reputable and experienced organic distributors and growers. Our principal year-round suppliers are Albert’s Organics, a national company, and Midwest-based Co-op Partners Warehouse and Goodness Greenness. The first two send us product from distribution centers in the Twin Cities, the last ships from Chicago. We have some local product, like mushrooms, all year, but during the growing season we receive goods from over two dozen local farms.

Our receivers deal with scores of boxes everyday. They quality check incoming produce and will immediately return or take credit for damaged or poor quality product. The boxes are marked with a source and date, and then moved quickly to the appropriate storage area, refrigerated items being our first priority. Broccoli and many greens arrive with a layer of top ice that has to be removed before storage, a cold and tedious task! The cooler is stacked carefully: like items together, wet items below dry items, and new boxes behind previous days’ boxes. We store and stock on a FIFO basis: first in, first out. The receivers also set aside a few boxes of avocados, pears and other fruits that are normally shipped less mature, for ripening. We don’t have a sophisticated ripening room for this purpose, but we do wrap and add an ethylene-generating ripe banana to the boxes tohelp out.

Most, but not all our produce is stocked straight from the box. Stockers quality check all items as they shelve, setting aside, or “culling,” lower-quality pieces for evaluation after stocking is finished. Some culls will become compost, some will be discounted for sale in what one customer calls our “scratch and dent bins.” I don’t know of any local retailers that reduce potential waste by sorting and offering less-than-perfect products to their customers, even to the point of bagging compost fodder. We also occasionally donate larger quantities of unsalable overripe bananas, winter squash and pumpkins to the zoo. Our displays are usually checked more than once a day for quality and restocking, so we can keep a close eye on quality.

Some items receive extra attention, or “prep.” Lettuces, greens, celery, bok choy, leeks, bunched roots, and green onions and others are in this category. We remove tattered leaves, trim and band lettuce and bok choy; trim and remove damaged bits from bundles of greens, green onions, celery, and leeks; remove withered tops and wash unusually dirty carrots, radishes and beets. We also “crisp” lettuce and other green produce as needed that may have lost some moisture in transit. “Crisp” is the industry term for soaking an item in water to restore lost moisture; it also restores appearance and keeping quality. We use only reverse-osmosis treated water when prepping organic produce.

We know that misting our produce is controversial to some of you. But, remember all that talk about respiration and transpiration, unavoidable natural processes that occur after harvest that degrade the produce? Misting is an essential tool used industry-wide to maintain produce quality in retail displays, including appearance, taste, texture and nutritional value (studies have shown misted produce retains more vitamin C). Misting counteracts the dehydrating affect of the cooling system (think how dried out you feel in air-conditioned environments.) The mist increases ambient humidity, assists with cooling and provides moisture imbibed by the plant tissue. However, as Megan Minnick, Westside Produce Manager wrote in the March Reader, “If you’d like lettuce that isn’t so wet, we can always get it for you from back stock.”

At store closing we also cover unmisted high-respiring produce to retard overnight water loss. Some leafy greens we remove at night and store in moisture-retentive bins.

The woman with the waste question? After explaining what we do to reduce loss and recycle waste in the produce department, she said, “That’s why I like to shop at the Co-op!” We sincerely hope you share her feeling.

We’ve included a chart with this article, developed by the folks at the Postharvest Technology Center at the University of California, Davis, of recommended home care of your produce. Of course, please feel free to direct any produce related questions to our store staff. If we don’t know the answer, we’ll be happy to research it and get back to you!

Bon appetit!