Postharvest FAQs

Harvested fruits

What is postharvest about?
The term ‘postharvest’ relates to any treatment, handling or natural biological processes that occurs after produce have been removed from the growing plant. ‘Postharvest’ is usually reserved for perishable specialty products such as fruits, vegetables and flowers.

These products are essentially alive and a major goal of growers, producers distributors and sellers is to keep them alive for as long as possible so that they havemaximal shelf life. Unlike grain and legumes, horticultural crops have a high water content and high metabolic activity. Perishables will use oxygen to ‘burn’ nutrients such as sugars, starch, acids, etc. and in the process produce carbon dioxide to support biological activity. They will also lose water through transpiration through the epidermis or outer skin. If this process is accelerated by keeping the produce at higher temperatures, then deterioration will quickly ensue and the product will “spoil”.

Keeping fruits and vegetables alive is a race against time. One way to ensure their longevity is to perform harvesting before the produce has reached horticultural maturity. This can extend the time needed before the product is consumed. The other way is to change the environment in which the produce is stored. Refrigeration is one of the most effective ways to this. Each crop may have a temperature optima that best preserves shelf life without destroying quality. Some high-value crops like apples may be further stored in controlled atmosphere rooms with high carbon dioxide and low oxygen to reduce respiration. Humidity in these rooms may also be controlled to slow down water loss. Some produce may even be treated with gases like 1-MCP (1-methylcyclopropene) to slow down or inhibit maturation, and then when closer to the point of sale be treated with the hormone ethylene that accelerates ripening. Enclosing fruits in waxes and other edible coatings can create a modified gas atmosphere (lower oxygen, higher carbon dioxide) close to the produce surface, minimize water loss and also reduce the probability of pathogen infestation.

Without intervening technologies, substantial losses of crops can occur at every point along the supply chain from field to consumer. Accurate estimates are hard to come by but values of between 2 – 25% in the USA or up to 50% in tropical countries have been reported. There are some drawbacks to applying postharvest technologies to prolong storage. These handling and storage practices may simultaneously, have a negative effect on the sensory attributes of the produce. There is usually a tradeoff between shelf life and sensory quality.

Why are the tomatoes in the supermarket not as good as the ones I grow myself? Is it because they are GMO?
First, there are no GMO (Genetically Modified Organism) tomatoes currently on the market.

Two things are at play here; the first is genetic and the second is postharvest treatment of the fruit.

Fruit destined for later sale in large supermarkets in distant markets are inherently different to those sold directly to consumers in farmers markets or stands. Commercial fruits are bred for uniform and slower ripening, higher firmness and better disease resistance, and these traits may come at the expense of those that promote good sensory quality. Most of the popular tomatoes such as the heirloom types have excellent flavor, but they tend to have a shorter shelf life and poor resistance to many diseases. If handled in the same manner as commercial tomatoes, they would deteriorate faster and the whole shipment may be lost. Such varieties require gentler handling and are more commonly found in specialty stores and farmers’ markets. Therefore, if you are comparing the quality of the types of tomatoes that will do well in your home gardens vs. those that are typically sold in large-scale operations for shipment, the specialty crop will win each time.

Heirloom Tomato beats Conventional Tomato in terms of taste, flavour, and texture

The second factor relates to harvesting and storage. Fruits destined for distribution to chain supermarkets are usually handled differently than those sold in farmers’ markets. A large degree of planning is required to calculate when fruits are to be harvested and to balance that with the desired shelf life. Growers are usually cautious and will harvest fruits while they are still firm (i.e. before they are fully ripe), so that they can survive physical damage during shipping and handling. The advantage is that these fruits will have a longer shelf life (up to 7 – 10 days) and will reach the supermarket intact, with a smooth surface, evenly ripened, with no physical or internal damage. Studies have shown that appearance is a major driver determining consumer preferences. In contrast, tomatoes grown at home can be harvested at the peak of ripening when the sugars and flavor compounds are at optimal levels and the fruits have softened and are juicy. The time traveled from plant to plate may be as little as 15 minutes. It is difficult for a typical tomato sold in the supermarket to be of similar quality to that grown in kitchen gardens even if it is the same variety.

Are all the fruits and vegetables in the supermarket GMO?
The answer is no.

The term GMO (Genetically Modified Organism) is used to refer to crops produced using transgenic technology. These are crops that have additional DNA specifically introduced into the genome. The only produce marketed in this way is papaya in Hawaii; sweetcorn, and a few varieties of summer squash in the US.

Heirloom Tomato beats Conventional Tomato in terms of taste, flavour, and texture

However, none of the fruits and vegetables we eat today can be considered natural and strictly speaking, even if they are not produced by transgenic technology, they are all GMOs. The genomes or genetic makeup of an organism contains the code or information that determines how that organism will look and function. The genome of many wild plants were modified as the result of random, spontaneous mutations, i.e. mistakes or changes in the DNA 1000s of years ago. These mutations arose naturally and the DNA code may have sequence substitutions, insertions, deletions and inversions. If they occur in regions that encode important traits for the plants, then that trait may be visually different. Early agriculturalists would have noticed these strange variants and selected the seed or plant and maintained them. These genetic mutants now form the basis for almost all of the food we eat today.

For example, a beefsteak tomato plant can produce twice as much fruit and these fruit can be 20 times larger than the naturally occurring wild tomatoes. What is really interesting is that these changes are largely the result of combining mutations in just a few genes that altered flowering, fruit production and fruit size.1

Often these changes would ‘handicap’ the plants in the wild, but through careful cultivation they thrive in farms and fields globally. Few modern crops—even open-pollinated or heirloom types—are fit enough to grow in the wild because their survival mechanisms have been disabled to make them suitable for agriculture. For example, wild tomatoes have a viny habit that allows them to efficiently compete with other plants to ensure maximal capture of sunlight. In most gardens we can grow plants evenly spaced, provide adequate water and fertilizer so that most energy is invested in the fruit and not in leaf and stem production. Compared side-by-side, our current crops would look like “frankenfruit” compared to the wild species.

Why shouldn’t I keep my tomatoes in the fridge? Won’t they spoil so much faster if I don't?
This is an interesting dilemma. Refrigeration normally slows down the metabolism of the fruit, which can substantially extend their shelf life. However, tomatoes, like many produce which originated in the tropics and sub-tropics, are sensitive to cold. If stored below 12.5 °C (54.5 °F) they may suffer from chilling injury.

Damage to tomato fruit after postharvest storage at temperatures below 10 °C (50 °F).

All fruit were harvested at mature green. Fruit on the left were stored in the cold for 3 weeks before ripening at room temperature. Fruit on the right were only stored at room temperature.

Alternaria infection in cold-stored fruit.

Chilling injury describes a complicated set of physiological reactions that can reduce the overall quality of the fruit. Mild chilling injury leads to reduced flavor and perhaps a mealy fruit texture, but in extreme cases, it can lead to pits on the surface of the fruit, water soaking and fungal infestation. The severity of the symptoms is related to storage conditions: lower temperatures for longer durations will amplify the problem. Also, fruits harvested at an earlier stage (e.g. when still green) will exhibit more damage when they are allowed to fully ripen at room temperature. If tomatoes are allowed to fully mature at room temperature, the metabolic processes in the fruit will still occur even when off the vine, and the fruit will taste better.

What is the localvore movement about?
Localvores are committed to consuming only foods produced in close proximity to their community—typically within a 100 – 250 mile radius.

Modern advances in postharvest technology, reduced shipping and freight costs mean that produce can be shipped globally and remain competitively priced. Now, it is not uncommon for consumers in temperate climates to have access to tropical fruits and vice-versa. This development is spurred by the growth of immigrant communities in major cities longing for food from home, increased international travel, and importers wishing to expand their markets. The benefits of this trend are that fruits and vegetables are readily available all year long at economical prices, and that consumption is not limited by seasonality.

However, there is a downside. Localvores point to the environmental, social and economic benefits of eating food produced within close proximity. Less greenhouse gases are generated when foods are purchased locally. This could be a major problem when food is shipped globally, often with refrigeration. Furthermore, money spent stays in the community and often the consumer has a better interaction with the grower and is more connected to the food chain. Since the supply chain is minimal, there are less chances of foodborne diseases and most of the profits should stay with the grower and is not be allocated to middlemen such as distributors, shippers, etc.

There is still some debate about the benefits of localvorism when analyzed in purely economic terms. Although, counterintuitive, some produce may be more expensive to consumers if purchased locally. This, plus seasonality may magnify the hurdle for people in lower socio-economic communities to access healthy foods.

References

  1. http://postharvest.ucdavis.edu/pfvegetable/Tomato/
  2. Frary et al (2000) Science 289: 85-89
  3. Powell et al (2012) Science 336 no. 6089 1711-1715
  4. Kedear (1989) Long shelf life heterozygous tomato plant US 4843186 B1
  5. Wieczorek, A. M. & Wright, M. G. (2012) History of Agricultural Biotechnology: How Crop Development has Evolved. Nature Education Knowledge 3(10):9
  6. California Agriculture vol 58 #2; http://CaliforniaAgriculture.ucop.edu
  7. Postharvest Technology of Horticultural Crops (2002). Third Edition. Editor Adel Kader
  8. Buzby et al (2011) The Value of Retail- and Consumer-Level Fruit and Vegetable Losses in the United States. Journal of Consumer Affairs 492-515