Fig. 1. These tomatoes are being sorted to ensure all the fruits in each shrink-wrapped polystyrene tray have a uniform size and appearance.

From sowing seeds to managing mineral nutrients and water quality, to controlling light and temperature, to proper training and pruning, our goal in producing hydroponic food is to harvest quality foliage and fruits for the consumer. However, our jobs are not done when we harvest. Rather, this begins one of the most critical time periods for maintaining quality. Throughout the harvest and postharvest chain, our goal is to minimize the loss of carbohydrates and water from produce, mold and bacteria development, and exposure to ethylene.

Grades and standards

What are you going to harvest and when? Do your customers want an 8 or a 12-ounce head of lettuce? Single tomatoes or clusters? The specifications that your buyers require will be the most important guidelines for you to meet, so you should get a clear understanding of what product they want to purchase so you know what and when to harvest. However, in addition to your client’s needs, the U.S. Department of Agriculture’s (USDA) Agriculture Marketing Service grades and standards makes it easy for producers to market and sell their tomatoes, cucumbers, and lettuce, among other crops.

Regardless if you are using specifications from your buyers or the USDA, you are going to need to consider the appearance, uniformity, and size or weight of your crops. For every crop, the appearance needs to be free from blemishes and diseases. If the foliage or fruits are marred, it makes the products less valuable or even unmarketable. Produce must also be uniform in appearance (Fig. 1); there is some room for variation, though the tolerance within a crop will depend on your buyer or the standards you are using. In addition to appearance, size can be important. For example, a greenhouse cucumber must be a minimum of 11 inches long to be marketable as U.S. Fancy, No. 1, or No. 2. Additionally, tomatoes are frequently sold based on weight and not size, but may be classified from “small” to “extra large”.

Cooling

One of the most important steps to maintain quality after harvest is to remove field heat (or for hydroponics, greenhouse heat) immediately after harvest in order to reduce respiration rate. Horticultural crops continue to respire after harvest. The rate of respiration is affected by the temperature of the produce; the warmer the temperature, the faster the rate of respiration, while the cooler the temperature, the lower the rate of respiration. Therefore, by cooling down our crops, we slow down the rate of respiration and maintain more carbohydrates in the plant extending the shelf life including saleable weight and taste.

There are two types of cooling methods used: forced air and hydrocooling. The simplest way to cool produce is to place it in a refrigerator, or room cooling. Over time, the cool air in the refrigerator reduces produce temperature. However, this can be a slow process, especially for produce in certain containers or packaging that restricts air flow around the produce. A similar, but more active, method of cooling produce using cool air is forced-air cooling (Fig. 2). By actively drawing air across the crop, forced-air cooling allows for more rapid cooling of produce compared to simply placing produce in a cooler.

Hydrocooling, as the name implies, is the use of chilled water to cool produce. Compared to forced-air cooling, hydrocooling can rapidly reduce produce temperatures, since heat is removed via conduction as the water removes heat from produce. However, hydrocooling cannot be used on all crops. For instance, hydrocooling soft fruits such as strawberries could promote botrytis due to the free water on the surface of the berry.

Storage temperature varies based on commodity and crops are commonly grouped based on chilling sensitivity. For example, tomatoes, cucumbers and basil should not be cooled to or stored below 50°F to avoid chilling injury. Alternatively, leaf lettuce and other cold-tolerant greens can be cooled to and stored at 32°F to minimize water loss and prolong shelf life without adverse effects.

In addition to keeping crops cool to reduce respiration, you want to manage the humidity during storage. Maintaining a humid environment will help minimize water loss and desiccation of crops. For most crops, target a relative humidity of 85 to 95%. For most leafy greens and herbs (except for basil), target a relative humidity of 90 to 98%.

Fig. 2. Freshly harvested mushrooms are placed in this vacuum cooler to remove heat and lower the temperature before placing them into a walk-in refrigerator.

Packaging

There are a number of different types of packaging that hydroponically grown produce is placed in, including cartons, clamshells, sleeves or bags, and shrink wrap. Cardboard cartons are commonly used for crops like beefsteak tomatoes. Cartons can be an efficient type of packaging because in some instances, crops can be harvested and placed directly into the cartons from which they will be sold, thereby reducing successive repackaging and extra labor associated with handling.

Clamshells and plastic containers are popular for living lettuce, loose greens, and berries. Living lettuce is when the heads of lettuce are harvested from hydroponic systems, but all or most of the root system is left on the plant. This is not only popular because it is a unique product to market, but it also enhances the postharvest longevity of lettuce for consumers; leaves can be harvested off the heads as needed and the remaining leaves continue to take up water through the attached root system. While clamshells for leafy greens should be airtight to minimize water loss, containers for berries should have adequate ventilation to discourage condensation in the container or on fruits.

Another popular method of packaging leafy greens and herbs is to use sleeves or bags. Additionally, sleeves are often used for containerized or living herbs by placing them around the roots of the containers. In addition to protecting the commodity, the sleeve can serve as an advertisement for your greenhouse or you can use the space to place instructions for postharvest storage.

Shrink wrap is most commonly used for packaging individual Dutch or European cucumbers, as well as polystyrene trays of smaller beit alpha cucumber and cluster, cocktail or cherry tomatoes. Shrink wrap is required to prevent desiccation of thin-skinned cucumbers, like Dutch cucumbers which can lose turgidity with water loss.

Fig. 3. This head lettuce has been harvested from nutrient-film technique systems, rinsed, dried and placed inside these plastic sleeves for marketing at their point of purchase.

Transport

Now we are in the home stretch — let’s not lose sight of our goals! Getting from the cooler, to a truck, and onto grocery store shelves is no easy task. Exposure to sub-optimal storage temperatures can occur from extended periods on the loading dock or too much movement in and out of storage areas preventing the cooling of the air in the storage area. In addition, ethylene is one of the concerns when loading trucks for transport. Since ethylene is a product of incomplete combustion, engine exhaust can contaminate produce. Try to avoid having produce sit on the loading docks with trucks running. Refrigerated trucks can help keep produce from warming during shipping and avoid condensation from cooling crops down after delivery.

Take-home messages

All of the effort that goes into producing hydroponic food leads up to the harvest, sale and marketing of the produce. Provide as much devotion and attention to detail to your crops after harvest as you did during production. Hydroponic food crops are usually marketed as a high-quality and value-added product, and you want your customers to know you as a source of consistently high-quality produce.

Christopher (ccurrey@iastate.edu) and Diana (dianac@iastate.edu) are assistant professors of horticulture in the Department of Horticulture at Iowa State University.