Harvest and postharvest success

Growing the best product is what drives greenhouse growers. Taking pride in what is shipped out is one of the most satisfactory parts of the job. But what are you doing to make sure consumers get to appreciate your hard work and the resulting quality herbs? By focusing on a few key elements of postharvest, you can capture and share the flavor and freshness your herbs offer consumers.

When should herbs be harvested? Unlike many fruiting crops grown in greenhouses, there are no size standards for fresh-cut culinary herbs. Producers have the flexibility to combine shoots from multiple plants to achieve 0.75-ounce bunches, or they can grow individual plants until they reach (or slightly exceed) the 0.75-ounce mark. Sometimes, the harvest can also be influenced by the customer. For example, some chefs have a culinary preference for the second harvest on cut-and-come-again basil due to the perception of improved flavor and harvest compared to the first harvest.

In addition to when in the crop cycle to harvest herbs, you can also consider what point in the day to harvest herbs. In the morning, the plant temperature is usually the coolest, so harvesting at this time makes it easy to cool.

However, the carbohydrate pool in herbs is also lowest in the morning, as they have spent the night respiring and depleting carbohydrates with no photosynthesis occurring to replenish them. Herbs have their highest carbohydrate concentration in the afternoon, as they have been photosynthesizing all day. But the heat is also greatest in the afternoon, which makes quick and efficient cooling harder compared to crops harvested in the morning.

Although we have been discussing time-of-day choices based on plant requirements, we are forgetting about the most important biological organism in this process: your workers! While harvesting in a greenhouse in the afternoon during the winter may be comfortable, harvesting herbs in the afternoon heat and sun in summer conditions may be anything but.

Reducing carbohydrate loss is a central goal in postharvest handling of food crops. To achieve this, the first step is to slow down the rate of respiration, the process that consumes carbohydrates. Due to the high water content of horticultural crops, produce is essentially still “living,” as it continues to respire after being severed from the root system. The rate of respiration is affected by the temperature of the produce: the warmer the temperature, the higher the rate of respiration; the cooler the temperature, the lower the rate of respiration. 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.

In controlled environment agriculture, the two primary cooling methods are forced air and hydrocooling. Hydrocooling uses chilled water to cool produce immersed in it. Hydrocooling can rapidly reduce produce temperatures, since heat is removed via conduction as the water removes heat from produce. However, hydrocooling is not useful for leafy crops such as herbs, where the introduction of free water just prior to storage can increase the risk of botrytis developing.

Herbs are cooled instead using cold air. 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 herbs in containers or packaging that restricts air flow around the produce.

A much more effective method of cooling herbs using cool air is through forced-air cooling. By actively drawing air across the crop, forced-air cooling allows for much quicker cooling of produce compared to simply placing produce in a cooler.

When it comes to storage temperatures following harvest and in the postharvest chain, it is important to recognize differences in culinary herb requirements. Appropriate storage temperature varies with species, and herbs can be grouped into one of two primary groups based on chilling sensitivity. Cold-tolerant herbs such as parsley and dill can be treated like other leafy crops and can be cooled to and stored at 32°F to minimize water loss and prolong shelf life. A very important exception is basil, which is cold-sensitive and should not be cooled to or stored below 50°F, which results in chilling injury and can make crops unmarketable.

Just like cooling crops to reduce respiration and subsequent carbohydrate depletion, 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 leafy herbs except for basil, target a relative humidity of 90% to 98%; for basil, target 90% to 95%.

Hydroponically grown produce is placed in a wide variety of packaging, including cartons, clamshells, sleeves, bags and shrink wrap. For culinary herbs, clamshells and sleeves are the most widely used. Clamshells and plastic containers are most popular for cut herbs, usually in 0.75-ounce packages. Unlike some containers used for small fruits, clamshells for leafy greens should be airtight to minimize water loss. Sleeves or bags are another popular method of packaging herbs, most common for “living” hydroponic herbs that are not severed at harvest but are simply lifted out of growing systems with their root system left intact.

Don’t roll out anything less than the red carpet for your herbs when it is time to harvest, ship and sell them. A few key practices and procedures can help extend the shelf life for your fresh-cut herbs further.

Christopher J. Currey (ccurrey@iastate.edu) is an associate professor in the Department of Horticulture at Iowa State University.