Maintain Your Harvest: Post-Harvest Handling and Storage
Growing a bumper crop of perfect plants is just the beginning—the job of the grower doesn’t end with the harvest!
You have made it this far—your crops are ready for harvest and the route to their final destination is all planned out. This is when many growers start partying, assuming the battle is over and they’ve won.
Even after harvest, though, your plants are not truly dead, as they actually continue to ‘breathe’ (exchange gases) for many days after they are picked. They can’t grow any more or increase their sugar content after they’ve been harvested but they can degrade, which means that after harvest a crop can only maintain or lose value.
I have had the opportunity to study in some depth what happens to plants after they are harvested and I know that typically as much money and effort can go into a natural product after it is harvested as went into growing it. The basics of these post-harvest processes involve cooling, processing and storage.
The cooling process pertains to temperature regulation of post-harvest crops, which is critical because it directly affects metabolism. Plants start preparing for death long before harvest, but live for some time after. Metabolic processes do not cease until the food source (water, carbohydrates and minerals) is no longer available. The harvested commodity is inherently susceptible to the elements after harvest, because without roots it can no longer self-regulate temperature or humidity.
With regard to the temperature of harvested plants you have to consider not only the ambient temperature of the environment where they are stored but also the heat given off by the harvested commodity itself. All living cells respire, producing heat by using sugar and releasing carbon dioxide.
Most people think plants only take in CO2 and give off oxygen, but the truth is most plants actually respire and give off CO2 as well, during the night. Metabolic processes like respiration produce heat and once harvested, most crops should be moved to an area with a lower temperature. This is especially true for leafy crops, where the heat of respiration is considerable.
Case in point—if you harvest basil plants and place them directly into a garbage bag and wait 20 minutes, when you reach into the bag again you will notice a dramatic increase in temperature. This is due to the plant’s respiration, resulting in an increase in heat and humidity.
Humidity is closely linked to temperature during post-harvest processes. Leafy plants left to be dehydrated should have plenty of air circulation and a moderately low humidity, which will slow the metabolism of the plants.
Once the metabolism slows, lowering the humidity until the moisture content is below 15 per cent will keep the product from molding after storage. For home hobby growers, the more slowly and gradually you can lower the humidity the less stress it will cause to the dying plant.
That ‘15 per cent or less’ mark is critical, so don’t stop too soon! Many growers get their vegetables or herbs nearly dry and begin storing them, only to return a week or two later and be horrified to find the entire stored commodity affected by mold. On the other hand, plant parts that are dehydrated too long with moisture levels that are too low tend to crumble, disintegrate and lose flavor and aroma. Balance is the key!
While most home hobby growers won’t actually bother to measure moisture content, it can be done quite easily. In the method explained here, the sample will be destroyed by drying to a theoretical zero moisture content. First, pull out a test sample and note the precise weight of the material.
Then bake the sample in the oven at a very high heat for a few hours. Measure the weight of the sample again, directly out of the oven—avoid waiting too long after taking the plant material out because it can actually reabsorb moisture and throw off your calculation.
Finally, for the calculation, subtract the smaller weight from the larger weight. This number is then divided by the original larger weight. The result will give you the moisture content of your original sample. Keep in mind the more accurate your measurement, the more accurate your moisture content calculation will be.
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The main goal of processing, regardless of method, is to extend the shelf life of a product while maintaining quality. There are other types of processing besides dehydration—the herbal cultivation industry also uses extraction processing, for instance. Extraction is achieved by using one of several different solvents at various temperatures and pressures. Other common types of processing are freezing, freeze drying and canning.
After cooling and processing are completed you’ll have to consider storage options. There are many variables that can degrade plant products during storage. Oxygen and other gases influence degradation the most, along with light, heat, moisture and pests.
Yes, we breathe it—but oxygen is probably the most destructive molecule on the planet! Several oxygen radicals are toxic to all living cells in both plants and animals. One example is ozone (O3), which we currently use in many sterilization processes.
Other examples of radical oxygen varieties are super oxide and even hydrogen peroxide, both byproducts of metabolic processes. Once you have reached a low enough water content (less than 15 per cent) you can store most products safely in an airtight container so that oxygen and humidity are excluded and cannot degrade the commodity.
As an added precaution nitrogen gas can be blown into the container, pushing out other gasses and thereby replacing the harmful oxygen with nitrogen—after which the container must be sealed quickly. Nitrogen gas is nonreactive and will not degrade plant products.
Have you have ever been told not to put apples and bananas together? There is actually a scientific reason for this. Apples produce a lot of ethylene gas, which actually causes ripening in many plants. In fact, most tomatoes in stores are picked nearly green and ripened with man-made ethylene. Ethylene gas is also produced by natural gas heaters and even some light ballasts—and should be avoided during storage of plant products to prevent further ripening.
Light can also degrade certain plant products. It is mainly known to degrade surface waxes and this can be a problem because many secondary metabolites are stored in the surface waxes. A solution for light degradation can be as simple as storing the product in opaque containers.
Once your products are in storage there are still a few more challenges to address. Moisture can creep into the smallest imaginable places and ruin a stored crop. Simple warming and cooling can cause condensation, which will produce just enough moisture to encourage mold.
Heat is also known to degrade the structure of all proteins and the amino acids (the building blocks of proteins) that make up plant tissue are also susceptible. If you can’t store your harvested crops in a cold room they should be at least be stored somewhere cool, where temperatures never exceed 75°F.
You should also be aware of the possibility of pests in any storage situation—if there is a food source, pests are adept at finding it. The best solution is to pay close attention and not let creatures like rats, roaches or ants sneak into your storage area while you are visiting.
Precision growing for production is very labor-intensive and those who achieve long-term success deserve an enormous amount of respect for their accomplishments. The grower’s work begins before planting and continues long after harvest and involves meeting many difficult challenges along the way to maintain the quality of the final product for the consumer.
Keep this in mind the next time you select fresh vegetables or herbs from your local grocery store—and perhaps you’ll begin to develop a newfound respect for the precision grower. And if you are that grower, pat yourself on the back—we salute you!
Written by Shane Hutto | Owner
Shane Hutto is the owner of Horticultural Solutions, a cannabis cultivation consulting company. He earned a bachelor’s degree in horticulture at Oklahoma State University and received a research assistantship for his master’s degree. During his graduate studies he researched production and extraction of surface waxes on horticultural commodities. His passion for growing is complemented by his experience in many types of controlled environment operations and design.