The compositional quality of hydroponic produce is something we don’t often give that much thought to. It’s easy to assume that all well grown, attractive, highly productive, and rapidly grown fruit and veg will automatically give a taste sensation we want to savor. Just as we expect that biting into a large, succulent red strawberry will give an incredible sensory burst of sweetness combined with that distinctive aroma. The reality doesn’t always live up to expectations and sometimes that attractive berry can be insipid and lacking in any real flavor. The same is true for a wide range of plants commonly grown in hydroponics, while we can maximize yields, size, and appearance quality, nailing that intense, distinctive, homegrown flavor and aroma profile can be more challenging. This is because many of the compounds contributing to sensory quality are influenced by a diverse range of factors and aiming to just maximize yield may not necessarily give the optimum taste experience.
Whether it’s fragrance from hydroponic blooms, the distinctive smell of crushed basil, or the Mediterranean scents of rosemary and oregano, aroma is an important aspect of sensory quality. Airborne volatile compounds, of which there may be many hundreds in each plant species, are detected by the olfactory nerve endings in the nose. These sensations are then combined with taste receptors on the tongue to provide an overall flavor experience. However, while the tongue can detect flavor compounds in parts per hundred, we can usually detect volatiles in parts per trillion. In tomato fruit, more than 400 volatile aromatic compounds have been identified that contribute to the overall flavor experience of the fruit. In many aromatic herbs and plants we grow, it is the production and concentration of essential oils in the foliage and flowers which give the characteristic aroma and flavor. Hydroponic herbs such as basil, marjoram, mint, oregano, thyme sage, and rosemary contain high concentrations of specific essential oils which we have come to associate with the distinctive aroma of each.
The Role of Stress in Flavor Profiles
Many of us have pleasant memories of great tasting and fantastically aromatic produce picked outdoors in the height of summer. There is some science behind this “summer flavor phenomenon” as it relates to plant physiology and it gives us insight into how plants respond to their environment. Plant stresses such as those experienced in summer include high light, lack of moisture, low humidity, warm conditions, salinity, and other issues that may all act to reduce plant growth and yields; however, they often have a positive effect on fruit flavor and aromatic compounds. Rosemary plants, for example, which originate from a hot, dry, high light climate, concentrate essential oils in the foliage under these conditions, however, when grown in a less stressful environment, flavor and aroma can be somewhat lacking.
Plants grown under some degree of climatic stress such as high light and reduced moisture also restrict their uptake of water; this concentrates many compounds within the plant tissue which are related either directly or indirectly to flavor and aroma. Field tomatoes, for example, are often grown with “deficit irrigation practices” to concentrate the dry matter, sugar compounds, and flavor in the fruit destined for processing or fresh consumption. This application of stress science applies to several fruiting crops, where increased brix (total soluble solids or sugars) is desirable. In hydroponics, we have traditionally aimed to provide the plant with more than sufficient water and nutrients, so all its requirements are fully met. However, application of slight moisture stress is a technique sometimes used by commercial growers to improve fruit compositional quality and to also restrict unwanted vegetative growth. Moisture stress in the root zone creates a higher osmotic potential, however, this can also be achieved by increasing the electrical conductivity (EC) of the nutrient solution. Electrical conductivity control is easier to measure and manage than imposing moisture stress in the highly restrictive root zone of most hydroponic crops, and is less likely to be taken too far, causing plant damage. Bringing EC levels in the root zone up just as the plant has passed the vegetative stage and has set the first fruit is a common practice in commercial hydroponic tomato crops. This has been shown to have a number of positive effects, such as improving not only the compositional quality in terms of flavor, sweetness, and volatiles, but to also prolong shelf life and firmness which are important post-harvest factors. Modern tomato hybrids bred for greenhouse and hydroponic production are particularly tolerant to high EC levels and respond well to this technique of quality improvement. Heirloom and older tomato varieties can also respond to increases in EC to maximize flavor, however, not to the same extent as commercial hybrids and they can become more prone to disorders such as blossom end rot when this is attempted. In hydroponic tomatoes, it has been found that the flavor profile, sugar, acid, and sodium content of fruit grown at an EC of 8.0 mScm-1 was far greater than fruit grown at an EC of 3.0. However, increasing the EC to improve flavor via a higher percentage of dry matter in the fruit tends to give smaller fruit and lower yields. So, there is often a trade-off between flavor improvements and yield potential.
With hydroponic chili crops, EC levels as high as 8.0 mScm-1 have been applied to boost pungency with good results, however, different chili cultivars will respond differently to increases in EC and growers need to determine for themselves which will give the biggest ‘kick’ in their fruit. When using a higher EC to increases the pungency of chilies, it’s best to do this by increasing only the macro nutrients in solution (nitrogen, phosphorus, potassium, sulfur, and calcium) and maintain the trace elements at normal EC strength levels. For crops such as onions, garlic, shallots, and chives, the strong flavor and aroma is derived from the present of organosulfur compounds. In hydroponic allium crops, these distinctive flavors have been shown to be boosted by using higher levels of nitrogen and sulfur in the nutrient solution. Brassica crops such as watercress, arugula, cabbage, kale, and others also derive much of this distinctive flavor from sulfur containing compounds called glucosinoles. Manipulation of sulfur in the nutrient solution can help boost these flavors, which range from cabbagey to peppery, and pungent in watercress to sharp and nutty in arugula. Increasing sulfur levels in hydroponics have also been shown to assist flavor or pungency in condiment herbs such as wasabi or horse radish which contain mixtures of volatile compounds such as isothiocyanates which give them their flavor and heat.
Other stresses that may be applied during hydroponic plant growth to improve sensory quality are also those which concentrate volatiles and influence the water balance of the plant. High light not only ensures maximum rates of photosynthesis, which produce assimilates or sugars which are imported into fruit, but also acts to provide another stress factor, increasing the overall dry matter accumulated by the plant. Studies have also shown that full spectrum light, including incorporation of some UV-B wavelengths, can improve the content of antioxidant compounds in plants such as spinach grown under protected cultivation. Low humidity, which speeds up the rate of moisture loss from the foliage, influences the water balance of the plant and the resulting osmotic adjustment gives a low level of continual stress and a concentration of volatile compounds within the plant. Warm conditions, with high rates of air flow, are also stressors in many plants which can all act to play a role in dry matter accumulation.
Other Flavor Factors
While stress application to improve compositional quality is a useful tool for hydroponic crops, other factors influencing flavor should not be overlooked. These include basic plant production principals such as providing a well-balanced nutrient solution, increasing potassium for plants in fruit, and containing all the essential elements. With indoor gardens, attention to light levels and leaf area determine the amount of sugar available for importation into fruit, and with many crops the leaf-to-fruit ratio is vital to ensure not only fruit size but also quality. Crops such as tomatoes, grapes, melons, and berries may need fruit thinning to ensure all fruit can receive sufficient assimilate for optimal compositional quality.
Sensory Quality and Variety Selection
While growing conditions and manipulation of the root zone EC can play a major role in quality determination, genetics are also an important tool for hydroponic growers. Varieties of the same plant species can differ considerably in flavor and aromatic profile. It’s a waste of energy putting a great deal of time and effort to boost flavor using cultivars which have poor flavor genetics in the first place. So, growers should select naturally high-flavored or aromatic varieties to experiment with stress techniques to maximize this even further.
Flavor Quality Assessment
To determine if the application of plant stress has been used successfully and achieved flavor improvement, there are a couple of different evaluation approaches. Taste testing, or “sensory evaluation,” is one method, however, it can be difficult to compare samples from crops grown months apart and flavor assessment is best carried out when there are two or three different treatment samples to compare at the same time. Having a number of different people assess the samples is also advisable and blind testing, where taste panellists don’t know which sample is which, is also standard practice. For a more analytical approach, lots of fruit such as berries, grapes, tomatoes, melons, and others can have sugar levels (total soluble solids) directly measured using a portable brix meter (refractometer). While industry standards for brix exist for most fresh products — as a basic guide for tomatoes, a great tasting beefsteak tomato will have a brix of at least seven, smaller cocktail types a brix of more than 10, and a poor testing fruit often have brix levels below five. Most people can usually taste a difference of just one degree in brix, however, taste is somewhat subjective with human evaluation, while brix meters give hard and fast data. Hand-held brix meters are not expensive and can be easily obtained by growers who are keen to assess their flavor improvement techniques.
Sensory quality in hydroponic produce encompasses a complex array of flavors from sweet, sour, salty, and bitter, which interact with a vast number of aromatic volatile compounds to give an overall taste experience. While indoor hydroponic gardens can provide a perfect environment for high yields of beautifully presented produce, flavor and aroma can be given a helping hand with the careful application of some well-timed plant stress. Understanding how stress, osmotic adjustment, and other plant processes influence that final sort-after flavor can lead to some interesting experimentation and great tasting results.