Microgreen

Visiting scientist Liping Kou (left) and technician Ellen Turner harvest different types of microgreens for shelf-life studies and nutrient analyses.

A microgreen is a tiny vegetable green that is used both as a visual and flavor component or ingredient primarily in fine dining restaurants. Fine dining chefs use microgreens to enhance the beauty, taste and freshness of their dishes with their delicate textures and distinctive flavors. Smaller than “baby greens,” and harvested later than “sprouts,” microgreens can provide a variety of leaf flavors, such as sweet and spicy. They are also known for their various colors and textures. Among upscale markets, they are now considered a specialty genre of greens that are good for garnishing salads, soups, plates, and sandwiches.^[1]

Edible young greens and grains are produced from various kinds of vegetables, herbs or other plants. They range in size from 1” to 3” including the stem and leaves. A microgreen has a single central stem which has been cut just above the soil line during harvesting. It has fully developed cotyledon leaves and usually has one pair of very small, partially developed true leaves. The average crop-time for most microgreens is 10–14 days from seeding to harvest.^[1][2]

History

Microgreens began showing up on chefs' menus as early as the 1980s, in San Francisco, California.^[1] In Southern California, microgreens have been grown since about the mid‑1990s. There were initially very few varieties offered. Those available were such as arugula, basil, beets, kale, cilantro and a mixture called Rainbow Mix. Having spread eastward from California, they are now being grown in most areas of the country with an increasing number of varieties being produced.^[3] Today, the U.S. microgreens industry consists of a variety of seed companies and growers.^[1]

Form

Microgreens have three basic parts: a central stem, cotyledon leaf or leaves, and typically the first pair of very young true leaves. They vary in size depending upon the specific variety grown, with the typical size being 1 to 1.5 in (25 to 38 mm) in total length. When the green grows beyond this size, it should no longer be considered a microgreen. Larger sizes have been called petite greens.^[3] Microgreens are typically 2–4 weeks old from germination to harvest.^[4] Both baby greens and microgreens lack any legal definition. The terms "baby greens" and "microgreens" are marketing terms used to describe their respective categories. Sprouts are germinated seeds and are typically consumed as an entire plant (root, seed, and shoot), depending on the species. For example, sprouts from almond, pumpkin, and peanut reportedly have a preferred flavor when harvested prior to root development. Sprouts are legally defined, and have additional regulations concerning their production and marketing due to their relatively high risk of microbial contamination compared to other greens. Growers interested in producing sprouts for sale need to be aware of the risks and precautions summarized in the FDA publication Guidance for Industry: Reducing Microbial Food Safety Hazards for Sprouted Seeds (FDA 1999).^[5]

Growing

Growing microgreens is relatively easy, many small "backyard" growers have sprung up selling their greens at farmers' markets or to restaurants. A shallow plastic container with drainage holes, such as a nursery flat or prepackaged-salad box, will facilitate sprouting and grow out on a small scale.^[6] Growing and marketing high-quality microgreens commercially is much more difficult.^[5]

Nutritional analysis

ARS scientists analyzed key nutrients in 25 different varieties of microgreens and found that red cabbage microgreens (shown here) had the highest concentrations of vitamin C. These nutritious microgreens are ready to harvest just 10 days after planting.

Researchers at the USDA Agricultural Research Service have published, as of early 2014, several studies that identify the nutritional make-up and the shelf life of microgreens. Twenty-five varieties were tested, key nutrients measured were ascorbic acid (vitamin C), tocopherols (vitamin E), phylloquinone (vitamin K), and beta-carotene (a vitamin A precursor), plus other related carotenoids in the cotyledons.

Among the 25 microgreens tested, red cabbage, cilantro, garnet amaranth, and green daikon radish had the highest concentrations of vitamin C, carotenoids, vitamin K, and vitamin E, respectively. In general, microgreens contained considerably higher levels of vitamins and carotenoids—about five times greater—than their mature plant counterparts, an indication that microgreens may be worth the trouble of delivering them fresh during their short lives.

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A nutritional study of microgreens was done in the summer of 2012 by the Department of Nutrition and Food Science, University of Maryland,^[7] indicating promising potential that microgreens may indeed have particularly high nutritional value compared to mature vegetables. Bhimu Patil, a professor of horticulture and director of the Vegetable and Fruit Improvement Center at Texas A&M University, agrees that microgreens may potentially have higher levels of nutrients than mature vegetables. But he says more studies are needed to compare the two side by side. "This is a very good start, but there can be a lot of variation in nutrients depending on where you grow it, when you harvest, and the soil medium," Patil says.^[8] When choosing a microgreen, researchers say to look for the most intensely colored ones, which will be the most nutritious.^[9] Results of the microgreens research project conducted by the University of Maryland and the USDA has garnered attention from several national media outlets including National Public Radio (NPR) and The Huffington Post.^[10]

Microgreens sprout comparison

Sprouts are germinated or partially germinated seeds. A sprout consists of the seed, root, stem. The FDA seeks to regulate all businesses that produce sprouts due to numerous outbreaks of food poisoning ("sproutbreaks"). Salmonella and Escherichia coli O157:H7 have been the major causes of sprout-associated illness outbreaks. Commercial sprout processors are asked to follow rigorous FDA Guidelines for production that include multiple laboratory tests of each batch for the presence of pathogenic bacteria, to minimize the threat of food borne illness. Despite these precautions, there continues to be regular recalls and alerts relating to sprouts with more and more restaurants removing them from menus to reduce their liability. In 2011, 52 people died and thousands became ill due to consumption of locally grown organic sprouts in Europe. The two largest U.S. retailers Walmart and Kroger are no longer selling sprouts. "Sprouts present a unique challenge because pathogens may reside inside the seeds where they cannot be reached by the currently available processing interventions," Payton Pruett, Kroger's Vice President of Food Safety, said in an October 19, 2012 news release.^[11]

Sprout seeds are soaked in water for usually eight hours and then drained. A high density of seed is placed inside of sprouting equipment or enclosed containers. The seed germinates rapidly due to the high moisture and humidity levels maintained in the enclosures. Seeds can also be sprouted in cloth bags that are repeatedly soaked in water. The sprouting process occurs in dark or very low light conditions. These dark, wet, crowded conditions are ideal for the rapid proliferation of dangerous pathogenic bacteria. After a few days of soaking and repeated rinsing in water (several times a day to minimize spoilage), the processing is complete and the sprouts are ready to consume.

Microgreens are grown in soil or soil-like materials such as peat moss. Microgreens require high light levels, preferably natural sunlight with low humidity and good air circulation. Microgreens are planted with very low seed density compared to sprout processing. Crop times are generally one to two weeks for most varieties, though some can take four to six weeks. Microgreens are ready to harvest when the leaves are fully expanded. Harvesting is usually with scissors cutting just above the soil surface, excluding any roots. Some growers sell them while still growing, rooted in the growing trays so that they can be cut later. Once removed from their growing environment, these trays of microgreens must be used quickly or they will rapidly begin to elongate and lose color and flavor.

The conditions that are ideal for properly grown microgreens do not encourage the growth of dangerous pathogens. These growing methods would not work for the production of sprouts.

With stronger flavors compared to sprouts, they are an excellent ingredient with a wide selection of leaf shapes, textures and colors.

The potential for food safety issues with microgreens may be increasing due to the number of indoor microgreen growing operations in which excessive seed density, low light intensity, low air circulation or most commonly, a lack of GAP (good agricultural practices) and GMP (good manufacturing practices) based food safety procedures. Certain provisions of the Guidance for Industry: Reducing Microbial Food Safety Hazards For Sprouted Seeds ^[12] may be beneficial and prudent for growers of microgreens to follow.

Storage and commercial transport

Microgreens have a short shelf life and better methods of storing and transporting microgreens are currently being studied, which at this time are mainly focusing on buckwheat. Commercial microgreens are most often stored in plastic clamshell containers, which do not provide the right balance of oxygen and carbon dioxide for any live greens to breathe. Among package materials called films, differences in permeability, (see Permeation), are referred to as the oxygen transmission rate.

The researchers found that buckwheat microgreens packaged in films with an oxygen transmission rate of 225 cubic centimeters per square inch per day had a fresher appearance and better cell membrane integrity than those packaged in other films tested. Following these steps, the team maintained acceptable buckwheat microgreen quality for more than 14 days—a significant extension, according to authors. This study was published in LWT-Food Science and Technology in 2013.^[1]

Light-emitting diodes. otherwise known as LEDs, now provide the ability to measure impacts of narrow-band wavelengths of light on seedling physiology. The carotenoid zeaxanthin has been hypothesized to be a blue light receptor in plant physiology. A study was carried out to measure the impact of short-duration blue light on phytochemical compounds, which impart the nutritional quality of sprouting broccoli microgreens. Broccoli microgreens were grown in a controlled environment under LEDs using growing pads. Short-duration blue light acted to increase important phytochemical compounds influencing the nutritional value of broccoli microgreens.^[13]

References