Postharvest Hints: Non-Destructive Evaluation Of Vegetable Quality
February 1, 2009 
Produce quality and quality retention during storage, transportation, and retailing is critical to successfully market fresh vegetables. However, the term “quality” has far reaching components that include sensory quality appearance, flavor, texture, etc.), nutritional quality (protein, vitamins, minerals, fiber, etc.), and food safety (chemical and microbial contaminants).
Currently, determination of fresh vegetable quality often requires destructive methods, such as cutting, to visually evaluate for internal defects and extracting juice to measure the concentration of important chemical components such as sugars, acids, etc. Product firmness is often measured using a penetrometer that punctures the product.
Because these tests are destructive, only a sample of the product is tested and the results applied to the entire lot. Since there is natural variation in product quality with some having inferior taste or containing internal defects or injury that are not apparent upon normal visual inspections, there is intense interest in methods to evaluate vegetables individually to assure buyers and consumers of the highest quality.
Measuring Without Injury
Substantial progress has been made developing technology to measure the quality of each fruit or vegetable — without injury — as they move past sensors at speeds of about 10 items per second. Non-destructive tests measure attributes of flavor, texture, and internal defects (i.e., internal bruising and insect injury). The following is a brief overview of some of the new technologies available or that are being developed to non-destructively measure the quality of fresh vegetables.
Measurements based on electromagnetic waves. Wavelengths used to measure produce quality range from X-rays (short wavelengths down to 0.01 nanometer or nm) that measure variations in water density within tissue, to radio waves (long wavelengths reaching as long a 100 kilometers) that are used for magnetic resonance imaging. How these waves are transmitted, reflected, absorbed, and scattered by produce tissues can provide useful information about the quality of the product as a whole.
Within this spectrum is the relatively small band of wavelengths (400 to 750 nm) that are visible. Images taken of produce are quickly analyzed by computers to measure product size, shape, volume, color, and surface defects. While measurements of light reflectance can identify surface attributes, measurements of light transmission through the produce can measure internal defects such as hollow heart of potato or impact damage of cucumber.
More recently, infrared and near infrared (NIR) light (750 to 2,500 nm) have been used to measure internal soluble solids (Brix) and acid content and even internal injuries and firmness. These measurements are nearly instantaneous and can be conducted relatively easily. Some companies now sell detectors that can be integrated into existing optical sizing and grading lines.
Multispectral and hyperspectral imaging systems combine measurements taken at different wavelengths (e.g., NIR and visible light) to not only measure attributes, but may also discriminate between specific types of blemishes. Such capabilities would be useful if, for example, the system could distinguish a lesion from a specific disease of quarantine significance from other ordinary surface grade defects.
Another method to evaluate the health of fresh produce is through the measurement of chlorophyll fluorescence. Researchers have found that measuring light fluorescing from chlorophyll within fresh vegetables can serve as an indicator of stress. Such stress might occur after exposure to chilling temperatures or as a result of bruising. While the technique can detect injury before becoming visible, some chlorophyll fluorescence measurement techniques may be difficult to use commercially because the equipment to measure fluorescence is expensive and the tissue must be kept in darkness for some time before taking a measurement.
Most are familiar with the capabilities of magnetic resonance (MR) imaging as a medical tool. The technology also has potential for evaluating the internal structures of fruits and vegetables. For example, it has been used on fresh vegetables to evaluate maturity and ripening and to measure internal quality. These include measurements of soluble solids, total solids, firmness, core breakdown, bruising, insect damage, and chilling and freezing injury in different products. While showing great promise, the technology is currently too expensive and difficult to operate for routine quality testing, but may be used for other applications as the technology advances.
Sound waves. Both acoustic sound waves in the range of human hearing (20 hertz or Hz to 20 kilohertz or KHz) and ultrasonic waves which are above human hearing (20 KHz to 1 megahertz or MHz) are being used to non-destructively evaluate the quality of fresh vegetables. For acoustic sound, a device is often used to lightly tap or thump the commodity to create a sound wave that moves through the product tissue. The characteristics of the sound waves as they pass though the product can indicate quality attributes such as firmness and maturity.
Solid state or biosensors. These sensors use solid-state or biological materials such as antibodies, enzymes, or molecular probes to measure different biochemical markers of vegetable quality such as those involved in fruit ripening, aroma, taste, etc. Sometimes called an electronic nose or tongue, sensors that “sniff” the air around a product can detect ripening, injury, or decay-related volatiles. We mentioned in a previous article how some sensors are already being used in packaging materials that allow monitoring of gasses.
While these methods offer the promise, the technology is often expensive and sometimes difficult to adapt. As prices come down and the technology is adapted to harsher commercial environments, the days of cutting up produce to evaluate internal vegetable quality may become a thing of the past. The act of cutting up produce will be replaced by a much more complete picture of vegetable quality taken non-destructively on every individual vegetable item.