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Evaluating Vegetable Quality Nondestructively

By:  Mark Ritenour, Associate Professor
Indian River Research and Education Center – Ft. Pierce
&
Thomas Burks, Associate Professor
Agricultural and Biological Engineering

Produce quality and quality retention during storage, transportation, and retailing is critical for the successful marketing of 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, quality determinations often require destructive methods, such as cutting, to visually evaluate fresh vegetables 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 nondestructive methods to evaluate vegetables individually to assure buyers and consumers of the highest quality.  

Substantial progress has been made developing technology and systems to measure the quality of each fruit or vegetable, without injuring them, as they move past sensors at speeds up to 10 items per second. Nondestructive tests have been developed measuring 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 currently 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 nm) that measure variations in water density within tissue, to radio waves (long wavelengths reaching as long a 100 km) 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 to the human eye. Images taken of produce as they pass by at rapid speeds 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 produce characteristics such as internal soluble solids (Brix) and acid content and even internal injuries and firmness. These measurements are nearly instantaneous and can be conducted relatively easily. A number of 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 as described above, but may also allow discrimination between specific types of blemishes (Fig. 1). Such capabilities would be extremely useful if, for example, the system could distinguish a lesion from a specific disease of quarantine significance from other ordinary surface grade defects (Fig. 2).

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 people are familiar with of the capabilities of magnetic resonance (MR) imaging as a medical tool that produces detailed images inside the human body. The technology also has great 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 of vegetables such as tomato, cucumbers, squash, and beans. 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 more for research and specialty application as the technology advances and the cost of MR comes down.

Sound waves. Both acoustic sound waves in the range of human hearing (20 Hz to 20 KHz) and ultrasonic waves which are above human hearing (20 KHz to 1 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, and internal defects such as bruising and hollow heart of potato.

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 incorporated into packaging materials that allow monitoring of gasses such as ethylene to provide an indication of product ripeness. Some sensors may even detect volatiles associated with human pathogens to prevent the distribution or consumption of contaminated produce.

While these methods offer the promise of peering inside individual produce without injury, the technology is often expensive and sometimes difficult to adapt to commercial conditions. 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, replaced by a much more complete picture of vegetable quality taken non-destructively on every individual vegetable item.

(This article is adapted from an article published in the Feb. 2009 American Vegetable Grower magazine.)

Fig. 1 Prototype optical multispectral optical grader for disease and blemish detection. (Photo by Thomas Burks)

Fig. 2 Hyperspectral image of a specific disease lesions (left = canker) compared to other peel diseases and blemishes of citrus fruit. Such technology could also be used for vegetables.  (Photo by Thomas Burks)