Postharvest engineering refers to the sorting, packaging, temperature management, transporting, and temporary storing of biological materials. Sorting and grading operations guard product quality. For example, grading determines if the product meets quality standards, separates products into different quality grades to determine the price paid to the grower or the sale value, and enables removal of off-grade products. Optimal maturity, color, sugar, solids, moisture content, size, and absence of defects are some of the factors considered for various fresh market products.
Most quality detection and sorting operations are still done by human vision. These operations are slow, tedious, and often yield inconsistent results. Thus, there is a need to develop quality detection devices which can operate consistently at high speed. In some cases, devices which “see” visible differences (for example, color, surface scars, shape) are desired, and, in other cases, devices which detect other types of characteristics which correlate with quality (for example, acoustic response, mass density, optical density, firmness) are desired. Current sorting projects include the following technologies: magnetic resonance spectroscopy and imaging, near infrared spectroscopy, acoustic response, impact response, and machine vision. Several of these projects are described in the section Sensor and Control Engineering.
Decay organisms are a major source of quality loss for processed and fresh market products. A project has recently begun to detect and quantify mold in tomatoes arriving at the processor. A biotechnology process which rapidly separates mold from a tomato sample is being developed as a procedure for grading truck loads of freshly harvested processing tomatoes. The process also holds promise as a method of measuring airborne fungal spores in citrus processing and storage operations. This will allow better use of the limited chemical control materials available to this industry. The technology could also be used to measure and identify airborne allergens and disease organisms.
There is an ongoing effort to model the heat and mass transfer in cooling and drying operations. The models have been used to improve the energy efficiency and predict process times and moisture variability of outgoing products. The modelling is supported by a continuing effort to measure the physical properties of numerous food products.
Engineers working with postharvest physiologists in several campus departments continue to work in a long-standing and successful program to improve the quality of fresh fruits, vegetables, and flowers. Over the years the interdepartmental team has developed improved cooling methods, better packaging and handling systems, and a basic understanding of the quality loss mechanisms in perishable foods.
Transporting fresh produce can result in mechanical injury and quality loss. The objectives of several current projects involving pears, strawberries and grapes are to characterize the transit environment and to determine fruit susceptibility to vibration of different frequency and acceleration components. The results from recent studies are leading to new packaging techniques and transit trailer design which will improve the quality of the produce upon its arrival. For example, a recently completed study on grapes indicates that the practice of over filling boxes results in considerably more compression damage and increased vibration amplifications in palletized units. For both grapes and pears the study has shown that packaging the fruit in polyethylene bags greatly reduces the vibration injury. A different study of cross-country transit tests revealed that the vibration level on the floor of an air-ride suspended trailer is much lower than that of a convention-al steel suspended trailer. As a consequence, it is now recommended that strawberries be transported in air-ride trailers whenever possible and never at the rear of a steel suspended trailer.
Insect damage to agricultural produce during storage can be significant. A recent project evaluated high temperature disinfestation and low temperature storage to maintain dried fruit and nuts free of insects without having to resort to chemical fumigation. Currently, radiant cooling is being assessed as a low cost method for cooling dried products.
Some of our postharvest research focuses on techniques for predicting fruit and vegetable quality when marketed. Since customers generally prefer firm cherries, we are developing a method of predicting the firmness of the fruit at market, based on measurements taken at the time of packing and subsequent handling procedures.
Removal of defective specimens prior to fruit shipment to other locations can lead to more efficient food distribution. A low-mass impact sensor has been used for evaluating fruit quality. The unit was capable of sorting fruits at the maximum conveyor belt speed of six fruits per second.
Magnetic resonance imaging can be used for sorting fruits, based on oil or sugar content, or internal defects. Our researchers are presently developing a real-time sorting machine with speeds of three to five fruits per second.
Different packaging and stacking techniques for cross-country transit are evaluated on a special vibration platform. The vibration frequencies and amplitudes which are damaging to a variety of fruits and vegetables can be determined and eventually applied to developing ideal transport conditions.
Cross country truck transit vibration tests were conducted to validate laboratory data on various experimental packaging designs. Pears were the focus of this particular study.
This container of cabbages is fitted with thermocouples for determining temperature profiles of different packaging configurations. Rapid cooling of produce is desirable prior to shipping.