Sensors for product characterization and quality of specialty crops—A review

Abstract

This review covers developments in non-invasive techniques for quality analysis and inspection of specialty crops, mainly fresh fruits and vegetables, over the past decade up to the year 2010. Presented and discussed in this review are advanced sensing technologies including computer vision, spectroscopy, X-rays, magnetic resonance, mechanical contact, chemical sensing, wireless sensor networks and radio-frequency identification sensors. The current status of different sensing systems is described in the context of commercial application. The review also discusses future research needs and potentials of these sensing technologies. Emphases are placed on those technologies that have been proven effective or have shown great potential for agro-food applications. Despite significant progress in the development of non-invasive techniques for quality assessment of fruits and vegetables, the pace for adoption of these technologies by the specialty crop industry has been slow.

Introduction

Fifty years ago, a new approach to characterizing fresh food materials was created, which treated food items as physical bodies to which conventional engineering concepts and methods could be applied. The aim was to maintain and enhance the quality of food products as they go through different stages of operation from harvest to postharvest handling to retailing.

Specialty crops are defined as “fruits and vegetables, tree nuts, dried fruits and horticulture and nursery crops, including floriculture” (USDA, 2004). Non-destructive (ND) testing for properties and characteristics of specialty crops is critical for monitoring and controlling product quality and safety. Sensors play the key role in identification of product properties, and thus they have been an active research area, as evidenced by thousands of engineering research publications during the past 50 years.

Quality sensing is needed or desired for most or all agricultural commodities or foods at different stages of the production/marketing chain. Specialty crops cover a wide, diverse variety of commodities, which differ greatly in morphology, composition, and physiology. Hence it is customary to classify them into different groups according to a specific criterion. Temperate fruits, including apple, peach, pear, citrus, and table grape, are harvested manually for fresh consumption, or mechanically for processing. Tropical fruits including avocado, banana, mango, and papaya are also hand harvested, while dry (shell) fruits or nuts are often machine harvested. We should also mention that olive and grape are two fruit crops of world significance; they are mainly machine harvested, where sensors are being introduced for quality monitoring. Vegetables contain even a much greater number of commodities; they are cultivated in different types of environments, including greenhouse. For example, there are fruit vegetables (e.g., tomato, bell pepper, zucchini) and green leafy vegetables (e.g., lettuce, spinach, cabbage, small greens). Finally, ornamentals refer to potted flowers, potted green plants and cut flowers.

After harvest, specialty crops may undergo all or part of these postharvest operations before being delivered to the consumer: pre-sorting, sorting, washing, refrigeration, grading (for quality classes), wrapping, and packaging (placing into cartons, small boxes, baskets, bags, nets, etc.), cold-storage (for short term, i.e., days) or controlled or modified-atmosphere storage (for long term, i.e., months). In addition, some commodities need such special treatments as ripening using gases and temperature (peaches, citrus, bananas), individual wrapping (lettuce, broccoli, cauliflower, bell peppers), cutting and small-bag wrapping (lettuce, mixed salads, fruits), or destruction (e.g., for olive oil and wine grape). Ornamental crops represent a large share of the total production for the specialty crop industry. But there is still a lack of research and progress on development of sensors for ornamental crops, except for automatic production and handling systems which have been widely adopted by the industry.

Specialty crops are living biological products that the consumer expects them to be in the best quality and safety condition. Freshness and quality, which are important to the consumer, are affected by time, handling procedure, environmental conditions, and the processes to which they undergo. At each of these steps, the freshness and quality of specialty crop products need to be monitored and controlled. Traditional manual expertise for quality inspection is no longer adequate nowadays, and only sensors can provide solutions for monitoring and controlling the quality of specialty crops.

A number of reviews on non-invasive, fast technologies for fruit and vegetables quality sensing have been published (Chen and Sun, 1991, Abbott et al., 1997, Studman, 2001, Butz et al., 2005). Many of these review papers covered a broad range of sensing techniques, with a selected few that also discussed the feasibility of these techniques for industrial applications (Abbott, 2004, Walsh, 2005). Moreover, several recent reviews are focused on selected techniques, such as mechanical methods for firmness measurement (García-Ramos et al., 2005), size characterization techniques (Moreda et al., 2009), computer vision (Brosnan and Sun, 2002, Du and Sun, 2006), near-infrared (NIR) spectroscopy (Nicolai et al., 2007), nuclear magnetic resonance (NMR) (Aristizábal, 2007), biosensing (Mello and Kubota, 2002, Patel, 2002), wireless sensing (Ruiz-Garcia et al., 2009), and plant diseases detection (Sankaran et al., 2010). The needs for this area of research are mainly driven by the specialty crop industries to meet increasing consumer demand for better quality and safer fresh products.

A large number of recent publications on non-destructive detection of food quality are related to the utilization of electromagnetic radiation in a wide range of frequencies. Electromagnetic radiation-based technologies have shown great potential; some of them have been successfully used for monitoring the quality of specialty crops. Successful application of these technologies requires the combination of effective sensors with sophisticated mathematical models and computer algorithms to establish relationships between selected physical/chemical properties and quality attributes of the product. As a result, a large number of papers published recently are focused on utilizing different non-destructive (ND) optical techniques for quality detection of agro-food products. Great advances have been made in spectroscopy and computer vision, and these techniques are being widely used for quality inspection and control of products in many industries including food. As technologies based on VIS, NIR, mid-infrared (MIR), and ultra-violet (UV) are becoming more affordable and equipped with more user-friendly data treatment and calibration capabilities, they have fostered further development of detection procedures for different quality- and composition-related properties of fruits and vegetables.

Over the past 10 years, a number of new technologies based on electromagnetic properties have emerged, whereas great progress has also been made on other existing technologies. They include X-ray, nuclear magnetic resonance (NMR) or magnetic resonant imaging (MRI), fluorescence, and with less success until now, electrical impedance and permittivity (mainly microwave), thermal sensing and selective gas/volatile sensing. These developments have opened new areas of research as well as new applications for sensing quality of specialty crops.

This review covers different sensing techniques, with emphasis on those emerging technologies like NMR, MRI, wireless sensor networks (WSN) and radio-frequency identification (RFID), for fruits and vegetables and their potential for industrial applications.