Introduction

The importance of high quality and efficiency in food processing is growing as consumer demand for new and better products at lower prices expands. The success of new product introductions lies with consumer acceptance of key food attributes such as texture, color, flavor, freshness, and nutrition. Automation in food processing allows for control of the consistency of these attributes by measuring a specific property, adjusting processing conditions to maintain the attribute associated with the property, comparing the measurement to predefined specifications and through computer control, adjusting the process to maintain overall consistency. Such automation is essential to the establishment of dynamic, flexible, and competitive manufacturing technologies. Well positioned, on-line+ sensor technology is at the core of efficient process control and assurance of high product quality. Measuring physical and chemical attributes of food materials in real-time must be achieved to meet process control targets and ensure product quality.

Traditional measurements, done off-line in a laboratory, are performed under controlled conditions. Since these situations do not mimic actual processing conditions, such measurements will obviously be of limited usefulness for realtime process control. In addition, with off-line measurements, a significant time lag will often occur for results to be translated into process adjustments, thereby creating the possibility that significant production runs will be compromised.

In-process measurements, in contrast, allow real-time determination of chemical and physical properties thus allowing immediate feedback for process control leading to optimization of attribute quality. On-line measurements, therefore, save money in terms of both loss of product and efficiency of process.

+ The term "on-line" is used to refer to real-time measurement during processing. In this sense, it includes both in-line measurement (within a processing system) and on-line measurement which may use a side-stream bypass of material for measurement.

This chapter will focus on the types of sensors currently available for use in food processing systems and the new technologies emerging from research laboratories throughout the world.

The importance of on-line sensors to the food industry was noted by the U.S. Department of Energy (DOE) in a January, 1990, report entitled "Assessment of Sensors Used in the Food Industry", prepared by the National Food Processors Association [1]. The DOE supported this study because of its interest in activities to improve the efficiency of energy conservation and utilization systems. The DOE believed that in-line sensors in food processing could achieve these goals. The study was based on interviews with more than 50 representatives of food companies, food equipment manufacturers, and representatives of sensor manufacturers and suppliers and 13 industry, academic and consortia sensor research and development organizations. These people were selected to determine the current status of food industry process control sensor technology. Instrument manufacturers were contacted to obtain information about current capabilities and market status; food processors were contacted to identify available technology and assess its strengths and weaknesses.

A consensus emerged from the DOE survey as to characteristics of an ideal on-line sensor. Table 1 lists the most important of these characteristics. Other studies of industrial needs found support for similar requirements [2-5], For example, Kress-Rogers [2] also found that an ideal sensor should be free of catalysts that might enhance oxidative processes in the food. In addition, total costs including capital, maintenance, and operating, should be low in relation to the benefits of overall process control.

Table 1

Characteristics of an ideal on-line sensor

• Easily maintained

• Tolerant of harsh food processing plant conditions such as vibration, environment laden with moisture, dust, or particles, high cooking temperatures, wide temperature cycles, chemical sanitizing and cleaning, exposure to oils and solvents, and abrasion and impact from passing product

The DOE survey also examined the most important in-process measurements to the food industry addressing process control as well as quality assurance. They are listed in Table 2 [1-6]. Giese [4] provides details about various tests and measurements that can assist in quality assurance during food processing for production of safe, wholesome foods. These tests include immunoassays, near infrared spectroscopy, chemical sensing, and color measurement.

Table 2

Priority sensing needs for food processing

• Moisture in solids

• Chemical composition

• Rheological properties

• Presence of foreign matter

• Soluble solids

Scientific understanding of material properties and sensor technological developments are emerging to address the needs identified by food processors. The food industry has generally lagged behind other process industries in the use of sensors and related microprocessor control instrumentation. Among the more important reasons for this are the variability in raw ingredients used in food processing, the complexity of food materials, the lack of availability of online sensing systems that can perform in harsh food processing environments, and the clean-in-place needs for safe food processing systems [2, 3, 7],

New sensors are being developed in areas such as moisture determination to meet these identified needs, by a variety of research laboratories and research centers worldwide [1], At consortia such as the Center for Process Analytical Chemistry (CPAC) at the University of Washington (Seattle, Washington, USA); the Center for Advanced Food Technology (CAFT) at Rutgers, the State University of New Jersey (New Brunswick, New Jersey, USA); the Japanese Research and Development Association for Sensing in the Food Industry (Tokyo, Japan); the Leatherhead Research Association (Surrey UK); and Campden Food and Drink Research Association, (Campden, UK); sensors are being developed through partnerships of material and sensor technology researchers, sensor manufacturers, and food processors.

For example, at the Center for Advanced Food Technology, faculty in food science and a variety of engineering disciplines work with sensor manufacturers to place the faculty research and technology developments in sensing systems which meets the needs described above. These systems are then tested in the food processing plants of Center industrial members. This overall effort allows the development of new sensors which address industrial needs, with shared technology development risk by the companies and sensor manufacturers, offset by government funds.

Many other contributions from linkage of individual research organizations and instrument and sensor manufacturers are also bringing new sensor technology forward. It is important to remember that it is only in recent times that sensor manufacturers developed the capability to manufacture sensors which can be used in food processing environments. This is due to the emergence of new materials, new micro fabrication and miniaturization strategies, and the development of knowledge-based computer technologies such as fuzzy logic [8] and expert system neural networks [9, 10] which help accommodate for the variability in process measurements.

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