Food Processing System Sensor Needs

As the research and development community of the food processor, academia, and the instrument manufacturer responds to the challenge of achieving more efficient processing and better product quality, novel on-line sensor technologies are emerging. The following sections offer suggestions for application of previously described sensing technologies to a variety of food processing systems. While any particular on-line sensor may not be applicable to a proprietary industrial process, the generic sensing suggestion may offer directions to the system and process engineers charged with improving production efficiency and product quality.

Besides the on-line measurements that a sensor can provide, smart sensor packages can improve the collected data and suggest better control strategies for the processor. Smart sensors are instruments that generally combine two or more measurements interpreted by a computer or microchip to improve the information output [55]. For example, if pH were the critical parameter in the process, multiple pH sensors might be desirable to assist in accurate process control. pH sensors can be multiplexed together and computer statistical analysis performed. In most on-line instruments today, a computer or computer chip are an integral part of the system to analyze, correlate, or make temperature corrections for the measured data.

3.1. Baking

Different types of measurements are important at different points in the baking process as indicated in the generic process shown in Figure 1. In particular, there are some unique requirements because of the high temperatures encountered during the actual baking. This is also the time when the ability to control processing parameters can influence final product color, texture, and flavor, so feedback to control earlier process steps can be important. For example, in a baking process, it might be important to know the moisture and protein content in the flour because of the critical effect on structure of the product. While other components are also of significance to the quality of baked products, protein has the principle effect on loaf volume. The accurate weight and volume of mixed ingredients and the viscosity of the blend are also important to quality [56]. Other important factors include knowledge of the stage of fermentation, moisture content, and color of the product.

There are a number of on-line sensors that are likely to be of use to the baking industry with its higher temperature requirements. For example, moisture and humidity measurement are of interest in many of the processing steps. Research at the Center for Advanced Food Technology is beginning to result in on-line sensor prototypes that address moisture with new sensors [57-59].

Other moisture sensors are in development to address the higher temperatures encountered in ovens. Infrared systems are being used to examine on-line product moisture at various process stages. Scotter describes the use of NIR for determinations of fat, protein, and carbohydrate [13]. To assist in making these measurements, instrument manufacturers have been developing unique on-line test cells and increasing the measuring distance from the test cell to the remainder of the instrumentation with better fiber optics and microelectronics to allow on-line use [60],

Other moisture sensor research efforts are aimed at developing lower temperature porous fiber optical sensors, fluorescence optical sensors suitable for food environments, and polymer-based thin film sensors capable of operating in environments to 180°C [58].

3.2. Meat/fish processing

In meat processing, the most significant areas of interest concern meat aging, decomposition, and bacterial inspection as part of overall quality assurance programs. In fish processing, moisture content and fat content are also significant. The Japanese Research and Development Association for Sensing Systems in the Food Industry, a partnership of food processors and sensor manufacturers, has undertaken studies in a variety of areas including meat and fish. They have reported significant progress in developing quality assurance sensors which promise to improve food processing operations [61].

For example, a multisensor biosensing system was developed that measures two diamine compounds, putrescine and cadaverine, as indicators of meat decomposition. For investigating bacterial growth, the Japanese R&D Association identified a fluorescence method that detects coliform colonies within six hours. Sensitive instruments are under development.

Kress-Rogers, et. al. [62] report on measurements of meat freshness using a prototype knife-like probe with a biosensor array that measures glucose concentration at depths of two to four mm below the meat surface. It is reported that the probe is ready for consideration for commercial development.

In fish processing, fat and water content are of interest. A compact microwave instrument to measure the fat content of a variety of fish species relies on a microstrip sensor which has been demonstrated with herring [63].

The Japanese R&D Association reports high interest in processing of surimi products. pH was shown to be a good measure of quality and the development and use of an ISFET type probe provides good data in short time periods [61].

3.3. Dairy processing

The ability to examine the composition of milk and milk products such as cheese and ice cream is of interest to meet standardization requirements. Determination of total fat and solids in milk, moisture and fat in butter and margarine, and moisture content in processed cheese are among important characteristics to be measured.

Wide variations in butterfat content of milk have been reported [60] and the production and cost savings derived from using a process control system utilizing on-line NIR measurement described. The Japanese R&D Association also describes the development of special sampling systems together with the use of NIR to establish on-line process control [61].

Two developments related to cheese manufacture are of significance. A fiber optic sensor was developed to measure the changes in diffuse reflection of coagulating milk using NIR reflectance. Time correlations between enzyme addition and signal pattern were developed to predict cutting time, defining feasibility of the technology [64]. Moisture measurement in cheese using NIR [60] and a new thermomoisture probe [39] were also reported.

Aseptic milk packages provide an interesting application for non-invasive sterility control using ultrasonic measurement. By transmitting an ultrasonic beam into the package inducing acoustic flow streaming, it was shown that the velocity spectrum measured by the Doppler shift of reflected sound decreased in packages with microbial growth [65].

3.4. Fermentation

Several organizations have reported sensor developments aimed at automation in fermentation processes. The Japanese R&D Association investigated techniques for quality improvement in yogurt production by lactic acid fermentation processes. A key part of the system includes the development of a fully automated pH sensing system [61].

Guenneugues, et. al. [66] report that an NADH fluorescent biosensor has been demonstrated to indicate the end of the process in lactic yeast fermentation of deproteinized whey substrates. Tests in batch processing were precise; continuous processing disturbances require further study related to each specific process.

In another research study, different types of biomass sensors were examined and their performance compared with the NADH fluorosensor [67], Results were inconclusive but the researchers suggest that a knowledge-based computer system might be feasible to better interpret the biosensor information.

3.5. Beverages

The beverage industry includes a wide variety of products with attendant varying measurement needs. The use of refractometers has been well accepted for determining component concentrations, for example, sucrose in water (for soft drinks) solids in orange juice, and solids in milk [28]. The potential for color sensing in conjunction with knowledge-based intelligent computer software has been reported in coffee processing [68].

More recently, the application of an electronic nose using an array of twelve tin oxide sensors was shown to discriminate between both the blend and roasting level of coffees. This technique depends on multivariant analysis of the sensor data for specific classification [34]. A multisensor array with lipid membranes supported on polymer films was also shown to discriminate between different beers and different coffees [35].

An interesting approach for screening fruit juice authenticity uses NIR and multidimensional discriminant analysis [69]. A new enzyme biosensor has been developed to measure aroma of green tea. Screening was of interest because of the value-added aspect of certain high quality green teas in Japan. The amino acid oxidase and oxygen electrode were found to be rapid and easy to use [70].

3.6. Fats and oils

Fats and oils are of interest as product ingredients as well as for use in frying processes. Studies conducted at the Leatherhead Food Research Association investigated monitoring of frying oil quality to ensure wholesome fried products and adequate shelf life. Kress-Rogers, et. al. [71] discuss the development of a prototype probe to monitor viscosity of frying oil. The probe incorporates two vibrating rods where dampening of the vibrations is indicative of viscosity. The use of NIR has also been reported for determination of fat and oil content [61].

3.7. Packaging

Packaging and package seal integrity directly affect the shelf life of products. On-line determination of integrity could significantly improve the efficiency of food packaging operations since off-line evaluation might cause rejection of an entire batch of a packaged product. The Center for Advanced Food Technology has surveyed leak detection technology and reported a need for new developments [72]. Other developments in this area include a new ultrasonic detection techniques for aseptic milk packaging [65].

3.8. Fruit and vegetable processing

Several new studies have been reported on the use of NIR and ultrasonic examination of fruits and vegetables for identification and freshness. The NIR sensor study showed that apple cell walls have a characteristic spectrum that indicates the capability of authentication of fruit-containing product using this technique [73]. Discriminant analysis was used for positive quantitative identification. Scotter and Legrand [69] also showed NIR screens of fruit juice authenticity using discriminant analysis.

An ultrasonic study of color and ripeness of melons indicated differing attenuation correlating with these characteristics. Further studies of this concept were recommended [47].

3.9. Pasta processing

In pasta processing, controlled drying is very important to ensure product quality. Moisture sensors that can be used on-line in production dryers will impact on processing capability. The Japanese R&D Association reported the development of a ceramic zirconia oxygen type moisture sensor capable of measuring moisture up to 150°C [61]. The Center for Advanced Food Technology has also reported development of a fiber optic sensor that can measure moisture to 300°C and is useful in both dryers and baking ovens [59].

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