Color Measurement in Foods as a Quality Parameter
LABORATORY
Color measurement is a critical objective quality parameter that can be used for the following applications: as quality index measurements of raw and processed foods for use in quality control documentation and communication; for determinations of conformity of food quality to specifications; and for analyses of quality changes as a result of food processing, storage, and other factors.
Since visual color judgments can be affected by a wide variety of factors, from plant lighting conditions and angle of observation to individual differences in color perception, instrumentation to measure color provides a subjective and consistent method of color quality control.
Visible light is found between 380 and 780 nm in the electromagnetic spectrum. It is bordered by ultraviolet light on the low end and infrared light on the upper end. When light strikes an object, it is reflected, absorbed, or transmitted. Because reflected light determines the color of a material, the appearance can change depending on amount of light, the light source, the observer’s angle of view, size, and background differences. Depending on how light acts, food products can be classified as opaque, translucent, or transparent.
A variety of color scales or schemes are used to describe color. Those most often used in the food industry include the CIE system, the Hunter L, a, b system, and the Munsell color solid.
The Commission International de l’Eclariage or CIE developed the most influential system for the description of color. The system is based on using a standard source of illumination and a standard observer. The system is used to obtain CIE standard-observer curves for the visible spectrum for the tristimulus values which are converted to the unreal primaries X,Y, and Z.
The Hunter L, a, b and CIELAB color scales are opponent-type systems commonly used in the food industry. The systems measure the degree of lightness (L), the degree of redness or greenness (+/-a), and the degree of yellowness or blueness (+/-b).
The Munsell color-order system is a way of specifying colors and showing the relationships among colors. Every color has three qualities or attributes: hue, value, and chroma. Munsell established numeric scales with visually uniform steps for each of these attributes. The Munsell Book of Color displays a collection of colored chips arranged according to these scales. Each chip is identified numerically using these scales. The color of any surface can be identified by comparing it to the chips, under proper illumination and viewing conditions. The color is then identified by its hue, value, and chroma. These attributes are given the symbols H,V, and C and are written in a form H V/C, which is called the Munsell notation. All colors lie within a specific region of Munsell color space called the Munsell color solid.
Color can be measured either colorimetrically or spectrophotometrically. Tristimulus filter colorimeters are designed to reproduce the “psycho-physical” sensation of the human eye’s view of color. Light is projected through red, green, and blue filters in front of a lens, and the colored beam is projected in a screen. A filter of unknown color is projected on the same screen, and the amount of red, green, and blue light is adjusted to match the unknown color. In practice, a set of glass filters with transmission curves shaped like those of the standard observer are used. A photocell and meter take a reading of the light reflected from an object through each of these filters, and X,Y, and Z values are obtained.
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Spectrophotometers measure a ratio of the light reflected or transmitted from a food product to that from a known reference standard. Spectrophotometers are more accurate and more expensive than colorimeters. The choice of which instrument to use will depend on the food material and type of application. Colorimeters can be used as a rapid QA technique to monitor product quality during each processing step. For instance, a colorimeter was used in a study of potato processing to monitor incoming materials, knife sharpness, degree of browning during frying and freezing, shelf-life aging, and potato respiration during storage.
Instrumental color measurements correspond to visual assessments of food color. It is important that sample preparation and presentation procedures are followed to obtain high repeatability of measurement. Grinding, mixing, milling, and blending are all preparation techniques that produce uniform samples, but also affect the light scattering properties. Indeed, the sample may no longer represent the visual appearance of the product. For instance, bulky vegetables such as broccoli heads can present a difficult color evaluation problem. Blending the heads with water and taking a color measurement may not correspond to how a consumer perceives them. Loosely fitting pieces of food trap light and reduce light scattering. In this case, an aperture larger than the incident light beam should be used to maximize light collection. Most color measurement procedures for food products recommend that the samples be turned and another reading be taken at a 90° angle from the first reading. The two readings can then be averaged.
Along with color measurement, it may also be useful to measure the surface appearance of foods. The appearance of a food surface is a property that can be detected by human vision. The appearance of a food surface can be classified as: (1) diffuse reflection—shiny; (2) specular reflection—glossy, mirror-like; (3) diffuse transmission—cloudy, opaque; and (4) specular transmission—translucent. Several companies make meters for the measurement of gloss. One food industry in which gloss measurement is important is confectionery.
A variety of companies offer color measurement instruments. ColorTec, Clinton, N.J., offers the ColorTec-PCM™, a hand-held color instrument which provides color measurement data to meet color QC and statistical process control (SPC) requirements. BYK-Gardner USA, Columbia, Md., offers several color meters, portable spectrophotometers, and spectrophotometers for use in a variety of inspection applications. Hunter Associates Laboratory, Reston, Va., has a line of color measurement systems including the ColorFlex color measurement system. This system is completely self-contained, including built-in software. It may be used for basic color measurement or more sophisticated applications. Minolta Corp., Instruments Systems Division, Ramsey, N.J., offers a line of instruments, including spectrophotometers and meters. The meters include the BC-10 Baking Contrast Meter that may be used for judging the acceptable color of finished baked goods.
PATENTS
This month’s patents include methods to collect and recover microorganisms from environmental samples, detect Giardia lamblia and Cryptosporidium, detect low levels of microorganisms, and detect prions.
Method to collect and recover microorganisms from environmental samples. U.S. patent 6,010,896, filed 7/23/1998, issued 1/4/2000 to J. Calomiris, assigned to the United States of America as represented by the Secretary of the Air. Describes an improved method to collect and recover microorganisms from environmental samples. The method of this invention consists of obtaining a sample for testing and, optionally, suspending the sample in a suitable liquid; amending the sample with sodium hexametaphosphate and, optionally, centrifuging the sample to remove solid sediments, insoluble salts, inert materials, and the other similar materials; centrifuging the sample through a separation column amended with sodium hexametaphosphate; and then recovering the microorganism-containing material remaining above the column for analysis. The invention relates to a method for concentrating and recovering microorganisms from difficult-to-separate samples, particularly environmental samples, and it should provide an improved method to collect and recover microorganisms from environmental samples.
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Automatic machine vision microscope slide inspection system and method. U.S. patent 6,005,964, filed 1/24/1998, issued 12/21/1999 to J. Reid and J. O’Brien, assigned to The Board of Trustees of the University of Illinois. Describes a system for automatically detecting the presence of contaminants in samples. The system includes a microscope, controllable stage positioner for holding slides under the microscope, a computer for controlling the stage positioner and a digital camera to capture images through the microscope. The system scans microscope views of regions of a slide sample and provides the digital images to the computer. Image processing routines stored in the computer analyze the digital images and determine whether they contain certain contaminants by comparing the characteristics of the objects in the image with the known characteristics of the contaminants. The system also includes a method for automatically determining the presence of contaminants. The presence of microscopic organisms in public drinking water is a threat to public health. Two particular pathogenic protozoa pose a significant threat, Giardia lamblia and Cryptosporidium. Since the current method requires manual detection, it would be helpful to have an improved system and method for automatically detecting contaminants such as pathogenic protozoa in samples, making the method more efficient and accurate.
Method for detecting low levels of microorganisms. U.S. patent 6,004,766, filed 5/16/1995, issued 12/21/1999 to V. Atrache et al., assigned to Biotechnology Australia Pty Ltd. Describes a method and kit for the detection of low levels of a microorganism in a sample in the presence of competing microflora. The kit consists of a solid support to which are adsorbed antibodies specific for the microorganism, a first container comprising a washing solution, a second container comprising a sterile nutrient broth, and, optionally, a third container comprising a solution comprising a releasing agent. The kit allows the user to detect low numbers of a particular microrganism or microorganisms in a mixed population, which overcomes the need for pre-selection in selective media by using an immunoimmobilization technique followed by non-selective growth and immunoassay or by cleavage of the antibody–microorganism bond and growth of the microorganisms on non-selective media. The method is said to especially useful in detecting Salmonella and Listeria spp. in mixed populations.
Detecting cow, sheep and human prions in a sample and transgenic mice used for same. U.S. patent 6,008,435, filed 9/27/1997, issued 12/28/1999 to S. Prusiner et al., assigned to the Regents of the University of Calif. Describes a method for producing transgenic animals, such as mice, rats, or hamsters, that have their endogenous PrP gene ablated and have an exogenous PrP gene from a genetically diverse animal. The exogenous PrP gene comes from a sheep, cow, or pig, with cow PrP genes being preferred. When a mouse of the invention is inoculated with a sample containing prions which generally only infect a genetically diverse species (e.g., a cow), the mouse will become ill within about 250 days or less. Methods of producing the transgenic animals are disclosed. Mice produced with the method are used to test samples for the presence of prions which generally only infect cows.
Additional University Research Centers
A variety of friends and other interested individuals pointed out that my November column, “University Centers Ease Product Development,”contained some serious omissions, outdated information, and other problems. So, without further ado, here are some corrections and additions:
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The Institute of Food Science and Engineering (University of Arkansas, Fayetteville, phone 501-575-4040) facilitates and encourages value-added research and seeks to improve the efficiency and effectiveness of processing of agricultural products. Major commodity research programs of its Center for Food Processing and Engineering include rice, fruits and vegetables, wine and other grape products, pickled vegetables, rheology and sensory, lipids and oils, and soy products. A fully automated Steritort and conventional retorts are used to conduct applied research projects assisting commercial food processors with thermal processing, quality programs and product development. Advanced analytical equipment is also available to measure the color, flavor, texture, and nutritional content of processed products. A permanent, professionally trained descriptive sensory analysis panel has the ability to precisely describe food products in terms of their appearance, aroma, flavor, and texture. The Center for Food Safety and Quality provides integrated multidisciplinary approaches to research, technology transfer and education/extension activities to enhance the safety and quality of foods. The Center also offers assistance to entrepreneurs who are developing and conmmercializing new products. The United Nation’s Food and Agriculture Organization (FAO) has designated the Institute as a Center of Excellence for Food Safety, Quality and Nutrition Secretariat. Activities include joint efforts with public and private agencies and other universities in support of FAO training, research, and infrastructure building to assure that imported products meet U. S. standards for quality and safety.
The Food and Agricultural Products Research and Technology Center (Oklahoma State University, Stillwater, phone 405-744-6071) is dedicated to providing basic research, applied problem solving, and needed training to support existing and new enterprises in the food and agricultural products processing industry in Oklahoma. The Center offers pilot processing facilities, research laboratories, educational programs, and marketing assistance. From a business standpoint, the Center can assist in the development of business plans, market identification and evaluation, product pricing and promotion, evaluation of financing options, identification of co-processors, and meeting state and federal regulations. Technical assistance is available in the areas of food microbiology, food chemistry, food engineering, horticultural processing, meat science, cereal and oilseed processing, quality control, and sensory analysis.
The Center for Advanced Food Technology or CAFT (New Brunswick, N.J.) has a new phone number: 732-932-8306. Also, the multidisciplinary cooperative research projects now have the titles: Flavor Science and Technology, Food Materials Science, and Process Control and Simulation. Additional multidisciplinary research projects in the Center are Quality Enhancement of Combat Rations, and Pioneer Nutraceutical Research. The Advanced Manufacturing and Outreach Program includes the COmbat RAtions NETwork (CORANET) Base Program, Statistical Process Control and Quality Database Management, Package Seal Integrity Inspection and Control, and Retort Operations Control and Material Tracking. The Targeted Research Program and Developmental Research Program carries out research and development focused on the needs of an individual member or non-member company, including incubator space for startup companies and production for new product introductions or test marketing. The Instrumentation Support Facilities support industry needs in mass spectrometry and chromatography; rheological measurements and extrusion cooking; and spectroscopy and calorimetry.
Products & Literature
INFRARED PROCESS ANALYZERS, part of the Xendos 2500 series, are said to deliver long-term accuracy and repeatability in hostile, dirty, or corrosive environments, and are certified for use in hazardous areas without purge requirements. These single-beam, dual-wavelength infrared process analyzers can be used for measurement of a single-infrared-absorbing component or component group (up to three components) in a process gas or liquid sample stream. Each analyzer is manufactured and tested in a configuration specific to the user’s required measurement and background sample stream. For more information, contact Servomex Co., Inc., 90 Kerry Pl., Norwood, MA 02062-4733 (phone 781-769-7710; fax 781-769-2834; www.servomex.com) —or circle 360.
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POCKET THERMOMETER may be used in many food quality control applications. A needle-tip probe features a reduced diameter at the insertion end with a larger diameter shaft for durability and strength. The miniaturized tip makes it possible to take internal temperatures of very small samples. The probe folds into the meter for safe transport or storage and it can be rotated for use at an angle convenient for the user. When the probe is folded in, the instrument shuts off. For more information, contact ThermoWorks, Inc., 221 S. Country Manor Ln., Alpine, UT 84004-7705 (phone 801-756-7705; fax 801-756-8948)—or circle 361.
MICROWAVE MOISTURE ANALYZER, the M2, utilizes a patented cylindrical applicator and microwave technology that is said to focus energy directly on the sample. The concentration of energy is said to result in analysis times that are up to four times faster than conventional microwave-oven-based analysis. The analyzer may be used for food and environmental samples. For more information, contact Microwave Research Center, 1285 Corporate Center Dr., Eagan, MN 55121-1256 (phone 651-456-9190; fax 651-454-6480; www.rubbright.com) —or circle 362.
SOLID FAT CONTENT ANALYZER, the QP20+, uses nuclear magnetic resonance to measure a range of components, including water, oil, fats, and chemical products. This benchtop analyzer is said to measure components from less than 0.01% to 100%, usually in less than 30 sec. This instrument features an optional PC-based operating system. For more information, contact Oxford Instruments, 130A Baker Ave. Extension, Concord, MA 01742 (phone 978-369-9933; fax 978-371-0204)—or circle 363.
ION TRAP, the Finnigan LCQ™DUO, is a benchtop ion trap LC/MS/MS featuring a small footprint, ESI probe, and several scan functions. The detector is said to offer high qualitative and quantitative sensitivity, combined with automated library-searchable MS/MS spectra. The MS/MS spectra can be used as component “fingerprints” and may be searched against commercial or customized spectral libraries. The detector runs on an “Open Lab” software concept, which is said to permit in-line acquisition and integration of photodiode array data along with mass spectral data. For more information, contact ThermoQuest Corp., 355 River Oaks Pkwy., San Jose, CA 95134-1991 (www.thermoquest.com) —or circle 364.
PADDLE BLENDER may be used in food quality labs when conducting bacteria tests where contamination reduction is essential. The sample, as it is being mixed, is contained in a sterile bag, which reduces the risk of cross contamination because the samples never touch the blender. Water-regulated models are able to warm or cool the bag and the sample in the blending chamber. Circulating water in the chamber door keeps the sample temperature at the same temperature as the fluid—even during long periods of blending. This application is ideal for fragile and temperature-sensitive samples. These blenders come with a cooling or heating circuit and two 8-mm-diameter stainless-steel tubing inlets. For more information, contact Cole-Parmer Instrument Co., 625 E. Bunker Ct., Vernon Hills, IL 60061 (phone 847-549-7600, www.coleparmer.com) —or circle 365.
THERMOMETRY BRIDGE, the ASF F300, is a metrology-grade AC direct-reading thermometry bridge designed for convenience and ease of use. The bridge has the ability to display absolute resistance or resistance ratio, operating with any platinum resistance thermometer that has an ice point resistance in the range of 0.25 to 1,000. This instrument is said to have virtually zero drift with changes in operating temperature and time. For more information, contact ASL, Inc., 100 Brickstone Sq., Andover, MA 01810 (phone 978-658-0000; fax 978-658-5444)—or circle 366.
OXYGEN ANALYZER, the Snack O2, is a sample-drawn oxygen analyzer that can be used to measure oxygen for the modified-atmosphere packaging industry. The 25-oz, battery-powered unit is said to display an accurate reading in seconds. The membrane keypad and backlit LCD are combined to provide flexibility to set operating modes, adjust sampling duration, and conduct simple calibration in ambient air. No warmup time is required, and the internal, rechargeable battery lasts for 2 hr of continuous use. For more information, contact Teledyne Electronic Technologies, P.O. Box 1580, City of Industry, CA 91749-1580 (phone 626-934-1500 or 626-961-9221; fax 626-961-2538)—or circle 367.
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PLATFORM ANALYZER is equipped with a horizontal attenuated total reflection sample stage. The Infracal IR Platform Analyzer is said to reproducibly detect dissolved oil and grease in water and soil at concentration levels from 1 to 4,000 ppm. It utilizes a hexane extraction sampling procedure and can be used with TOG extracting solvents such as Freon or Perchloroethylene. This unit is portable, weighs less than 5 lb, and can be operated from a 12-volt DC power source. For more information, contact Spectra-Tech Inc., P.O. Box 869, Shelton, CT 06484-0869 (phone 203-926-8998; fax 203-926-8909)—or circle 368.
MICRO PUMP may be used by OEM designers and manufacturers of scientific and electronic instrumentation. The fixed-displacement diaphragm pump can be factory set to handle discrete fixed output flows in volumes from 3 to 8 μL per solenoid actuation. This miniature self-priming 5-μL micro pump is said to provide a discrete dispense to replace pump and valve combinations, or a continuous pulsating flow to replace maintenance-intensive peristaltic pumps. For more information, contact Bio-Chem Valve Inc., 85 Fulton St., Boonton, NJ 07005 (phone 973-263-3001)—or circle 369.
AMINO ACID ANALYZER, the Biochrom 20, is said to be the only system available to meet the requirements of the pharmaceutical and life science industry. The analyzer is a benchtop system that combines post-column ninhydrin derivatization with full automation and on-line monitoring. The system allows for the analysis of both physiological fluids and protein hydrolysates, with a range of chemical reagent and buffer kits. Amino acids, small peptides, and amino sugars are said to be determined with quantitation down to 15 picomoles. The system features an integrated 96-vial autosampler and data handling software. For more information, contact Transgenomic, 5600 S. 42nd St., Omaha, NE 68107 (phone 402-738-5480; fax 402-733-1264; www.transgenomic.com) —or circle 370.
pH METERS, called the V ® 600 Series, may be used to measure pH, mV (ORP), relative mV, and temperature with five-point calibration and RS 232 interface. These benchtop meters feature a menu-driven user interface, settable alert limits, and compensation for isopotential pH point. This series recognizes five standard and four replacement buffers, as well as five custom buffers. For more information, contact Beckman Coulter, Inc., P.O. Box 3100, Fullerton, CA 92834-3100 (phone 714-871-4848; fax 714-773-8283; www.beckmancoulter.com) —or circle 371.
PROTEIN DETERMINATION in dairy products may be performed with the Kjeltec 2300. This testing system offers PC-controlled downloading, pre-programing for 10 distillation routines, proactive safety systems to protect against handling errors, and a calorimetric titration system which is AOAC, EPA, DIN, and ISO approved. For more information, contact Foss North America, 7682 Executive Dr., Eden Prairie, MN 55344 (phone 612-974-9892; fax 612-974-8923; www.fossnorthamerica.com) —or circle 372.
COMPACT SCALES, part of the HL Series, combine a solid and portable design with readings over a wide temperature range. These new scales are battery operated and may be used for industrial, warehouse, foodservice, office, and laboratory applications. The plastic weighing pan is detachable for cleaning and offers stable weighing of items of various shapes and sizes. An optional stainless-steel pan is also available. Other features include a sealed front key panel, low-battery indicator, automatic power off, and an optional AC adapter. For more information, contact A&D Weighing, 1555 McCandless Dr., Milpitas, CA (phone 408-263-5333 or 800-726-3364; www.and1.com) —or circle 373.
by JAMES GIESE
Associate Editor