Comparison of Subjective and Objective Methods for Determining the Color of Reconstituted Frozen Concentrated Orange Juice1

One subjective and two objective methods were used to determine the color of reconsti tuted frozen concentrated orange juice. The USDA color scores for 21 samples ranged from 32 to 37 points when this subjective method was used. Two objective methods were used: (a) the Hunter Color and Color Difference Meter to ob tain tristimulus color values and (b) the Bausch and Lomb Spectronic 505 recording spectrophotometer to obtain spectral curves. Hunter Rd values ranged from 20.3 to 27.1; the a values values from —7.5 to —2.8; and the b values from 28.1 to 29.8. Dominant wavelengths computed from the spectral curves for the reconstituted juices, ranged from 576 to 581 m^; purity from 59 to 90%; and brightness from 20.3 to 33.8%. As the visual color score increased, the Hunter a value increased and the Rd value decreased; also, brightness decreased but no trend was evi dent between either the dominant wavelength or purity and the color score. Florida Agricultural Experiment Stations Journal Ser ies No. 2529. lCooperative research by the Florida Citrus Experiment Station and Florida Citrus Commission. 2Florida Citrus Experiment Station, Lake Alfred. 3Florida Citrus Commission, Lake Alfred. Introduction Color to different disciplines of science means various things. To the chemist it is dye and pigments. To the physicist color is a phenomena in the field of optics and electromagnetc radia tion. To the physiologist and psychologist color denotes a sensation to the human observer. There are many procedures in use today that compare the color of one object with that of another or give a value for the color difference between them. Such methods are either sub jective or objective, the former depending upon a visual evaluation while the latter uses differ ent instruments. Subjective methods.—The Maerz and Paul Dictionary of Color (5) contains examples of many colors. Plate 10 on page 43 of this book shows various colors that could apply to orange juice. The Macbeth-Munsell Disk Colorimeter (3, 4) can be used so that overlapping color wedges are spun and the resulting color compared to that in samples of orange juice. By varying the amount of white, gray, yellow, and orange, the color of orange juice may be matched. Another example of a subjective method is the use of USDA color comparator tubes to obtain a color score for orange juice. A trial set at such tubes consisted of colored viscous plastic in capped glass tubes. These were avail322 FLORIDA STATE HORTICULTURAL SOCIETY, 1966 able in Florida for a number of years and were numbered 1, 2, 3, and 4 corresponding to color score points 32, 34, 36, and 38. This trial set of color comparator tubes wes replaced in 1963 with a set of tubes made entirely of colored plastic (6). These tubes were designated as 0J1, 0J2, 0J3, 0J4, 0J5, and 0J6 and were referred to as USDA Orange Juice Color Stan dards. The procedure for evaluating the color of reconstituted frozen concentrated orange juice by using the 0J2, 0J3, 0J4, and 0J5 tubes is given in the U.S. Standards for Grades of Froz en Concentrated Orange Juice (7). Objective methods.—Kramer and Twigg (3) and MacKinney and Little (4) described many instruments for measuring objectively the color or color difference of substances. Instruments described include spectrophotometers, Gardner Color and Color Difference Meter, Hunterlab Color and Color Difference Meter, Colormaster Differential Colorimeter, Color Eye, Photovolt Reflection Meter, and Agtron. Color terminology.—Kramer and Twigg (3) list some of the physical and sensory terms used to denote different color attributes in the following manner. Physical Measurement Radiant energy Reflectance Dominate wavelength Purity Sensory Term Equivalent Light Lightness, value Hue, color Chrome, intensity, strength The purpose of this paper is to present to the citrus processing and other related indus tries information pertaining to the use of sub jective and objective methods for determining the color of reconstituted frozen concentrated orange juice. Data are needed so that undesir able variations, occurring when color scores of juices are subjectively determined by visual comparison, may be eliminated by the use of objective instrumentation. Experimental Procedures Sample preparation.—A set of reconstituted frozen concentrated orange juices were prepared by thawing and mixing together different sam ples of commercial frozen Florida orange con centrate so that the reconstituted juices would have a wide range of USDA color scores and Hunter Color Difference Meter values. Use of USDA orange juice color standards. —The set of USDA plastic color standards (0J2, 0J3, 0J4, 0J5, and 0J6) were used visually to obtain the color scores for these reconstituted orange juices. The juices were placed in 1-inchOD screw-cap culture tubes. The juices and the standard tubes were viewed together in a Mac beth Examolite daylight model EBA-220 with a rated color temperature of 7400° Kelvin. The averages of the total score points given by five judges to each juice were used. Use of Hunter Color and Color-Difference Meter.—The Hunter Color and Color Difference Meter (3, 4) was used in the objective evalua tion of the color of the 21 reconstituted orange juices. This instrument is a photoelectric tristimulus colorimeter. It can measure small differ ences in color with its three filters, which ap proximate the standard observer of the Inter national Commission of Illumination. Most of the information (1, 2, 8) on the color of citrus concentrates and juices has been obtained by the authors using this instrument. Three readings are obtained: the Rr, a, and b values. The Rd value indicates the lightness (whiteness) of a juice sample. Readings of either 0 or 100 mean that the sample is black or white, respectively. Values for Rd between 0 and 100 indicate differ ent shades of grayness. The a values are meas ures of redness when positive or greeness when negative. Yellowness or blueness are indicated by positive or negative b values, respectively. Use of Bausch and Lomb Spectronic 505 re cording spectrophotometer.—A Bausch and Lomb Spectronic 505 (B & L gO5) recording spectro photometer was loaned to the Citrus Experiment Station for a short time by W. H. Curtin and Company, Jacksonville, Florida. This instru ment made it possible to obtain color data by another objective method. Equipped with a re flectance accessory attachment, the B & L 505 recorded the reflectance from a sample of juice as compared to the reflection from magnesium oxide. Thus, a spectral reflectance curve was obtained over the wavelengths from 440 to 700 m^u,. This instrument recorded directly on a trichromatic coefficient computing chart. These charts made it simple for conversion to C.I.E. trichromatic coefficients (3, 4) which are needed to determine the dominant wavelength (DWL), purity, and brightness of the light reflected from the juice. EDWARDS ET AL: CONCENTRATE COLOR 323 Results and Discussion Spectral curves for USDA plastic-in-glass color comparator tubes.—Spectral curves from the B & L 505 of the USDA plastic-in-glass col or comparator tubes are shown in Figure 1. Characteristics of these special curves were cal culated and are presented in Table 1. The dom inant wavelength increased with the color scores (Table 1) but there was only 1 m^ difference between any two adjacent scores. Purity in creased and brightness decreased as the color score increased. USDA color scores and Hunter Color Dif ference Meter alues for reconstituted frozen concentrated orange juices.—Color scores, Hun ter Oolor Difference Meter values and spectral characteristics for 21 samples of reconstituted frozen concentrated orange juice are listed in Table 2. Color scores ranged from 32 to 37 points when the OJ set of USDA plastic color comparator tubes were used. The Rd values ranged from 20.3 to 27.1; the a values from —7.5 to —2.8; and the b values from 28.1 to 29.8. In general, the Rd values decreased and the a values increased as the color scores increased. Spectral curves for reconstituted frozen con centrated orange juices.—Some typical spectral curves for reconstituted orange juices, ranging in score from 32 to 37, are shown in Figure 2. Spectral curves for the orange juices had higher dominant wavelenghs (Table 2) than those for the USDA color comparator tubes (Table 1), indicating more redness in the juices as com pared to that in the plastic-in-glass tubes. The dominant wavelengths of the spectral curves for these comparator tubes ranged from 571 to 574 m^u, while those for the juice curves ranged from 576 to 581 m^. This shift in dominant Table 1. Dominant wavelength, purity, and brightness of the color of USDA plastic-in-glass color comparator tubes U4FO" Wavelength n mp Color