Gloss of Fruits and Vegetables

Gloss of Fruits and Vegetables

Lebensm.-Wiss. u.-Technol., 29, 184–186 (1996) Research Note Gloss of Fruits and Vegetables A. Nussinovitch, G. Ward and E. Mey-Tal Institute of Bio...

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Lebensm.-Wiss. u.-Technol., 29, 184–186 (1996)

Research Note

Gloss of Fruits and Vegetables A. Nussinovitch, G. Ward and E. Mey-Tal Institute of Biochemistry, Food Science and Nutrition, The Hebrew University of Jerusalem, Faculty of Agriculture, P.O. Box 12, Rehovot, 76100 (Israel) (Received February 13, 1995; accepted April 18, 1995)

A glossmeter, suitable for measuring the gloss on curved surfaces has been constructed. The gloss of ripe banana, green bell pepper, orange, tomato, eggplant and onion were checked with an incident laser light at 45° and 60°. Reflected light was collected by photosensors and the image was analysed using appropriate software. L, a, b, °Bx, pH and gloss of fruits and vegetables were checked once they reached the stage of ready to eat. Eggplant and green bell pepper presented the highest gloss properties under investigating conditions, while banana and unwaxed orange exhibited the lowest sheen. Results from our glossmeter were compared with a commercial glossmeter, designed to check gloss of flat surfaces. In such operations it was necessary to peel and flatten the skin. Gloss of the fruits and vegetables were ranked in a similar way by both apparatus. Linear correlation was found between results of gloss measured at 60° by both apparatuses. It must be emphasized that intact fruits cannot be checked by the commercial glossmeter because of their curved surfaces.

©1996 Academic Press Limited

Introduction Gloss characteristics of surfaces are important physical aspects of food quality detected by human vision (1). Apples, cucumbers, rice and fruit pie fillings are food products where a shiny surface is valued, whereas unwaxed oranges, green beans and mushrooms are expected to have a dull surface (1). Surface appearance is associated with the distribution of light by the object. The four types of light distribution and their respective relationships to surface appearance are: diffuse reflection — shiny, specular reflection — glossy, mirror-like, diffuse transmission— cloudy or opaque and specular transmission — translucent (2). It was suggested that by using Pacific Scientific Spectroguard Color System at 10° observer angle, Illuminat C and 2.2-cm-diameter viewing port, indirect measurement of surface gloss of watermelons can be estimated by calculation of (∆L spin – ∆L spex). When L is the Hunter L value, spin indicates that spectral reflectance is included, and spex indicates that spectral reflectance is excluded (3, 4). These authors found that the rind gloss of Blue Belle melons increased with age. In contrast, rind gloss of Charleston Gray melons remained nearly constant until about 29 days after harvest and then decreased as the fruit ripened (4). It was estimated also that gloss differences are a result of differences in quantity and structure of the watermelon wax cultivars (4, 5). However, similar information for other fruits and vegetables is either unavailable or hardly documented in the food and agriculture literature. 0023-6438/96/010184 + 03$18.00/0

Until now, gloss measurements have been performed with glossmeters designed to execute measurements only with flat surfaces. This manuscript will describe an apparatus capable of measuring gloss properties of non-flat surfaces and to apply that method to check gloss of several fruits and vegetables.

Materials and Methods A variety of fruits and vegetables with obvious differences in gloss properties (banana, tomato, eggplant, onion and bell green pepper) were purchased from a local supermarket, with the exception of oranges freshly picked from a local orchard to eliminate the possibility of checking a waxed commodity, which could influence gloss results. Total soluble solid content (°Brix) of the juice or pulp from the studied fruits and vegetables was checked using a digital refractometer PR-100, Atago Japan. pH was measured using a pH meter, Al Hama, Israel. Colour was analysed and monitored using a Minolta Chroma Meter CR-100, Japan. All colours within the range of human perception were converted and compared on the basis of a common numerical colour notation system, using parameters L, a and b. The instrument was calibrated with the white standard plate provided by the Minolta manufacturer. Five pieces of each fruit and vegetable were in turn positioned in the gloss meter (Fig. 1) and illuminated by a light beam from a helium-neon laser, at incident angles of 45° and 60°. Photosensors collected all reflected light from the surface of the fruit or vegetable. ©1996 Academic Press Limited


lwt/vol. 29 (1996) Nos. 1 & 2

Table 1 Characterization of fruits and vegetables measured for gloss properties



Photodiodes Laser Cable Power source Rotatable support

Cable Amplifier





Aubergine Green pepper Tomato Onion Banana Orange

5.3 6.4 3.9 4.9 4.9 3.6

4.8 4.6 5.3 11.2 14.1 9.7

26.6 43.0 41.6 69.3 67.0 62.0

a sx


b x–


0.15 24.9 21.1 21.9 34.3 58.2

0.5 6.0 1.0 1.9 2.9 2.6


1.4 3.0 0.5 3.9 –15.1 1.4 1.2 15.9 0.8 3.1 3.4 1.3 2.0 –0.9 0.3 1.3 29.2 2.4

L, a and b are the average of 50 readings taken at different locations around the surface of the fruit or vegetable. Results are presented as mean (x–) and standard deviations (sx).

Support plate


Fig. 1. The curved surface glossmeter

The recorded image was analysed by a specially designed computer program, and translated into graph form of light intensity (arbitrary units) vs. distance or light scattering (pixels). The width of the curves at 50% intensity was measured and served as an indicator of gloss. A total of 10 separate readings at different positions around the surface were taken for each fruit or vegetable to obtain a more representative value of overall gloss. The glossmeter (Fig. 1) consists of a support plate as the base. The object to be tested sits on a disc in the centre that rotates to allow surface scanning. Two arches, attached to the base, are lined with photodiodes, a halfarch holds an adjustable light source, preferably a laser that can be beamed directly at different surface angles of the revolving support plate. The laser is connected via cable to a power source and the output of photosensors passes via cable to amplifying means connected to a computer. For comparison, the peels of the same commodities were removed, flattened and examined using a flat surface glossmeter (Triple Angle-Gloss, Rhopoint Instrumentation Ltd. Germany), capable of measuring gloss at illuminating angles of 20°, 60° and 85°.

with observation of the products clearly indicated either the ready-to-eat or buy stage of the fruits and vegetables. Numerical L values (lightness–darkness) are presented. Aubergine (eggplant) was the darkest while onion and banana exhibited the lighter surface commodities. Variations in a (hue) values of the commodities were observed; in particular, green bell pepper (–15.1) proved to be the greenest commodity and orange (29.2) the reddest one. Eggplant had the lowest b (chroma) value (more bluish) and orange the highest b value (more yellowish). Produce with skin defects were deliberately excluded in this study. Figure 2 describes typical gloss curves, derived at illumination angle of 45°. The fruit or vegetable gloss was checked after illumination of 45° and 60°. An angle of 45° was chosen since in a few previous attempts at practical gloss measurements, this incident angle was used (e.g. the Ostwald–Klughardot Method, the Pfund Method, etc) (8). An angle of 60° was also chosen since the commercial glossmeter for flat surfaces is designed to measure gloss only at three predetermined angles, namely 20°, 60° and 85°. Since our glossmeter can be implemented at every angle between 30 and 60°; it was important to compare results in this particular case where the angle of illumination is

80 70

The main attributes influencing vegetable grading include a near-to-perfect surface, colour intensity, flavour, character, consistency, uniformity, texture and maturity (6). Until now, regular quality control studies relied on the light source used to observe the crops during lab quality control tests (7). Yet, gloss was not among those properties considered in rating fruits and vegetables, probably due to the lack of appropriate means of studying gloss factors of curved surfaces. We know that consumers view the degree of gloss as a major determinant in acceptance or rejection of a commodity, especially when the appearance is appealing and taste can be judged only after purchase. °Bx, pH and colour indices of the tested fruits and vegetables are provided in Table 1. These results along

Intensity (arbitrary units)

Results and Discussion

60 50 40 30 20 10 0


100 150 200 Distance (Pixels)



Fig. 2. Typical goniophotometric curves obtained from the curved surface glossmeter. The curves from the outermost to the innermost represent fruits and vegetables in the following order: orange, banana, onion, tomato, pepper and aubergine


lwt/vol. 29 (1996) Nos. 1 & 2

Table 2 Gloss measurements of fruits and vegetables Glossmeter for flat surfaces Commodity Aubergine Green pepper Tomato Onion Banana Orange

20°C – x sx 2.1 1.7 3.1 1.0 0.7 0.6

0.2 0.2 0.3 0.1 0.05 0.05

60°C – x sx

85°C – x sx

Glossmeter for curved surfaces 45°C – x sx

60°C – x sx

11.6 10.9 9.6 2.5 2.0 2.4

1.6 8.6 1.5 41.7 1.7 37.7 8.8 1.2 6.9 0.7 51.7 3.3 43.5 8.7 0.4 4.2 0.3 54.6 0.9 37.2 8.1 1.0 1.8 0.1 84.6 6.5 86.9 3.9 0.1 1.5 0.1 80.7 6.5 92.3 2.2 1.0 1.4 0.1 95.2 0.9 93.3 0.8 Each result is the average of 50 readings (x–) and standard deviations (sx) taken at differen locations around the surface. Results given by the commercial glossmeter are dimensionless whereas those for the curved surface glossmeter are given in pixels.

equal and could validate our results. A typical graph of intensity of light (arbitrary units ) vs. distance or light scattering (pixels) is presented in Fig. 2. The width of the curve at 50% intensity was measured and served as an indication of gloss (Fig. 2). Results calculated from such graphs are shown in Table 2 where mean and standard deviations of gloss of the tested commodities are presented with results derived by the commercial glossmeter. Eggplant and green bell pepper presented the highest gloss properties under investigating conditions while banana and unwaxed orange exhibited the lowest sheen. With the commercial glossmeter the higher the value the higher the gloss. In our instrument the smaller the number (in pixels) the higher the gloss. Although both apparatuses differ in design, the order in which the gloss of the fruits and vegetables increases (Table 2) is similar. In addition a linear correlation between gloss readings from both instruments was found at 60°, having a regression coefficient (R2) of 0.986. This finding strengthens the validity of our results. However, it should be borne in mind that in order to use the commercial glossmeter on fruits or vegetables, it is necessary to peel and to flatten their skin. In using our glossmeter there is no need to interfere with the integrity of the fresh produce (i.e. they are examined intact). In addition, the custommade apparatus (9), was easy to handle and could facilitate future studies on gloss during the ripening period or when investigating gloss of man-made curved surfaces.

References 1 SZCZESNIAK, A. S. In: PELEG M. AND BAGELY, E. B. (Eds), Physical Properties of Foods. Westport, CT: Avi Publishing Company, pp. 11–12 (1983) 2 HUNTER, R. S. The Measurement of Appearance. New York: John Wiley & Sons (1975) 3 ANONYMOUS. Instruction Manual # 4, Spectroguard Color System. Silver Spring, MD: Pacific Scientific, Gardner/ Neotech Instrument Division (1983) 4 COREY, K. A. AND SCHLIMME, D. V. Relationship of rind gloss and groundspot colour to flesh quality of watermelon fruits during maturation. Scientia Horticulturae, 34, 211–218 (1988) 5 COREY, K. A., SCHLIMME, D. V. AND CHANEY, N. A. Changes in epicuticular wax on watermelon fruits during ripening. HortScience, 23, 730–731 (1988) 6 LUH, B. S. AND WOODROOF, J. G. Commercial Vegetable Processing. Westport, CT: The Avi Publishing Company, pp. 131, 501 7 NATIONAL CANNERS ASSOCIATION. Laboratory Manual for Food Canners and Processors, Vol. 1. Westport, CT: Avi Publishing Co. (1968) 8 HARRISON, V. G. W. Gloss: Its definition and measurement. Brooklyn, NY: Chemical Publishing Co, pp. 74–116 (1949) 9 NUSSINOVITCH, A. AND MEY-TAL, E. Novel Glossmeter for the Measurement of Food Appearance Including Fresh Produce and Man-Made Materials. Israel Patent Application # 109,033 (1994)