Volume 63, No. 2, 1998—JOURNAL OF FOOD SCIENCE 317

ABSTRACTABSTRACTABSTRACTABSTRACTABSTRACTSurface topography and roughness of garlic and onion skins were studied byatomic force microscopy in order to estimate the surface area. Image-processingand Arc/Info software were used to interpret the data. The calculated ratio be-tween apparent and measured surfaces (roughness factor) deviated from 1.11 to1.15 for untreated and chloroform-treated onion skin, respectively. For garlic,higher values were detected for the untreated skin. The higher the roughnessfactor, when the coating solutions are easily spread on the fruit or vegetablesurfaces, the better the adhesion between coating and skin . A knowledge of truesurface areas can help to better estimate required coating-solution volumes.

Key Words: surface topography, coatings, contact angle, adhesion, rough-ness

established examples of surface roughnessenhancing adhesion. Early researchers con-sidered adhesion as two main types: specificand mechanical and the former had to devel-opment the adsorption theory. Mechanicaladhesion was hypothesized to occur when aliquid set in the pores and cracks of a sub-strate, providing a mechanical key. Whenmechanical keying occurs, adhesion is ex-pected to increase with substrate roughness(Packham, 1983). The effect of surface to-pography on measured adhesive-bondstrength, as detailed in published reports, iscomplex and to some extent contradictory.Roughening a surface can lead to incompletewetting and reduce the area of contact, andvoids at the interface may increase stresswhich, with brittle adhesives, could weakenthe joint. The effect of surface roughness oncoatings has been studied in food and agri-cultural commodities (Hershko et al., 1996),but data on roughness of fruit and vegetablesurfaces is rarely found (Ward and Nussino-vitch, 1996). More effective coatings couldbe developed by exploring properties of fruitand vegetable surfaces such as roughness.Therefore, the objective of this research wasto design a method for obtaining a reliable,valid “map” of vegetative surface skin, inorder to calculate the true surface area.

MATERIALS & METHODS

Preparation of skin specimensFresh garlic (Allium sativum) and onion

(Allium cepa) bulbs were collected ~4 h af-ter harvest, each weighing 60–75 g (garlic)or 80–100 g (onion). Skin was peeled manu-ally from the bulb for atomic force micros-copy (AFM) studies. Epicuticular wax wasremoved from the onion skin by pouringchloroform over each peel 6 times (Burdon

ENGINEERING/PROCESSING

et al., 1993). Skins were studied by AFM 1 hafter the wax extraction. Ten different loca-tions were studied on each skin-surface spec-imen.

Atomic force microscopyAFM was carried out using a Nanoscope

II atomic force microscope equipped with aSiN cantilevered scanner with a 12-mm2 scansize and a 4-mm2 vertical range. Force mea-surements were made on small (5 µm × 5 µm)“smooth” garlic-skin surfaces, located be-tween the skin’s vasculature. The AFM forcefields between the probe and the skin wereused to guide the probe over the surface andstudy its topography.

False-colored AFM images were analyzedusing the public domain NIH-Image version1.59 (available from NTIS, 5285 Port RoyalRd., Springfield, VA 22161, part no. PB93-504868) to calculate the x, y scale in nm/pixeland the z scale in nm/grey level. The z scalewas determined by measuring the grey-scalevalue of the lowest and highest points (partA of each figure represents the original im-age). The image was imported into the Arc/Info program (Environmental Systems Re-search Institute, Inc., 380 New York St., Red-lands, CA), where the Arc/Grid and Arc/Tinmodules were used to construct a 3-dimen-sional model of the surface, the area of whichcould later be measured. The image wascropped so that only the surface would beused, and not the surrounding markings (i.e.,the frame or tick marks). This step was veryimportant, because the 3-dimensional surfacebecomes seriously distorted if spurious dataare included.

The “Tin” is a set of adjacent, non-over-lapping triangles computed from irregularlyspaced points with x, y coordinates and z val-ues. The Tin model stores the topologicalrelationship between triangles and their ad-joining neighbors. This data structure enablesthe efficient generation of surface models toanalyze and display terrain and other surfacetypes. The scaling information determinedwith NIH-Image was then added.

The “latticetin” procedure was used tocreate a 3-dimensional model made up of tri-angles, and “tinarc” was used to project the3-dimensional surface onto a two-dimension-al polygon framework, where each trianglehas a surface measurement associated withit. The “regiondissolve” and “statistics” com-mands gave the sum of the areas of all trian-

The authors are with the Hebrew University of Jerusa-lem, Inst. of Biochemistry, Food Science & HumanNutrition, Faculty of Agricultural, Food & Environmen-tal Quality Sciences, P.O. Box 12, Rehovot 76100, Is-rael. Direct inquiries to Dr. A. Nussinovitch.

Method For Studying Surface Topographyand Roughness of Onion and Garlic Skins

For Coating Purposes

V. HERSHKO, D. WEISMAN, and A. NUSSINOVITCH

INTRODUCTIONFRUIT AND VEGETABLE COATINGS HAVEbeen used for many years and their advan-tages have been reviewed (Kester and Fen-nema, 1986; Nussinovitch and Lurie, 1995;Nussinovitch, 1997; Krochta and DeMulder-Johnston, 1997). Such coatings can extendthe postharvest shelf life of fruits and vege-tables (Baldwin, 1994), improve their me-chanical handling properties (Mellenthin etal., 1982), maintain structural integrity, re-tain volatile flavor compounds (Nisperos-Carriedo et al., 1990) or serve as a carrier ofadditives such as antioxidants and antimicro-bial agents. Hydrocolloid coatings for fruitsand vegetables are generally based on car-boxy methyl cellulose (CMC), sodium algi-nate or gellan (Banks, 1984a, b, 1985; Nussi-novitch and Kampf, 1993; Nussinovitch etal., 1994; Hershko and Nussinovitch, 1998a).Our lab has reported the incorporation of in-gredients which improve film adhesion to thecommodity (Nussinovitch and Hershko,1996). We have reported on the relationshipbetween the hydrocolloid coating and thefruit or vegetable skin, checked penetrationof the hydrocolloid and cross-linking agentsin the skin, and studied the skin’s delicatestructure. Top-view pictures of sectionedvegetable-skin surfaces provided informationon skin roughness (Hershko et al., 1996).

The adhesion of polymeric coatings hasbeen studied for the metal industry (Mittal,1983). Many reports have dealt with the re-lationships between ineffective adhesion androugher surfaces. However, there are well-

318—JOURNAL OF FOOD SCIENCE—Volume 63, No. 2, 1998

gles composing the model. Each measure-ment was repeated several times, using boththe whole surface image and random subsetsof each image. This 3-dimensional surfacemeasurement was compared with the 2-di-mensional area of each image. The largestsample studied was 225 �m2, and the diam-eter of the whole bulb was about 5 cm. Thusthe samples were small enough that the over-all curvature of the whole bulb could be ne-glected.

RESULTS & DISCUSSIONTHE MICRO-SURFACE AREA OF FRUITS ANDvegetables is generally not smooth. Food-surface roughness has been demonstrated byour group for several commodities (Hershkoet al., 1996; Nussinovitch and Hershko, 1996;Ward and Nussinovitch, 1996). We tried tofollow the surface curvatures using two dif-ferent methods: surface roughness testing andthe more sensitive method using several crosssections of the produced AFM images. Fromsuch studies Ra values, (defined as the arith-metic mean of absolute values of the rough-ness profile deviation from center within agiven evaluation length, Mitutoyo Guide,

1994) at different locations could be calcu-lated. These values are of limited practicaluse due to the natural variability of food sur-faces, i.e. coefficients of variance could bevery high. Therefore, to get a more represen-tative l and accurate impression of an area’stopography, Ra values of a segment were notadequate, unless an infinite number of veryclose cross sections were considered. A moreeffective method may be to study the topog-raphy of the entire area, as has been tradi-tionally done in geography, meteorology,metallography and satellite imagery. To per-form such studies, AFM images have beeneffective.

Four images, 2 of onion peel and 2 ofgarlic peel, taken from AFM studies, wereselected for this study (Fig. 1 to 4). The first(Fig. 1A) is a typical top view of natural on-ion skin. The area studied was 64 µm2 andits maximal height 300 nm. This area (as wellas that of garlic skin) is covered with amor-phous epicuticular wax which influences skinroughness. The morphology of the epicutic-ular wax differs among species and organs,as well as throughout the plant’s life cycle(von Wettstein-Knowles, 1979; Baker, 1982).

Surface Topography of Onion/Garlic Skins . . .

The main components of onion wax are �-sitosterol and stigmasterol (Rabinowitch andBrewster and Rabinowitch, 1990; see also listof references). A typical onion skin after chlo-roform treatment was viewed) Fig. 2A(. Thestudied area was 225 µm2 and its maximalheight 1507 nm. Generally, the same speci-men was studied by AFM before and afterchloroform treatment . Since it is very diffi-cult to re-map the same area, a few repeatsof the same measurement for a single speci-men were conducted, as well as 10 repeatson different areas of each skin specimen. Inthe imaging experiments, the size of the stud-ied area was limited by the AFM technique.That is, the natural topography determinedthe area sizes, especially when the vascula-ture of the onion and garlic epidermis bulgedhigher than the maximum height detectableby the AFM probe.

A magnification of the original AFM im-age (Fig. 1B), gave a better view of the waxdeposits. Fig. 1C was colored with hill-shad-ing to give an easily understood sense of sur-face. A Tin model (Fig. 1D) of the entire mea-sured surface was followed by polygon cov-erage (Fig. 1E) and gave a resultant perspec-

Fig. 1—Top view of onion-skin surface by AFM. The maximum height is 300 nm. (A) Original image. The bar at right is the false-color zscale. (B)Choice of subset of image to be measured. (C) Hillshade of lattice, to give an impression of the surface. (D) Three-dimensionalTin model of the entire measured surface. The area in the small black box is magnified in Fig. 1E. (E) Magnified view of the small blackbox in D. (F) A view of the surface model in perspective.

A B C

D E F

Volume 63, No. 2, 1998—JOURNAL OF FOOD SCIENCE—319

tive of the whole surface. A comparison ofthe real surface of the onion skin to the ap-parent area or envelope, revealed an11.5±2.5% increase in surface area. Each re-sult was the average of 10 determinations±standard deviation (SD), and as such indi-cated the natural variability of such surfacetopographies. This increase can lead to a bet-ter approximation of the solution volumenecessary to wet and coat a predeterminedfruit or vegetable skin. Other surface-mod-eling programs such as IDL, AVS, or IrisExplorer could probably be used to performsimilar calculations.

The software procedure found a 15.3±2.0% (each result was the average of 10determinations±SD) increase in surface area,after chloroform treatment, i.e. an increasein skin roughness occurred after wax remov-al. This was expected in view of similar stud-ies aimed at determining the roughness ofuntreated and chloroform-treated fruits andvegetables (e.g. eggplant and tomato whichyielded a similar conclusion, i.e. a generalincrease in roughness following wax extrac-tion). An exception was found in the case of“Granny Smith” apples, where no increase

in roughness was detected (Ward and Nussi-novitch, 1996). A comparison of Fig. 1F andFig. 2F demonstrates the contribution of waxto smoothing the skin.

A typical garlic skin sample was takenfrom between the natural garlic vasculature(Fig. 3). The studied area was 100 µm2 andthe maximal height 1369 nm. The increasein calculated surface area was 18.1±1.9%.Each result was the average of 10 determi-nations ±SD. A piece of smoother garlic skintaken from an area between the finer vascu-lar tubing was compared (Fig. 4). Its area was4 µm2 and its maximal height was 121 nm.Wax cleaning shown (Fig. 4F) the uniformi-ty of the garlic-skin surface as well as itssmoothness. After the computer graphics pro-cedure, only a slight increase (0.5%) in sur-face area was observed. The areas betweenthe fine vasculature appeared to be smootherand more uniformly distributed (Fig. 3, 4).

The calculated ratio between the appar-ent and measured surfaces has been definedas r (roughness factor) (Bikerman, 1970).This factor has been defined (Wenzel, 1936)as the ratio of the true area of the solid to becoated to the apparent area or envelope. From

the Young equation (Adamson,1976) and thedefinition of r, the relationship (r = Cos �'/Cos �) was derived, where �' is the measuredcontact angle between liquid and envelopeto the surface of the solid and � is the truecontact angle between liquid and surface atthe air/liquid/ solid contact boundary. A per-fectly smooth surface would have an r of 1.0,but in practice such a surface is rarely en-countered, r usually being considerablygreater than 1.0.

The roughness factor for onion skin de-viated from 1.11 to 1.15 for untreated andchloroform-treated skin, respectively. Foruntreated garlic skin, higher values(r � 1.18) were detected. When �90°, andfrom r � Cos �'/Cos �, it seems that ���'.Hence, roughening the solid surface madethe measured contact angle less than the truecontact angle (MacRitchie, 1990), resultingin the liquid appearing to spread more easi-ly when the solid surface was roughened.When >90°, in accordance with the r equa-tion, �'>�. In this case, roughening the sur-face would tend to increase the contact an-gle and consequently inhibit spreading.Spreading is enhanced by rough surfaces for

A B C

D E FFig. 2—Top view of chloroform-cleaned onion-skin surface by AFM. The maximum height is 1507 nm. (A) Original image. The bar at rightis the false-color z scale. (B) Choice of subset of image to be measured. (C) Hillshade of lattice, to give an impression of the surface. (D)Three-dimensional Tin model of the entire measured surface. The area in the small black box is magnified in Fig. 2E. (E) Magnified viewof the small black box in D. (F) A view of the surface model in perspective.

320 JOURNAL OF FOOD SCIENCE—Volume 63, No. 2, 1998

Surface Topography of Onion/Garlic Skins . . .

liquids that have contact angles <90° andinhibited by rough surfaces in the case of non-wetting liquids (Oliver and Mason, 1977).

In all tested specimens taken from garlicor onion skins, r values were >1.0, in accor-dance with any such surface in practice. Gumsolutions usually used for coating includeCMC, cellulose ethers, carrageenan, alginateand pectin, as well as many other natural andsynthetic water-soluble polymers. The con-tact angles of such materials were measuredon garlic and onion skins (Hershko andNussinovitch, 1998b) yielding values of <90°in all cases. Combined with the observed r

>1.0, this indicates enhanced spreading of thecoating solution on the vegetable or fruit sur-face better contact between solution and sur-face, resulting in good adhesion. These re-sults indicate that changes in fruit-surfaceroughness by delicate mechanical or chemi-cal treatments could be beneficial in termsof more effective coating adhesion.

REFERENCESAdamson, A.W. 1976. Physical Chemistry of Surfaces.

3rd ed., p. 1-43. John Wiley & Sons,. New York.Baker, E.A. 1982. Chemistry and morphology of plant

epicuticular waxes. In The Plant Cuticle, D.F. Cutler,K.L. Alvin, and C.E. Price (Ed.), 0. 139-165. Academ-

ic press, New York.Baldwin, E.A. 1994. Edible coatings for fresh fruits and

vegetables: past, present and future. Ch. 2 in EdibleCoatings and Films to Improve Food Quality, J. Kro-chta, E. Baldwin, and M. Nisperos-Carriedo (Ed.), p.25-64. Technomic Publ. Co., Basel, Switzerland.

Banks, N.H. 1984a. Some effects of Tal Pro-long coat-ing on ripening bananas. J. Exp. Bot. 35: 127-137.

Banks, N.H. 1984b. Studies of the banana fruit surfacein relation to the effects of TAL Prolong coating ongaseous exchange. Sci. Hort. 24: 279-285.

Banks, N.H. 1985. Responses of banana fruit to Pro-longcoating at different times relative to the initiation ofripening. Sci. Hort. 26: 149-157.

Bikerman, J.J. 1970. Physical Surface, p. 1-33. AcademicPress Inc, New York.

Brewster, J.L. and Rabinowitch, H.D. 1990. Onion andallied crops. Vol 1. Botany, Physiology and Genetics.CRC Press, Inc., Boca Raton, FL.

Burdon, J.N., Moore, K.G., and Wainwright , H. 1993.The peel of plantain and cooking banana fruits. Ann.

A B C

D E FFig. 3—Top view of garlic-skin surface by AFM. The maximum height is 1369 nm. (A) Original image. The bar at right is the false-color zscale. (B) Choice of subset of image to be measured. (C) Hillshade of lattice, to give an impression of the surface. (D) Three-dimensionalTin model of the entire measured surface. The area in the small black box is magnified in Fig. 3E. (E) Magnified view of the small blackbox in D. (F) A view of the surface model in perspective.

Volume 63, No. 2, 1998—JOURNAL OF FOOD SCIENCE 321

Appl. Biol. 123: 391-402.Hershko, V. and Nussinovitch, A. 1998a. Physical prop-

erties of alginate-coated onion (Allium cepa) skin.Food Hydroc. (In press).

Hershko, V. and Nussinovitch, A. 1998b. How to tailor asuitable hydrocolloid coating for vegetable material.(submitted).

Hershko, V., Klein, E., and Nussinovitch, A. 1996. Rela-tionships between edible coating and garlic skin. J.Food Sci. 61(4): 769-777.

Kester, J.J. and Fennema, O.R. 1986. Edible films andcoatings. Food Technol. 40: 47-59.

Krochta, J.M. and De Mulder-Johnston, C. 1997. Edibleand biodegradable polymer films: challenges and op-portunities. Food Technol. 51(2): 61-74.

MacRitchie, F. 1990. Chemistry at Interfaces, p. 112-114. Academic Press Inc., San Diego, CA.

Mellenthin, W.M., Chen, P.M., and Borgic, D.M. 1982.In line application of porous wax coating materials toreduce friction discoloration of ‘Bartlett’ and ‘d’Anjou’pears. Hort. Sci. 17: 215-217.

Mittal, K.L. 1983. Adhesion Aspects of Polymeric Coat-

A B C

D E FFig. 4—Top view of garlic-skin surface by AFM. The maximum height is 121 nm. (A) Original image. The bar at right is the false-color zscale. (B) Choice of subset of image to be measured. (C) Hillshade of lattice, to give an impression of the surface. (D) Three-dimensionalTin model of the entire measured surface. The area in the small black box is magnified in Fig. 4E. (E) Magnified view of the small blackbox in D. (F) A view of the surface model in perspective.

ings. Plenum Press, NY and London.Mitutoyo Co. 1994. Surface texture parameters. Mitu-

toyo Co., Tokyo, Japan.Nisperos-Carriedo, M.O, Shaw, P.E., and Baldwin, E.A.

1990. Changes in volatile flavor components of pine-apple orange juice as influenced by application of lipidand composite film. J. Agri. Food Chem. 38: 1382-1387.

Nussinovitch, A. 1997. Hydrocolloid Applications. Ch.10 in Gum Technology in the Food and Other Indus-tries, p. 169-1899. Blackie Academic & Professional.An imprint of Chapman & Hall, London.

Nussinovitch, A. and Hershko, V. 1996. Gellan and alg-inate vegetable coatings. Carb. Polymers 30: 185-192.

Nussinovitch, A. and Kampf, N. 1993. Shelf-life exten-sion and conserved texture of alginate coated mush-rooms (Agaricus bisporous). J. Food Sci. & Technol.26: 469-475.

Nussinovitch, A. and Lurie, S. 1995. Edible coating forfruits and vegetables:. a review article. PostharvestNews and Info. 6(4): 53N-57N.

Nussinovitch, A., Hershko, V., and Rabinowitch, H.D.1994. Protective coatings for food and agricultural

products, method for producing same and productscoated by same. Israel Patent Application # 111495.Nov.1994. PCT/US 95/14252 (1994)

Oliver, J.F. and Mason, S.G. 1977. Microspreading stud-ies on rough surface by scanning electron microscopy.J. Colloid Interface Sci. 60(3): 480-487.

Packham, D.E. 1983. The adhesion of polymers to met-als: the role of surface topography. In: Adhesion As-pects of Polymeric Coatings, K.L, Mittal (Ed.), p. 19-41. Plenum Press New York and London.

von Wettstein-Knowles, P. 1979. Genetics and biosyn-thesis of plant epicuticular waxes. In Advances in theBiochemistry and Physiology of Plant Lipids, L.-A. Ap-pelquist and C. Liljenberg (Ed.), p. 1-33. Elsevier NorthHolland, Amsterdam.

Ward, G. and Nussinovitch, A. 1996. Gloss propertiesand surface morphology relationships of fruits. J. FoodSci. 61: 973-977.

Wenzel, R.N. 1936. Surface roughness and contact an-gle. Ind. Eng. Chem. 28: 988-994.

Ms received 4/14/97; revised 8/19/97; accepted 10/3/97.