Scanning Electron Microscopy of Carrot Stick Surface to Determine Cause of White Translucent Appearance YASUO TATSUMI, ALLEY E. WATADA, and WILLIAM P. WERGIN ABSTRACT thickness) and stored at SC, 95% RH for subsequent daily analysis. The amount of whitish translucent tissue on the surface of each stick Carrot sticks that were prepared with a sharp culinary knife exhibited was scored using the following index: 1 = None: 2 = Slight; 3 = a whitish translucent appearance on the surface. This condition was Moderate; 4 = Severe and; 5 = Extreme. A severity index for each not readily apparent with carrot sticks sliced with a razor sharp blade. lot was calculated by multiplying the index score by the percentage Scanning electron microscopic examination of the translucent tissue of sticks in each category. revealed that the knife tended to shear, separate and compress the For SEM observation, 1 x 1 x 0.5 cm sections of tissue were cells and tissues of the root. Dehydration of the large mass of exposed sampledfrom either the freshly preparedsticks, (referred to as “fresh”), cells probably was responsible for the appearanceof the whitish trans- or the 3-day stored sticks, (designated “aged”). Tissues were pre- lucent tissue. Development of this condition is undesirable because pared as described for cucumbers (Tatsumi et al., 1987), with mod- consumers associate this with aged or nonfresh carrot sticks. ifications. The tissue pieceswere fixed in 3% glutaraldehydein distilled water for 24 hr at room temperature (18-20°C) and dehydrated in a standard ethanol solution series (20, 40, 60, 80, 90, and 100%). The INTRODUCTION samples were critical-point dried from liquid carbon dioxide. The cut DEMAND for partially processedfruits andvegetables by in- surface was sputter-coated with gold/palladium 60/40 and examined stitutiondining rooms,restaurants and fast food franchiseout- with a Hitachi S530 scanning electron microscope operated at 15 kV. For EDX, samples were fixed, dehydrated, critical-point dried, lets hasincreased substantially. This hasresulted in part from mounted, carbon coated and examined at 20 kV in the SEM equipped consumersgrowing’ appetitesfor fresh fruits andvegetables, with a Kevex 8000 EDX system. Samples were tilted 30” toward the asevidenced by the popularityand growth in numbersof salad X-ray detector. Analysis provided a qualitative and semi-quantitative bars.To meetthese demands without increasingcost of prod- elemental composition of an 8-10 rnk layer of surface tissue. ucts, the food industry has mechanizedmany labor intensive operationssuch as peeling,slicing, coring etc. However,the successof suchmechanizations in food preparationis depen- dent on maintainingfresh appearance and high quality. RESULTS & DISCUSSION Recently,the quality of partially processedcarrot sticks has CARROT STICK that were cut with the culinaryknife or the beenquestioned both by processorsand venders. Carrot sticks, razorblade did not exhibitany visible translucentwhitish tissue which are sold in grocery storesand are availableon most or discoloration on the day of slicing. Surfaces of sticks pre- saladbars, may be unsatisfactoryor unacceptablebecause they paredwith the blade, however,were visibly smootherthan sometimesexhibit a whitish translucentappearance on the sur- thosecut with the knife. The whitish tissue, similar to that face. Consumerstend to associatethis conditionwith agedor notedon grocerystore products, developed on the surfacedur- nonfreshcarrot sticks. This hasbeen observed in nonvacuum ing storageof the sticks that were cut with the knife. This packagedcarrots that were storedat 10” or 15°C for 2 to 4 discolorationwas associatedwith a noticeableamount of loose days,but not in carrotsthat were stored5 daysat 4°C (Buick or sloughingtissue. By day 1, (=24hr after slicing) 65% of and Damoglou,1987). The causeof the discolorationand the the sticks exhibiteddiscoloration, resulting in a total severity possibleways to alleviatethe condition have not beenreported. indexof 455 (Table1). Thosesticks also contained slight cracks Our objectivewas to determinewhether there might be a alongthe cambiumlayer betweenthe cortex and the vascular physicalcause for the effect. We examinedthe cut surfaceof cylinder. By day 2, 90% of sticks scoreda severityindex of carrotsticks visually, and the cells and tissueswith scanning 490, nearthe maximum severity index. No increasein severity electronmicroscopy (SEM). The elementalcontent of the cut index was notedon day 3; however,the separationbetween surfacewas also examinedusing an energydispersive X-ray thecortex and the vascular cylinder had widened and the sticks microanalysis(EDX) in an attemptto explainthe appearance of the whitish tissue. Table 1 -Relation between severity of white translucent tissue on surface of carrot sticks and method of slicing or storage MATERIALS & METHODS Davs at 5°C 2 WHOLE CARROTS (cultivar unknown) packaged in ventilated poly- Slicing Severity 1 3 ethylene bags were purchased at a local produce market and stored in tool score’ %V S.I. % S.I. % S.I. a waxed fiberboard container at 0°C until use. To prepare the sticks, 1 0 0 0 0 0 carrots were washed, cut to 6 cm lengths, and quartered with a sharp 2 0 0 0 0 : 0 culinary knife or a razor blade. Each quarter was placed in a separate Knife 3 10 30 0 0 0 0 lot with each lot containing 20 sticks. One lot of each prepared with 40 a knife or blade was analyzed immediately. The remaining lots of 20 45 fE 325100 10 45040 A: 450 ml 455 g 490 100 490 sticks each were placed in unsealed polyethylene bags (0.025 mm Total 1 55 ix 50 50 35 35 45 50 100 50 100 Author Watadais with the HorticulturalCrops Quality Labora- Razor blade 3 0 0 8 0 15 45 tory and Author Wargin is with the Electron Microscopy Labo- 4 0 0 0 0 0 5 0 0 0 0 0 0 ratory, USDA/ARS, Beltsville, MD 20705. Author Tatsumi’s present Total 100 145 100 150 100 175 address: Laboratory of Postharvest Horticulture, Faculty of Ag- ricultur& Miyazaki Univ., l-l, Gakuen Kibanadai-nishi, Miyazaki, z l-None, 2-Slight, 3-Moderate, 4&w% 5-Extreme. Y Percentage of 20 carrot stick sample. 889-21, Japan. Address inquiries to Dr. Watada. X Severiw Index (S.I.) = Score of severity x %. Volume56, No. 5, 1991-JOURNAL OF FOOD SCIENCE-1357 SEM OF CARROT STICKS. Fig 1 -Scanning electron micrographs of the surface of carrot sticks immediately after slicing (fresh) or after aging for 3 days at 5°C (aged). The fresh (Fig. 1-a) and aged (Fig. l-b) sticks prepared with a razor blade had evenly cut surfaces with well defined parenchyma cells that did not show distortion from pressure of the blade. Fresh (Fig. l-c) and aged (Fig. l-d) sticks prepared with a culinary knife showed considerable shearing of the tissue and compression of cells. Sticks fresh-cut with a knife showed some separation of tissue between cambium and vas- cular cylinder (Fig. l-e) as well as layers of collapsed cells (Fig. 1-q. Collapse was also evident in the vascular cylinder of aged sticks (Fig. 1-g). All micrographs 40 x . became curved along the longitudinal axes, a condition similar About 55% of the sticks that had been cut with the razor to that noted with carrot sticks from the grocery store. blade exhibited some loose tissue on day 1. This resulted in a 1358-JOURNAL OF FOOD SCIENCE-Volume 56, No. 5, 1991 severity index of 145, only one-third that scoredfor knife- had been cleanly cut by the blade were susceptibleto dehy- preparedsticks. The amountof loosetissue increased slightly dration. Drying of a single layer of cells apparentlywas not eachday resultingin 15% of the stickshaving a severityindex sufficient to causethe carrot stick surfaceto have the whitish of 175by day 3. However,the sticks did not exhibit any tissue translucentappearance. separationalong the cambiallayer or longitudinalcurving. Ten elementswere identified on the cut surfaceof carrot SEM examinationof the surfaceof the fresh tissueprepared tissuesby the energydispersive X-ray microanalysis,and as with a razor blade revealedthat the cortical region appeared reportedby others,(Hopkins, 1959;Haytowitz andMatthews, evenly cut, was not compressedand consistedof well orga- 1984), K and Ca accountedfor 70% of the total content(data nized parenchymacells which did not show mechanicaldam- not shown). The contentof none of theseelements differed age (Fig. la). The appearanceof the cortical region and the betweencarrots sliced with a knife or razor blade (data not organizationof the cells remainedunchanged during subse- shown).This implied that theseelements had no influenceon quent aging (Fig. 1). However, the vascularcylinder, espe- the tissuesurface changes. cially thexylem andpith, appearedto sustainsome mechanical We concludedthat the abrasiveaction of the culinary knife damagefrom the blade,which was not apparentto the naked causedconsiderable tearing and shearingof tissues.The re- eye (Micrographnot shown). sultingcells, which werecompressed and sloughed, were highly Freshtissue prepared with the knife appearedvisually sim- susceptibleto dehydrationwhich in turn causedthe white dis- ilar to that preparedwith the blade, but SEM examination colorationthat was characteristicof the agedcells cut with the revealedin the knife-cut samplesa serratedsurface character- knife. No consistentrelationship was apparentbetween changes ized by tearing,shearing, and compression of cells and tissues in elementalcomposition on the surfaceof the slices and the (Fig. l-c and d). Furthermore,cracks were presentalong the preparationmethod or aging process.Results tend