Journal of the Science of Food and Agriculture J Sci Food Agric 88:2087–2092 (2008)

Effect of production site and storageon antioxidant levels in specialty potato(Solanum tuberosum L.) tubersSarah Rosenthal1 and Shelley Jansky2∗1University of Wisconsin-Madison, Department of Horticulture, 1575 Linden Drive, Madison, WI 53706, USA2USDA-ARS and University of Wisconsin-Madison, Department of Horticulture, 1575 Linden Drive, Madison, WI 53706, USA

Abstract

BACKGROUND: The potato can make a significant contribution of antioxidants to the human diet. In this study,antioxidant levels in tubers of 14 specialty potato clones grown at four production sites (two conventional, twoorganic), both fresh and stored, were examined across 2 years.

RESULTS: Antioxidant activity of fresh tubers at all locations was higher in 2006 than in 2005. Cooler late-seasontemperatures in 2006 may have been responsible for the increased levels of antioxidants. Stored tubers had higherlevels of antioxidant activity than fresh tubers, with a larger storage effect in 2005, when antioxidant levels infresh tubers were lower. There was no consistent effect of production system (organic versus conventional) onantioxidant activity in tubers.

CONCLUSION: For the specialty potato clones we evaluated, antioxidant levels were generally highest in potatoesgrown in high-yielding production environments, and they increased during storage. Therefore, potatoes withhigh nutritional value, in terms of antioxidant activity, can be produced using conventional production and storagesystems. 2008 Society of Chemical Industry

Keywords: antioxidants; organic production; potato; Solanum tuberosum

INTRODUCTIONAntioxidants in potato tubersThere is an increasing interest in the role ofantioxidants in the human diet, largely due to epi-demiological studies in which the consumption ofvegetables has been reported to protect against manytypes of cancers.1 Several commonly consumed fruitsand vegetables are high in antioxidants.2 Total antiox-idant activity is strongly correlated with cancer cellinhibition.3

Potatoes, among the four staple crops of the world,are a significant source of antioxidants.4 The mostabundant antioxidants in potatoes are anthocyanins5

and phenolics.6 Differences in antioxidant activity,and compounds that act at antioxidants, exist amongpotato market types that differ in tuber flesh andskin color. Antioxidant activity is highest in purple-skinned, intermediate in red-skinned, and lowest inyellow-skinned cultivars.7 In red- and purple-fleshedpotatoes, chlorogenic acid (a type of phenolic acid) andanthocyanins are the main compounds contributing toantioxidant capacity.8–10

Abiotic and biotic stress effects on thephenylpropanoid pathwayAs reviewed in Hahlbrock and Scheel,11 and Dixonand Paiva,12 abiotic and biotic stresses induce thephenylpropanoid pathway. In potato, this pathway hasbeen induced by wounding,13 light14 and disease,15

which cause the accumulation of chlorogenic acid.13,16

The extent of accumulation of chlorogenic acid dur-ing processing and storage varies among cultivars.17

Chlorogenic acid accumulates in response to Phytoph-thora infestans Mont., the causal agent of late blight, inconventional potato cultivars,15 and the wild speciesS. sanctae-rosae.18 A higher amount of chlorogenicacid has been found in the roots of cultivars resistantto Verticillium wilt.19 Additionally, chlorogenic andcaffeic acids have been identified in potato tissue andskin with infected by Helminthosporium carbonium.20

The accumulation of phenolic acids in potato tubershas also been induced by wounding stress.9

Tubers grown at high light intensities and low tem-peratures have enhanced levels of anthocyanins andtotal phenolics.21 Potatoes cultivated in warmer anddrier regions with lower altitudes, in predominantly

∗ Correspondence to: Shelley Jansky, USDA-ARS and University of Wisconsin-Madison, Department of Horticulture, 1575 Linden Drive, Madison, WI53706, USAE-mail: shjansky@wisc.edu(Received 28 December 2007; revised version received 16 April 2008; accepted 13 May 2008)Published online 1 August 2008; DOI: 10.1002/jsfa.3318

2008 Society of Chemical Industry. J Sci Food Agric 0022–5142/2008/$30.00

S Rosenthal, S Jansky

loam soils, produce lower amounts of polyphenoliccompounds than tubers cultivated in cooler and morehumid regions with sandy loam soils.22 Similarly,cooler production sites with higher levels of soil organicmatter have been found to produce potatoes withhigher levels of chlorogenic acid.23

Effect of production environmentFew studies have addressed the effect of productionsystem on potato tuber antioxidant levels. Potatoesgrown in an ecological production system producedhigher polyphenol levels than those grown in a conven-tional system, with varietal influence more importantthan locality.22 Cultivar, year and geographic loca-tion were determined to be equal or greater factorsdetermining tuber quality than the farming system.24

However, an inverse relationship between nitrogeninput in the production system (and nitrate levels intubers) and ascorbic acid levels in tubers was noted. Inaddition, an inverse relationship between soil potas-sium levels and antioxidants has been reported.25,26

Effect of storageDuring cold storage, increases in levels of bothanthocyanins27 and phenolic acids28 have beenreported. Phenolic acids also accumulated in sweetpotatoes stored at 5 ◦C for 4 weeks.29 In contrast,levels of another antioxidant, ascorbic acid, werefound to decrease rapidly during cold storage.30 Ina study based on potato and a cultivated relative(Phureja Group), total carotenoid levels decreasedfollowing nine months of storage at 4 ◦C. Zeaxanthinand antheraxanthin levels decreased, while luteinincreased.31

Previous studies on the effect of environmenton antioxidants in potato tubers have focused onstandard potato cultivars. This study was carried out todetermine the effects of production environment andstorage on antioxidant activity of potato tubers fromspecialty clones selected for high levels of antioxidants.

MATERIALS AND METHODSPlant materialSpecialty potato clones developed by C. Brown(USDA-ARS, Prosser, WA, USA), with intenselypurple or red skin and flesh, were used inthis study.32 These clones have been selectedfor high antioxidant activity. In 2005 and 2006,three replications (4-hill plots) of 14 clones [B1(DEANA), B3 (PA96RR01-193), B4 (PA96RR01-220), B7 (POR00PG2-1), B8 (POR00PG2-16),B9 (POR01PG1-5), B12 (POR01PG8-3), B13(POR01PG10-1), B14 (POR01PG12-2), B15(POR01PG12-4), B17 (POR01PG16-1), B20 (POR-01PG45-3), B21 (POR01PG45-4), and B23 (S48-6)] were grown in Wisconsin on two conventionalfields near Rhinelander (45.64 ◦N) and Hancock(44.13 ◦N), and two commercial organic fields nearRosholt (44.63 ◦N) and Baraboo (43.45 ◦N). In 2005,

tubers were planted on 9, 10, 11 and 11 May andharvested 112, 127, 114 and 139 days after plant-ing, respectively. In 2006, tubers were planted on 4,1, 24 and 8 May, and harvested 116, 136, 99 and135 days after planting, respectively. Each field wasmanaged using standard conventional or commercialorganic practices and harvested with a digger. Tuberswere collected and bagged by hand. Fresh tubers werediced (as described below) 17–19 days after harvest.Additional tubers were stored at 5.6 ◦C (95% relativehumidity) for 5.5 months. This is a typical storagetemperature and time period for fresh market pota-toes. No sprout inhibitors were applied to the tubers.Soil samples were collected from each location in lateAugust in both years and analyzed for pH, percentorganic matter, potassium, and phosphorus by theUniversity of Wisconsin Soil and Plant Analysis Lab-oratory. Three samples were collected from each siteon each date.

Antioxidant extractionTwo samples per clone were collected for eachreplication. For each sample, three tubers were rinsed,dried and cut in half from stem to bud end. Onehalf of each tuber was thrown away; the other threehalves were diced into 1 cm cubes without removingthe skins. The cubes were placed in a bag, frozen inliquid nitrogen, and placed in a −80 ◦C freezer.

A 20 g sample of the frozen tissue was ground in50 mL of cold 100% ethanol with a hand blenderfor approximately 10 s. The slurry was then filtered,diluted, frozen immediately in liquid nitrogen, andstored at −80 ◦C. Before analysis, samples werethawed on crushed ice. The ABTS (2,2′-azino-di-[3-ethylbenzthiazoline sulfonate]) assay using hydrogenperoxide was adapted for tuber tissue from the96-well Caymen Chemical Antioxidant Assay Kit(Ann Arbor, MI, USA; catalog no. 709001). Inthis assay, metmyoglobin acts as a peroxidase toform an intermediate in the presence of hydrogenperoxide, leading to the formation ABTS•+ as a radicalcation. The antioxidant capacity was determinedby generating a standard curve for each plateusing Trolox, a water-soluble tocopherol analogue.Throughout this paper, units will be expressed asTrolox equivalents (TE, micromoles) per gram offresh tuber tissue. A standard curve was generated foreach plate.

Statistical analysisStatistical analyses were performed using the mixedprocedure in the SAS System for Windows (version9.1). Significant differences were determined usingtype 3 tests of fixed effects, and Tukey–Krameradjusted P values were used when possible. Fourseparate analyses were performed: 2005 fresh versusstored, 2006 fresh versus stored, 2005 versus 2006fresh, and 2005 versus 2006 stored. Additionalanalyses include specific gravity for Hancock 2006tubers, average tuber weight (all clones, all fields), and

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Antioxidants in potato tubers

tuber water loss over storage for four clones at all four2006 locations (three replications per field).

RESULTS2005 fresh versus stored tubersIn 2005, significant differences in antioxidant activitywere detected among clones (P = 0.0226), betweenstorage times (P < 0.0001), and across locations(P = 0.0002). Significantly higher antioxidant levelswere found in stored tubers than fresh tubers whendata were combined across locations and also whenanalyzed at each location (P < 0.01 for each analysis)(Fig. 1).

Fresh tubers from Hancock had significantly higherlevels of antioxidant activity (1061 ± 98) than thosefrom Rhinelander (601 ± 101), Rosholt (805 ± 113),and Baraboo (610 ± 128). Stored tubers from Rosholt(1636 ± 93), Hancock (1618 ± 107), and Baraboo(1615 ± 94) contained significantly higher antioxidantlevels than those from Rhinelander (1205 ± 97).

2006 fresh versus stored tubersIn 2006, significant effects were detected due toclone (P < 0.0001), location (P = 0.0002), storage(P = 0.0038), and the location by storage interaction(P = 0.0213). Similar to 2005, when data werecombined across all locations, antioxidant levels werehigher in stored tubers than in fresh tubers. However,while a significant storage effect was observedfor Hancock (P = 0.0018), it was not found forBaraboo (P = 1.000), Rhinelander (P = 0.9988), orRosholt (P = 0.9911). In fresh tubers from Baraboo,antioxidant activity was higher (1590 ± 102) than inthose from Rhinelander (1249 ± 102), but not higherthan Hancock (1433 ± 108) or Rosholt (1464 ± 99).After storage, antioxidant levels from Hancock tubers(2045 ± 105) were significantly higher than those fromRosholt (1584 ± 105), Rhinelander (1331 ± 94), andBaraboo (1620 ± 101) (Fig. 2).

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Figure 1. Average antioxidant values for 2005 fresh and storedtubers (5.6 ◦C for 5.5 months) at four locations. Error bars are 1standard deviation above and 1 standard deviation below the mean.TE, Trolox equivalents.

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Figure 2. Average antioxidant values for 2006 fresh and storedtubers (5.6 ◦C for 5.5 months) at four locations. Error bars are 1standard deviation above and 1 standard deviation below the mean.TE, Trolox equivalents.

2005 versus 2006 fresh tubersComparing data from fresh tubers across years,significant effects were detected due to clone (P <

0.0001), year (P < 0.0001), location (P = 0.0011),and the year by location interaction (P = 0.0068).Antioxidant levels in tubers harvested in 2006 werehigher than those in 2005 in the combined analysisand for each field analyzed individually (P < 0.03 foreach analysis) (Figs 1 and 2).

2005 versus 2006 stored tubersWhen analyzed across years, significant effects weredetected among stored tubers due to clone (P <

0.0001) and field (P < 0.0001), but not year. Overall,2006 stored tubers had higher antioxidant levels thanthose in 2005.

Soil propertiesSignificant effects of year were detected for the amountof organic matter in soil samples (P = 0.0062),but not for pH, phosphorus content or potassiumcontent. There were no differences among locationsfor soil pH in 2005, but in 2006, soil pH washighest at Rosholt (6.77), followed by Hancock(6.37) and Baraboo (6.17), which were higher thanRhinelander (4.37). In 2005, organic matter washighest at Baraboo (2.73%), followed by Rhinelander(1.90%) and Rosholt (1.83%). Hancock had thelowest organic matter (0.73%). In 2006, Baraboo hada higher organic matter content (3.77%) than Rosholt(2.37%), Rhinelander (2.37%) and Hancock (1.47%).In 2005, all four sites differed in soil phosphorus levels(Rhinelander 335 ppm, Rosholt 185 ppm, Hancock135 ppm and Baraboo 36 ppm). In 2006, the highestphosphorus levels were detected again at Rhinelander(319 ppm), followed by Rosholt (170 ppm), andthen Hancock (94 ppm) and Baraboo (72 ppm). In2005, Rhinelander had the highest soil potassiumlevels (233 ppm), followed by Rosholt (115 ppm) andBaraboo (107 ppm) and then Hancock (43 ppm). In2006, Baraboo and Rosholt had the highest levelsof soil potassium (176 and 159 ppm, respectively),

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followed by Rhinelander (109 ppm) and then Hancock(72 ppm).

Specific gravity, tuber weight, and water lossThese analyses were performed in 2006 to determinewhether they are likely to contribute to observeddifferences. Specific gravity was measured in Hancocktubers, but no differences were detected among clones(P = 0.1291). Significant location (P < 0.0001) andclone (P < 0.0001) effects were detected for averagetuber weight. However, no significant location by cloneinteraction was detected (P = 0.3689). Rhinelanderhad the highest tuber weight per 4-hill plot (132 ±31 g), followed by Rosholt (118 ± 31 g), then Hancock(109 ± 28 g), and Baraboo (64 ± 21 g). Only Hancockand Rosholt were not significantly different from eachother (P = 0.6503). All other location combinationswere different from each other (P = 0.0372 to<0.0001).

Significant location (P = 0.0009) and clone (P =0.0001) effects were detected for tuber water lossduring storage. Rosholt tubers exhibited the greatestwater loss (19.5 g kg−1), followed by Baraboo (18.9 gkg−1), Hancock (15.6 g kg−1) and Rhinelander (10.8 gkg−1). Specifically, differences were detected betweenRosholt and Rhinelander (P = 0.0015) as well asBaraboo and Rhinelander (P = 0.0036). The waterloss for three of the four clones was similar [B12 (13.8 gkg−1), B13 (12.4 g kg−1) and B14 (16.3 g kg−1)]. Oneclone, B3, was significantly higher than the others(22.4 g kg−1, P < 0.04 for each comparison).

DISCUSSIONAntioxidant activity increased following cold storage,especially in 2005 (Figs 1 and 2). Ascorbic acid levelsdecrease during storage.30 However, anthocyaninlevels of colored flesh clones increase slowly butsignificantly during cold storage (4 ◦C).27 In addition,total phenolic and chlorogenic acid contents increaseafter storage at low temperatures.28,33 Since theamount by which antioxidant levels increased wasvariable by location and year, it seems likely thatthe relative proportions of individual members of theantioxidant pool varied. It is also interesting to notethat, while antioxidant levels in most clones increasedduring storage, they decreased in one clone (B19) at alllocations. We are following up on these observationsby analyzing individual antioxidant components infresh and stored tubers in a subset of clones from allfour sites in both years.

When antioxidant levels of fresh tubers wererelatively low, such as those grown in 2005, antioxidantlevels increased dramatically during storage. Incontrast, when antioxidant levels were relativelyhigh in fresh tubers, such as the 2006 growingseason, the change during storage was less dramatic.Perhaps the antioxidant levels in fresh tubersin 2006 were near the maximum potential forthese clones, so additional increases were not

physiologically possible during storage. In 2005,antioxidant levels were lower at harvest, so differencesdue to environments were detected. After storage,though, mean antioxidant levels across environmentswere similar, again possibly reflecting a physiologicallimit to antioxidant activity. Perhaps that limit isbetween 1500 and 2000 µmol TE g−1 of fresh tubertissue.

Production year had a large impact on antioxidantlevels in fresh tubers (Figs 1 and 2). This effectwas lost following storage because tubers harvestedin 2005 gained more antioxidants while in storagethan those in 2006 (Figs 1 and 2). At all locations,antioxidant levels were higher in 2006 than in 2005.Since production practices were similar in both years,and the trial was carried out at the same sites in bothyears, it seems likely that weather was responsible forthose differences. Commercial potato production datareveal that conditions for potato growth were betterin 2006 than in 2005. Average yields in Wisconsinwere 46.5 t ha−1 in 2005, but rose by nearly 9% to50.5 t ha−1 in 2006. While mean summer temperatureswere similar in both years, a more detailed analysisof the data reveals some differences that may beimportant. At all locations, there were fewer warmdays (above 30.0 ◦C, 80 ◦F) and warm nights (above18.3 ◦C, 65 ◦F) in August and September in 2006. Theonly exception was daytime temperatures at Hancockin August. Levels of anthocyanins and phenolics arehigher in tubers from potato plants grown in coolerregions.21,22 Cooler growing environments also resultin higher levels of chlorogenic acid in potato tubers.23

Antioxidant levels were higher at all locations in 2006,when cooler days and nights prevailed later in thegrowing season.

It is interesting to note that soil organic matterwas significantly (P = 0.0062) higher in 2006 thanin 2005. The highest levels of organic matter wereobserved at Baraboo (27.3 g kg−1 and 37.7 g kg−1 in2005 and 2006, respectively) and the lowest levelswere at Hancock (7.3 g kg−1 and 14.7 g kg−1 in2005 and 2006, respectively). A positive associationbetween soil organic matter and chlorogenic acid levelshas been reported previously.23,34 However, if highlevels of organic matter were associated with highantioxidant levels, then we would expect tubers fromlocations with high soil organic matter to have highantioxidant activity. However, tubers from Hancockhad the highest levels antioxidant activity in 2005, butthe lowest organic matter content. Perhaps the highorganic matter levels in 2006 were due to lower soilmicrobial activity, since late season temperatures werelower in that year.

In 2005, fresh Hancock tubers had the highestantioxidant activity. It is interesting that the amount ofapplied nitrogen fertilizer was much higher at Hancock(260 kg ha−1) than the other sites (approximately170 kg ha−1). The synthesis of organic acids, includingchlorogenic and ascorbic acids, has been reported toincrease with increasing levels of nitrogen fertilizer.35

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When antioxidant levels in tubers are relatively low, asthey were in 2005, nitrogen availability may influenceantioxidant activity. In addition, soil potassium levelsat Hancock were significantly lower than those atthe other sites. In spinach hydroponic cultures, highantioxidant levels were reported in cultures with highnitrogen and low potassium levels.25 In addition, aninverse relationship has been reported between soilpotassium levels and chlorogenic acid levels in potatotubers.26

In all analyses, field by clone interactions werenot significant, indicating that clonal rankings amongfields were similar. Specific gravity differences among2006 Hancock clones were not significant, so they arenot likely to contribute to differences in antioxidantactivity levels. Although differences were detectedamong both clones and fields for water loss duringstorage, they were typically small (∼10 g kg−1) soare not likely to be a major contributor to large(∼500 g kg−1) differences in antioxidant levels betweenfresh and stored tubers.

We began this study with the hypothesis that tubersgrown under stressful environments will contain higherlevels of antioxidants than those grown in optimalenvironments because biotic and abiotic stressesinduce the phenylpropanoid pathway.11,12 In general,organic production systems are more stressful thanconventional ones due to disease, weed, and insectpressure; lower fertilizer inputs; and in some cases, alack of supplemental irrigation. The organic Baraboosite was considered to be the most stressful sitebecause it was the only site that is not irrigated, itis the southernmost site so temperatures are highestthere, nitrogen applications were low, and leafhopperpressure was strong due to neighboring alfalfa fields,which were cut during the growing season. In addition,in 2005, the trial at Baraboo was severely stressedby drought. The least stressful site was Rhinelander.That was the northernmost location in the study, withmoderate temperatures during the day and cool nighttemperatures. In addition, supplemental irrigationwas used and conventional production practices wereavailable to control disease and pests and to providenutrients. However, in both years, there was nodifference in antioxidant levels between Baraboo andRhinelander. Hancock is located in the central sandsregion of Wisconsin, where most commercial potatoesare produced. Because the soil is sandy, with loworganic matter, frequent overhead irrigation is used. In2005, when antioxidant levels in tubers harvested fromthe four sites varied, they were highest in Hancocktubers. In addition, antioxidant levels were highestin the best production year, 2006. It appears that,for the clones we evaluated, antioxidant levels arehighest in potatoes grown in high-yielding productionenvironments.

CONCLUSIONSFor the specialty potato clones we evaluated, antioxi-dant levels increased during cold storage, especially in

the relatively low antioxidant year. Antioxidant levelswere affected by both production year and productionenvironment. They were generally highest in pota-toes grown in high-yielding production environments.Therefore, potatoes with high nutritional value, interms of antioxidant activity, can be produced usingconventional production and storage systems.

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