Machado 2007 Food-Control

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Food Control 18 (2007) 503–508

Effect of light and temperature on the formation of glycoalkaloidsin potato tubers

Rita M.D. Machado *, Maria Cecılia F. Toledo, Lucila C. Garcia

Department of Food Science, Faculty of Food Engineering, State University of Campinas, UNICAMP, Caixa Postal 6121,

CEP 13083-862, Campinas, SP, Brazil

Received 28 December 2004; received in revised form 16 December 2005; accepted 16 December 2005

Abstract

This study was designed to determine the influence of two light sources and temperature on the total glycoalkaloid (TGA) content ofpotato tubers cultivated in Brazil. Tubers of cv Monaliza were exposed during 14 days to the following conditions: (1) indirect sunlight,(2) fluorescent light, (3) storage in darkness under refrigeration, and (4) storage in darkness under room temperature. The glycoalkaloidsa-solanine and a-chaconine were determined using a reversed phase C18 HPLC column with a photodiode array detector. Exposure ofpotato tubers to fluorescent light resulted in the highest TGA levels. Smaller tubers presented the highest TGA concentrations, irrespec-tive of the light source and temperature. Although the TGA levels at the end of the experiments were higher than the initial content, asteady increase of TGA concentration was observed only in tubers exposed to fluorescent light. The levels of TGA found in the analysedpotato samples were below 200 mg kg�1, value that has been considered to be safe for human consumption.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Glycoalkaloids; a-Solanine; a-Chaconine

1. Introduction

Potato is included among the main horticultural crops inBrazil. Due to its broad availability and nutritional charac-teristics, it has been considered one of the most importantcomponents of the human diet. Nevertheless, potato tubers(Solanum tuberosum L.) contain two naturally occurringtoxins, a-solanine and a-chaconine, which comprise over95% of the total glycoalkaloids (TGA) present in thepotato plant (Bushway & Ponnampalam, 1981). Accordingto Maga (1980), the glycoalkaloids can be found in all partsof a potato plant. Among the tissues that contain glycoal-kaloids, the skins and sprouts present the highest concen-trations (Morris & Lee, 1984; Smith, Roddick, & Jones,1996). Symptoms of glycoalkaloid poisoning include colicpain in the abdomen and stomach, gastroenteritis, diar-rhea, vomiting, fever, rapid pulse, low blood pressure,

0956-7135/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.foodcont.2005.12.008

* Corresponding author. Tel.: +55 19 3788 2168; fax: +55 19 3788 2153.E-mail address: [email protected] (R.M.D. Machado).

and neurological disorders (Morris & Lee, 1984; Slanina,1990). Toxicity of glycoalkaloids results from anticholines-terase activity on the central nervous system and mem-brane disruption adversely affecting the digestive systemand general body metabolism (Jadhav, Sharma, & Salun-khe, 1981; Maga, 1980; Roddick, 1989). In vitro experimentsshowed that a-solanine and particularly a-chaconine arepotent cytotoxins, acting rapidly to induce cell lysis(Phillips et al., 1996). The cytotoxic potency of potato gly-coalkaloids is considerable, particularly in the case ofa-solanine: a-chaconine mixtures, which present differenti-ated synergistic action in disrupting membranes (Keukenset al., 1996; Rayburn, Friedman, & Bantle, 1995; Roddick,Rijnenberg, & Osman, 1988). Moreover, these synergismshave important implications not only for the defensive roleof glycoalkaloids in potato plant but also for the safety andacceptability of potatoes and potato products (Smith, Rod-dick, & Jones, 2001).

The TGA content in potato varieties commerciallyavailable is usually below 200 mg kg�1 of fresh weight

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Table 1Parameters monitored during the period of 2 weeks

Condition T (temperature)a

(�C)Light intensityb

(Lux)

Indirect sunlight exposure 23–29 131–868Fluorescent light exposure 24–30 458–960Storage in darkness under

refrigeration7–8 –

Storage in darkness underroom temperature

19–26 –

a Measured during the experiment.b Measured using the Luximeter (testo 545-Lux, fc).

504 R.M.D. Machado et al. / Food Control 18 (2007) 503–508

(FW), the recommended safety level of TGA in unpro-cessed potato tubers for human consumption (FAO/WHO, 1999; Slanina, 1990). In the study conducted in Bra-zil by Machado and Toledo (2004), 82% of the samples ofwhole potato tubers of different commercial varieties pre-sented levels of TGA below 100 mg kg�1 FW. This resultis consistent with those findings from other countries(Friedman, Roitman, & Kozukue, 2003; Peksa, Gol-ubowska, Rytel, Lisinska, & Aniolowski, 2002) and indi-cates that the commercial potatoes are usually safe forhuman consumption. In addition, potato glycoalkaloidsappear to be largely unaffected by home processing condi-tions such as baking, boiling, frying and microwaving(Bushway & Ponnampalam, 1981; Maga, 1980).

However, the content of the glycoalkaloids can varygreatly in different potato cultivars and the biosynthesisof glycoalkaloid can be rapidly stimulated by environmen-tal factors such as light, mechanical injury, and storagetemperature (Friedman & Dao, 1992; Griffiths, Bain, &Dale, 1997; Jadhav & Salunkhe, 1975; Percival, 1999). Gly-coalkaloid synthesis due to exposure to light may occureither in the field, at harvest or during storage (Jadhav &Salunkhe, 1975). Several authors have demonstrated thatthe TGA concentration of potato tubers exposed to lightcan increase twice or three times. This means that individ-uals consuming large quantities of light-exposed tuberscould theoretically present adverse effects (Dale, Griffiths,Bain, & Todd, 1993; Griffiths, Bain, & Dale, 1998; Jadhav& Salunkhe, 1975).

It has been shown that the rate of glycoalkaloid accumu-lation in potato tubers can also be significantly influencedby the spectral composition of the light source. In a studyconducted with the cultivar Pentland Hawk, the levels ofglycoalkaloids were four to six times higher in tubersexposed to fluorescent or sodium light sources than intubers exposed to mercury light sources (Percival, Dixon,& Sword, 1994).

The aim of this work was to study the effect of lightexposure and temperature on the total glycoalkaloid(TGA) content of potato tubers of cv Monaliza cultivatedin Brazil.

2. Materials and methods

2.1. Plant materials

Tubers of cv Monaliza, harvested three days before,were acquired at the ‘‘Central de Abastecimento de Campi-nas’’ (CEASA) in April/2001 (50 kg).

Tubers were washed under running water and allowedto dry at room temperature (�25 �C). After this, eachtuber was weighted and classified as small or medium,corresponding to average weights of 68.8 ± 4.1 and117.6 ± 5.9 g, respectively. Batches of 30 tubers each wereselected by size (small or medium). Each batch was placedunder the following conditions: (a) indirect sunlight expo-sure; (b) fluorescent light exposure (lamps of 40 W, at a

distance of 2 m); (c) storage in darkness under refrigeration(7–8 �C); (d) storage in darkness under room temperature,with no ventilation (19–26 �C).

With exception of the tubers exposed to indirect sun-light, which were maintained in this condition for an aver-age period of 10 h (daytime), all other tubers werecontinuously exposed to the specified condition. Tubersexposed to indirect sunlight and fluorescent light wererotated at 24 h intervals to ensure complete exposure tolight. Measurements during the sunlight exposure weremade in days with different conditions of temperatureand luminosity (sunny and cloudy days). Table 1 summa-rises the range of variation of the parameters monitoredduring the experiments.

2.2. Sample preparation

From each batch, samples of four tubers were with-drawn at 0, 3, 7, and 14 day intervals for glycoalkaloidanalysis. The whole tubers were sliced, combined, frozen,and freeze-dried (Freeze-drier LabConco Lyph Lock 18).Samples were minced and stored at �15 �C until analysis.

2.3. Extraction of the glycoalkaloids

The extraction procedure was based on Percival andDixon (1996). The freeze-dried samples (0.5 g) were placedin a centrifuge tube and a volume of 10 ml of extractionsolution (1 l water, 20 ml acetic acid, 5 g sodium bisulphite)was added. The samples were shaken during 15 min andthe solution clarified by centrifugation (Centrifuge Hitachihimac CR 21) at 4000 rpm for 20 min. The analysis of eachsample was made in duplicate.

2.4. Clean-up

This procedure was accomplished according to Hellenasand Branzell (1997). A SepPak Plus C18 cartridge (WatersAssoc., Milford, MA, USA) was conditioned with 5 ml ofacetonitrile, followed by 5 ml of extraction solution. Fivemillilitres of the clarified tuber extract was passed throughthe column followed by washing with 4 ml water/acetoni-trile, 85:15 (v/v). The glycoalkaloids were then eluted fromthe cartridge with 4 ml of acetonitrile/0.022 M potassium

R.M.D. Machado et al. / Food Control 18 (2007) 503–508 505

phosphate buffer, pH 7.6, 60:40 (v/v). The eluate wasfinally adjusted to 5 ml with the same solution. Beforeinjection into the liquid chromatograph, all samples werefiltered through a Millex HV 0.45 lm filter (MilliporeCo.).

2.5. HPLC analysis

The analysis was carried out in a HPLC apparatusequipped with a Waters 600 Controller pump, a Watersin-line degasser, a Waters 717 plus autosampler, and aWaters 996 photodiode array detector. The analytical col-umn was an ODS-Hypersil 5 lm (25 cm · 4.6 mm), and themobile phase consisted of 65% water, 35% acetonitrile and0.05% ethanolamine at pH 4.5–4.6, adjusted with ortho-phosphoric acid (10 vol.%). The injection volume was20 ll, the flow rate 1.0 ml min�1 and the effluent monitoredat 200 nm. HPLC grade acetonitrile was purchased fromMallinckrodt and the water was previously purified in a

Table 2Alfa-solanine and a-chaconine content (mg kg�1 FW) of potato tubers of cv M

Condition Sizea Ex

Indirect sunlight exposure Small a-a-TGC:

Medium a-a-TGC:

Fluorescent light exposure (lamps of 40 W) Small a-a-TGC:

Medium a-a-TGC:

Storage in darkness under refrigeration temperature (7–8 �C) Small a-a-TGC:

Medium a-a-TGC:

Storage in darkness under room temperature (19–26 �C) Small a-a-TGC:

Medium a-a-TGC:

a Small: tubers with average weight of 68.8 ± 4.1 g, medium: tubers with avb Mean value of duplicates for each sample.c Standard deviation of the duplicates for each sample.d Total glycoalkaloid concentration.e Ratio a-chaconine:a-solanine.

MilliQ-Plus purification system. All other chemicals wereof analytical grade.

2.6. Glycoalkaloid quantification

Quantification was performed by comparison of thepeak areas with those from pure a-solanine and a-chaco-nine (Sigma Chemical Co., St Louis, USA) dissolved in0.05 M phosphate buffer (KH2PO4). The external standardplot method was used. The concentration range of the stan-dard curves was 0.7–61 lg ml�1 for a-solanine, and 0.8–64 lg ml�1 for a-chaconine. Recovery of the glycoalkaloidsadded to freeze-dried samples averaged 102%.

3. Results and discussion

Table 2 shows the levels of a-solanine and a-chaconinedetermined in the analysed samples, expressed on a freshweigh (FW) basis.

onaliza exposed to various conditions

posure time (days)

0 3 7 14

Solanine 14.3b ± 0.2c 28.5 ± 0.2 21.8 ± 0.9 29.2 ± 0.6Chaconine 37.1 ± 0.4 68.4 ± 0.7 51.5 ± 1.4 63.3 ± 0.7

Ad 51.4 ± 0.6 96.9 ± 0.5 73.3 ± 2.4 92.5 ± 0.1Se 72:28 71:29 70:30 68:32Solanine 14.3 ± 0.2 11.7 ± 0.8 21.2 ± 0.4 17.6 ± 0.6Chaconine 37.1 ± 0.4 31.0 ± 0.2 43.4 ± 4.3 41.3 ± 1.6

A 51.4 ± 0.6 42.7 ± 1.0 64.6 ± 4.8 58.9 ± 2.3S 72:28 73:27 67:33 70:30

Solanine 14.3 ± 0.2 17.0 ± 0.0 36.8 ± 2.4 42.2 ± 3.9Chaconine 37.1 ± 0.4 42.9 ± 0.7 66.8 ± 0.9 65.7 ± 7.7

A 51.4 ± 0.6 59.9 ± 0.7 103.6 ± 1.4 107.9 ± 11.6S 72:28 72:28 64:36 61:39Solanine 14.3 ± 0.2 14.2 ± 1.4 22.3 ± 0.2 34.6 ± 1.2Chaconine 37.1 ± 0.4 35.6 ± 3.2 45.6 ± 1.8 68.6 ± 7.2

A 51.4 ± 0.6 49.8 ± 4.6 67.9 ± 1.6 103.2 ± 8.4S 72:28 71:29 67:33 67:33



A 51.4 ± 0.6 58.7 ± 3.5 50.8 ± 4.8 65.0 ± 1.2S 72:28 72:28 68:32 70:30



A 51.4 ± 0.6 72.2 ± 3.0 59.6 ± 7.4 57.0 ± 1.4S 72:28 73:27 71:29 72:28

erage weight of 117.6 ± 5.9 g.


Regardless of the tuber size and exposure conditions,TGA concentrations were higher at the end of the experi-ments when compared to the initial levels (Fig. 1). Smallertubers, irrespective of the treatment, presented the highestconcentrations of total glycoalkaloids at 14 days exposure.This behaviour was also observed in studies accomplishedwith cultivars Pentland Dell and Estima where the glycoal-kaloid concentration was highly affected by the genotypeand an inverse relationship between concentration of totalglycoalkaloids and weight of individual tubers occurred(Papathanasiou, Mitchell, & Harvey, 1999).

As shown in Fig. 1, the concentrations of glycoalkaloidduring most of the experiments fluctuated; only the tubers(small and medium sizes) exposed to fluorescent light pre-sented a continuous increase of TGA levels. In addition,fluorescent light exposure was the condition that mostinduced glycoalkaloid formation, independent of the tubersize. This treatment doubled the initial concentration oftotal glycoalkaloids at the end of the experiment (maxi-mum of 107.9 mg kg�1 at 14 days).

The total glycoalkaloid concentration of smaller tubersexposed to indirect sunlight increased from 51.4 to96.9 mg kg�1 (FW) in the first three days, then decreasedto 73.3 mg kg�1 (FW) at 7 days and, finally, increased to92.5 mg kg�1 (FW) at 14 days. A similar behaviour wasobserved by Percival, Dixon, and Sword (1996) for the cul-

30405060708090

100110

0 3.5 7 10.5

Time (days)

Tot

algl

ycoa

lkal

oids

(mg.

kg-1)

a

30405060708090

100110

0 3.5 7 10.5

Time (days)

Tot

algl

ycoa

lkal

oids

(mg.

kg-1

)

b

Fig. 1. TGA concentration in potato tubers of cv Monaliza exposed to the founder refrigeration (7–8 �C), and storage in darkness under room temperaturetubers: average weight of 117.6 ± 5.9 g. The initial ratio chaconine:solanine w

tivars Kerrs Pink, Pentland Hawk and Desiree. Althoughan increase of the TGA level was verified after exposureof the tubers to natural light for 21 days, the authorsreported that the TGA contents fluctuated during theexperiment, with no continuous accumulation of thesecompounds by the potato varieties studied.

The increase of glycoalkaloids levels in potato tubers oflower size submitted to indirect sunlight and fluorescentlight was approximately 4–6 times higher than that ofpotato tubers stored in darkness under room temperature.Similar results were found by Salunkhe, Wu, and Jadhav(1972) who reported that potato exposure to sunlight orartificial light can increase the glycoalkaloid synthesis tolevels 3 or 4 times higher than those found when the pota-toes are maintained in darkness.

The potatoes stored in darkness under refrigeration (7–8 �C) synthesised glycoalkaloids in higher levels than thosefound in tubers stored in darkness under room temperature(19–26 �C). Griffiths et al. (1998) also observed that gly-coalkaloid accumulation can occur with potato tubersstored under low temperatures (4–10 �C), and that theintensity of this effect on the glycoalkaloid biosynthesisdepends of the potato variety.

According to literature, growth residual activity ofpotato may occur in darkness, allowing synthesis and deg-radation reactions such as the glycoalkaloid biosynthesis

14

Indirect sunlight

Fluorescent light

Darkness under refrigeration temperature

Darkness under room temperature

61:39

68:32

69:31

73:27

14

Indirect sunlight

Fluorescent light

Darkness under refrigeration temperature

Darkness under room temperature

67:33

70:30

70:30

72:28

llowing conditions: indirect sunlight, fluorescent light, storage in darkness(19–26 �C). (a) Small tubers: average weight of 68.8 ± 4.1 g; (b) medium

as 72:28 and the final values are represented in the figure.

R.M.D. Machado et al. / Food Control 18 (2007) 503–508 507

(Percival et al., 1996; Spoladore, Zullo, Teixeira, Coelho, &Miranda, 1985). Although some authors (Haddadin,Humeid, Qaroot, & Robinson, 2001; Maine, Bain, & Joyce,1988; Percival, 1999) have used storage in darkness as acontrol treatment when studying the glycoalkaloid forma-tion in potato tubers exposed to light, our results suggestthat this procedure may not be appropriate for all potatovarieties.

Exposure to fluorescent light decreased the ratio of a-chaconine:a-solanine from 72:28 to 61:39 for smaller tubersand from 72:28 to 67:33 for the larger size ones. Theseresults indicate enhancement of a-solanine synthesis to agreater degree than a-chaconine. This observation wasreported earlier by others for the cultivars Pentland Hawk,Desiree, and King Edward (Percival, 1999; Percival et al.,1994). As the toxicological potency of a-chaconine isaround 3–10-fold higher than that of a-solanine, a-chaco-nine may be more prominent in cases of potato poisoning(Friedman, Rayburn, & Bantle, 1991). Furthermore, it isbelieved that the relative proportion of glycoalkaloidsmay influence toxicity more than the absolute total glycoal-kaloid concentrations with consequential implications forthe overall recommendation of 200 mg kg�1 FW for foodsafety (Friedman et al., 2003).

The present investigation indicated that in the case of cvMonaliza, irrespective of the exposure condition there isalways an accumulation of TGA by the potato tubers.Moreover, as during marketing and sale potatoes are usu-ally displayed under fluorescent light, a substantial increaseof glycoalkaloids may occur, compromising of the tuberquality. Although the levels of TGA found in the tubers ofcv Monaliza exposed to the different conditions were below200 mg kg�1, this level has been questioned in relation to itssafety for long term exposures. Therefore, it is recom-mended that cultivars for human consumption possess slowrates of glycoalkaloid accumulation in response to light andbe selected based on a low initial concentration of glycoalka-loid. Further investigation is needed to determine the TGAlevels in other cultivars commercialised in Brazil.

Acknowledgement

The authors would like to acknowledge FAPESP forsupporting this project (proc. 99/08476-5).

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