Chemical Markers for Processed and Stored Foods - American

Chapter 17

Glycoalkaloids in Fresh and Processed Potatoes

Downloaded by UNIV OF GUELPH LIBRARY on September 7, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0631.ch017

Mendel Friedman and Gary M. McDonald Agricultural Research Service, U.S. Department of Agriculture, Western Regional Research Center, 800 Buchanan Street, Albany, CA 94710

As part of a program designed to improve food safety by controlling the biosynthesis of glycoalkaloids in potatoes, we define conditions of sampling, handling, storing, shipping, and processing that influence the biosynthesis of potentially toxic glycoalkaloids in potatoes after harvest. This brief overview also suggests research needs to develop a protocol that can be adopted by potato producers and processors to minimize post-harvest synthesis of glycoalkaloids in potatoes. Reducing glycoalkaloid concentration in potatoes will provide a variety of benefits extending from the farm to processing, shipping, marketing, and consumption of potatoes and potato products. Minimizing pre-andpostharvest glycoalkaloid production in potatoes, including new cultivars, requires an integrated multi-disciplinary approach.

Steroidal glycoalkaloids have been found in potatoes (7), green tomatoes (2), and eggplants (3). Symptoms of glycoalkaloid toxicity experienced by animals and humans include colic pain in the abdomen and stomach, gastroenteritis, diarrhea, vomiting, burning sensation about the lips and mouth, hot skin, fever, rapid pulse, and headache (4). The reported toxicity of these glycoalkaloids may be due to such adverse effects as: (1) anticholinesterase effects on the central nervous system (5); (2) induction of hepatic ornithine decarboxylase, a cell proliferation marker enzyme (6), and (3) disruption of cell membranes affecting the digestive system (7-9). Toxicity does not seem to occur at the genetic level (10). One manifestation of these adverse effects may be alkaloid-induced teratogenicity (77,72). The estimated highest safe level of total glycoalkaloids for human consumption is about 1 mg/kg body weight, a level that may cause gastrointestinal irritation (73). The acute toxic dose is estimated to be about 1.75 mg/kg body weight (4). A lethal dose may be as low as 3-6 mg/kg body weight (14).

This chapter not subject to U.S. copyright Published 1996 American Chemical Society In Chemical Markers for Processed and Stored Foods; Lee, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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CHEMICAL MARKERS FOR PROCESSED AND STORED FOODS

With respect to potential toxicity of glycoalkaloids to humans, Hopkins (75) points out that although glycoalkaloids are present in foods such as potatoes consumed daily by animals and humans, their toxicological status is poorly defined. Moreover, Rayburn et al. (16-18) point out that depending on the variety, potatoes may contain the glycoalkaloids α-chaconine and α-solanine at concentration ratios of 74:26 to 40:60 (a-chaconine: α-solanine). Therefore, additional studies are needed to address the interactions of glycoalkaloids when consumed as mixtures of varying proportions. Human consumption of potatoes, an excellent source of carbohydrates and goodquality protein (79), varies by country. For example, the average daily per capita intake in the United States is about 167 g (20); in the United Kingdom, 140 g (75); and in Sweden, 300 g (75). The cited amount for the United Kingdom is estimated to contain 14 mg of glycoalkaloids. Although the glycoalkaloid content of most commercial potato varieties is usually below a suggested guideline of 200 mg/kg fresh potatoes (27), the content can increase significantly on exposure to light and as a result of mechanical injury, including peeling and slicing (22-23). After processing, further accumulation of glycoalkaloids is halted as the necessary enzymes for biosynthesis have been deactivated. However, since glycoalkaloids are largely unaffected by home processing conditions such as baking, boiling, frying and microwaving (1,2,24-29), any glycoalkaloids present in the tubers before cooking will still remain afterwards. Since the two major potato glycoalkaloids α-chaconine and α-solanine differ in biological potency, may act synergistically, and their ratio may vary in different cultivars (18,30,31), care should be exercised in relating dose-response data for individual glycoalkaloids to the total amount in potatoes. New varieties may also contain glycoalkaloids of unknown structure and function inherited from their progenitors. These considerations suggest a need to reduce steroidal glycoalkaloid levels in the diet, possibly through suppression of enzymes responsible for their biosynthesis in plants (32-34). For the purposes of this study, we define the following terms: glycoalkaloids naturally occurring, nitrogen-containing plant steroids with a carbohydrate side chain attached to the 3-hydroxy position, e.g. α-chaconine and α-solaninefrompotatoes, atomatine from tomatoes, and solasonine from eggplants; aglycones - the steroidal parts of the glycoalkaloid lacking the carbohydrate side chain, e.g. solanidinefromachaconine and α-solanine, tomatidine from α-tomatine, and solasodine from solasonine; alkaloids - glycoalkaloids and aglycones. Table I summarizes glycoalkaloid content of some potato varieties grown in the United States, Table Π lists the content of different varieties grown in Sweden, and Table III shows glycoalkaloid content of widely consumed potato products. Figure 1 illustrates structures of common and uncommon Solanum glycoalkaloids, Figure 2 depicts structures of hydrolysis products of α-chaconine and α-solanine, Figures 3 and 4 show HPLC chromatograms of α-chaconine and asolanine and hydrolysis products, and Figure 5 correlates HPLC and immunoassay measurements of potato glycoalkaloids.

In Chemical Markers for Processed and Stored Foods; Lee, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

17. FRIEDMAN & McDONALD

Fresh & Processed Potato Glycoalkaloids 191

Table I. α-Chaconine and cc-Solanine Content of Various Potato Parts and Cultivars (7) α-chaconine (mg/100gfreshwt.)

a-solanine (mg/100gfreshwt.)

ratio a-chaconine/ a-solanine

sprouts (no. 3194)

150.4

123.4

1.22

berries

22.1

15.9

1.39

tubers (Lenape)

13.5

5.9

2.28

peel (Lenape)

62.3

23.0

2.70

flesh (Lenape)

8.02

3.95

2.03

tubers (no. 3194)

3.68

1.95

1.89

tubers (Simplot I)

3.85

1.72

2.24

tubers (Simplot Π)

2.75

1.07

2.57

tubers (commercial red)

2.72

1.09

2.31

tubers (commercial white)

1.17

0.58

2.02

tubers (Idaho Russet)

1.34

0.65

2.06

tubers (Washington Russet)

1.30

0.58

2.24


material

Table II. Glycoalkaloid Content of Commercial Swedish Early Potato Varieties (Adapted from ref. 35) variety

total glycoalkaloids (mg/100gfreshwt)

ratio a-chaconine/a-solanine 1.46

16.2 ± 2.6

min=max 15.4 - 34.4 11.6-21.5

Early Puritan

14.3 ± 2.4

11.0-19.9

1.63

Maria

9.8 ± 2.3

5.7 -14.3

1.41

Evergood Eldorado

9.5 ± 2.9

4.9 -13.9

1.86

Provita

8.5 ± 2.2

5.8-13.9

1.77

Ulster Chieftan

mean + SD 22.1 ± 5.0

Silla

1.86


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CHEMICAL MARKERS FOR PROCESSED AND STORED FOODS

Table III. α-Chaconine and α-Solanine of Processed Potato Products (mg/100g of fresh weight) (2)


sample

a-chaconine

a-solanine

ratio a-chaconine/ a-solanine

french fries, freeze-dried

0.42

0.42

1.00

wedges, freeze-dried

2.39

2.01

1.18

skins, sample A

3.89

1.74

2.23

skins, sample Β

4.40

2.36

1.86

skins, sample C

11.61

7.23

1.60

skins, sample D

11.95

8.35

1.43

pancake powder, brand A

2.05

2.41

0.82

pancake powder, brand Β

2.48

1.94

1.27

mushed flakes

3.17

3.29

0.96

chips, brand A

1.30

1.05

1.23

chips, brand Β

3.16

1.76

1.79

chips, brand C

5.88

5.02

1.17

The main objective of this paper is to briefly describe the fate of potato glycoalkaloids α-chaconine and α-solanine in potato tubers and processed potatoes after harvest. Handling and Sampling of Potatoes to Minimize Adverse Effects The biosynthesis of glycoalkaloids in potatoes continues long after harvest. Factors which influence glycoalkaloid formation include light, storage conditions, and mechanical injury. Possible relationships of other post-harvest events, such as blackening, blighting, and browning on glycoalkaloid formation are not well defined. In order to standardize handling and sampling of potatoes to minimize glycoalkaloid formation, it would be helpful to know the biochemical basis for the post-harvest changes which affect quality and safety. The following sections offer a brief review of some of these changes and the research approaches for lessening or eliminating such adverse effects.



17. FRIEDMAN & MCDONALD

Fresh & Processed Potato Glycoalkaloids 193

Glycoalkaloids, Flavor, and Taste. Experiments with human taste panels revealed that potato varieties with glycoalkaloid levels exceeding 14 mg/100 g fresh weight tasted bitter (38,39). Those in excess of 22 mg/100 g also induced mild to severe burning sensations in the mouths and throats of panel members. In a related study, Kaaber (40) demonstrated that the Norwegian potato variety Kerrs Pink was quite susceptible to greening-related glycoalkaloid synthesis and accompanying increases in bitterness and burning sensations, whereas the Bintje variety was not. Zitnak and Filadelfi-Keszi (41) describe the isolation of the diglycoside f^chaconine, a so-called potato bitterness factor. ^-Chaconine and other glycoalkaloid hydrolysis products (Figure 2) are readily formed on exposure of the glycoalkaloids in pure form or in potatoes to acid conditions (42,43). Effect of Light on Glycoalkaloid Synthesis. Exposure of post-harvest potato tubers to light, whether incandescent, fluorescent, or natural, can dramatically enhance glycoalkaloid synthesis. For example, exposure of the cultivar Sebago to a 15 watt incandescent lamp for 10 days resulted in an increase in glycoalkaloid content from 4.8 to 19 mg/100 g fresh weight (44). Exposure of peeled Russet Burbank potato slices tofluorescentlight of 200 foot-candles for 48 hr caused an increase from 0.2 to 7.4 mg/100 gfreshweight. Exposure of commercial White Rose potatoes tofluorescentlight for 20 days induced a time-dependent greening of potato surfaces. In addition to increases in chlorophyll content, chlorogenic acid and glycoalkaloid levels also increased, but no changes were observed in the content of inhibitors of the digestive enzymes trypsin, chymotrypsin, and carboxypeptidase A (23). Generally, the increases in chlorophyll and glycoalkaloid synthesis seem to depend on the wavelengths of the light to which the tubers are exposed (45). Since potato cultivars differ significantly in their ability to produce greeningrelated glycoalkaloids (46), it should be possible to find and use varieties with low rates of post-harvest glycoalkaloid synthesis. Mechanical Damage, Pest Resistance, and Glycoalkaloids. Bruising, cutting, and slicing of potato tubers induces the formation of glycoalkaloids (47,48). The response increases with the extent of injury and is cultivar-related (49). For example, Fitzpatrick et al. (50) showed that the glycoalkaloid content of potato slices increased from 5.5 to 99.4 mg/100 g fresh weight after storage for 4 days. Injury such as slicing before storage induces a burst in glycoalkaloid synthesis. It is therefore evident that storing whole tubers and cutting them just before cooking is much preferred over cutting and then storing them to await cooking. Fungi such as Fusarium solani and Phoma foveata damage potatoes by causing storage rot (51). Olsson (49,52,53) attempted to find out whether differences in initial levels of glycoalkaloids of various cultivars and their differing ability to respond to mechanical damage by increasing glycoalkaloid synthesis influenced their resistance to Fusarium and Phoma fungi. The extent of damage correlated with original glycoalkaloid content - a genotype with a high initial level of glycoalkaloids resulted in a greater increase than one with a low initial level. Cultivars most susceptible to




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Figure 1. Structures of the common Solanum glycoalkaloids.


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Chemical Markers for Processed and Stored Foods - American

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