Chapter 5
Potato Polyphenols: Role in the Plant and in the Diet Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 3, 2015 | http://pubs.acs.org Publication Date: April 1, 1997 | doi: 10.1021/bk-1997-0662.ch005
Mendel Friedman Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 800 Buchanan Street, Albany, CA 94710
Potatoes and other plant foods accumulate a variety of secondary plant metabolites including phenolic compounds, phytoalexins, protease inhibitors, and glycoalkaloids, as a protection against adverse effects of bruising and injury by phytopathogens including bacteria, beetles, fungi, insects, and slugs. Since these phytochemicals are consumed by animals and humans as part of their normal diet, a need exists to develop a better understanding of the role of these compounds in both the plant and in the diet. To contribute to this effort, this integrated overview describes the biosynthesis and the role of phenolic compounds such as chlorogenic acid and tyrosine in host-plant resistance and their beneficial effects as antioxidants, antimutagens, anticarcinogens, and as antiglycemic agents. Also covered are analytical and compositional aspects of phenolic compounds in potatoes; ferrous ion- and heat -induced discolorations such as after-cooking blackening which seems to affect organoleptic but apparently not nutritional properties of potatoes; polyphenol oxidase-catalyzed browning reactions and their prevention; effects of baking, cooking, microwaving, light, and γ-radiation on the stability of chlorogenic acid; and recommendations for future research. The possibility that the net antioxidative potency of structurally different potato polyphenolic compounds is related to their net electrochemical oxidation-reduction (redox) potential merits validation. Understanding the multiple, overlapping roles of polyphenols in plant physiology and in food science and nutrition should stimulate interest in maximizing beneficial nutritional and health effects of polyphenols in the diet.
This chapter not subject to U.S. copyright. Published 1997 American Chemical Society
In Antinutrients and Phytochemicals in Food; Shahidi, F.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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ANTINUTRIENTS AND PHYTOCHEMICALS IN FOOD
Figure 1. Effect of stirring on browning of a 5% slurry of Russet potato flesh measured at 420 nm (79).
In Antinutrients and Phytochemicals in Food; Shahidi, F.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
5. FRIEDMAN
Potato Polyphenols
Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 3, 2015 | http://pubs.acs.org Publication Date: April 1, 1997 | doi: 10.1021/bk-1997-0662.ch005
Introduction Polyphenolic compounds are secondary plant metabolites found in numerous plant species including potatoes (1). The oxidation products of phenolic compounds appear to be involved in defense of the plants against invading pathogens including bacteria, fungi, and viruses. Polymeric polyphenolic compunds seem to be more toxic to potential phytopathogens than the phenolic monomers such as chlorogenic acid from which they are derived. The polyphenol oxidase-catalyzed polymerization helps seal the injured plant surface and begins the healing process, analogous to the formation of fibrin blood clots in injured humans. Enzyme-catalyzed browning reactions (2-5; Figure 1) of polyphenols continue after the food is harvested resulting in deterioration in flavor, color, and nutritional quality. For this reason, prevention of enzymatic browning in fruits and vegetables has been a major concern of food scientists. Polyphenolic compounds have also been shown to possess antimutagenic, anticarcinogenic, antiglycemic, and antioxidative beneficial properties. These properties can be utilized in the prevention of rancidity and as health-promoting food ingredients. In this overview, I attempt to integrate and correlate the widely scattered literature on the role of polyphenols in potatoes before and after harvest (1-140). Specifically covered are the following relevant aspects; analysis, biosynthesis, host-plant resistance, food browning and its prevention, and beneficial and adverse effects on food quality and safety. Suggestions for future research are also mentioned in order to catalyze progress in minimizing adverse effects and enhancing desirable ones of potato polyphenols. Since chlorogenic acid constitutes up to 90% of the total phenolic content of potato tubers, most of the discussion centers around this compound. Figure 2 shows the structures of potato phenolics including chlorogenic acid and its isomers. Analysis and Composition Analytical methods for potato-polyphenols include gas-liquid chromatography (GLC), high-performance liquid chromatography (HPLC), thin-layer chromatography (TLC), and U V spectrophotometry, but surprisingly, apparently not immunoassays. Before analysis, the compounds have to be extracted and purified. Voigt and Noske (6) describe optimized extraction of chlorogenic acid from potatoes with the aid of acetone, ethanol, and methanol. The order of effectiveness was methanol > ethanol > acetone. They found that the chlorogenic acid content of stewed potatoes was the same as that of raw potatoes. Reeve et al. (7) discovered in our laboratory that phenolic compounds are distributed mostly between the cortex and skin (peel) tissues of the potato (Figure 3). About 50% of the phenolic compounds were located in the potato
In Antinutrients and Phytochemicals in Food; Shahidi, F.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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ANTINUTRIENTS AND P H Y T O C H E M I C A L S IN F O O D
cinnamic acid:
R, = R
p-coumaric acid: caffeic a c i d :
R1 = R
ferulic a c i d :
R1 = O H ,
chlorogenic acid:
2
= R
3
= H
sinapic
R i = O H , R2 = R3 =H 2
= O H , R3 = H R
= OCH , R
2
R1 = R
3
2
3
= H
= O H , R3 = q u i n i c a c i d
chlorogenic acid (5-O-caffeylquinic acid)
cryptochlorogenic acid = 4-O-caffeoylquinic acid neochlorogenic acid = 3-O-caffeoylquinic acid isochlorogenic acid "a" = 4,5-di-O-caffeoylquinic acid isochlorogenic acid "b" = 3,5-di-O-caffeoylquinic acid isochlorogenic acid "c" = 3,4-di-O-caffeoylquinic acid
Figure 2. Structures of potato polyphenols and of chlorogenic acid isomers.
In Antinutrients and Phytochemicals in Food; Shahidi, F.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 3, 2015 | http://pubs.acs.org Publication Date: April 1, 1997 | doi: 10.1021/bk-1997-0662.ch005
5. FRIEDMAN
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Potato Polyphenols
periderm v
cortex i_ -—-*
heel or stem end tr-—
