The Effects of Boiling and Leaching on the Content of Potassium and Other Minerals in Potatoes

JFDS
jfds?782
Dispatch: 4-22-2008
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Journal
MSP No.
No. of pages: 6
PE: Marie
JFS H: Health, Nutrition, and Food
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The Effects of Boiling and Leaching
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on the Content of Potassium
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and Other Minerals in Potatoes
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P.C. BETHKE AND S.H. JANSKY
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ABSTRACT: The white potato (Solanum tuberosum L.) is a valuable source of potassium in the human diet. While
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most consumers benefit from high levels of potassium in potato tubers, individuals with compromised kidney func-
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tion must minimize their potassium intake. This study was undertaken to determine the effects of leaching and
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boiling on levels of potassium and other minerals in potato tubers. Leaching alone did not significantly reduce lev-
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els of potassium or other minerals in tubers. Boiling tuber cubes and shredded tubers decreased potassium levels by
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50% and 75%, respectively. Reductions in mineral amounts following boiling were observed for phosphorus, mag-
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nesium, sulfur, zinc, manganese, and iron. There was no difference between the leaching and boiling treatment and
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the boiling treatment. In addition, mineral levels in tubers of 6 North American potato cultivars are reported. Sig-
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nificant differences in mineral levels were detected among cultivars, but they were too small to be nutritionally im-
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portant. Individuals wishing to maximize the mineral nutrition benefits of consuming potatoes should boil them
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whole or bake, roast, or microwave them. Those who must reduce potassium uptake should boil small pieces before
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consuming them.
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Keywords: boiling, leaching, minerals, potassium, potato
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and feet (Krishnan and others 2006). Patients with kidney disease
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Introduction
are advised to moderate their intake of K through careful selec-
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Mineralsareanimportantpartofahealthydiet.Nutritionpro- tion of foods, and by preparing foods in ways that minimize K
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fessionals recommend that they be consumed as part of a
consumption.
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balanced diet, primarily as fruits and vegetables, rather than in the
The white potato (Solanum tuberosum L.) is an important source
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form of dietary supplements. The recommended daily allowance
of dietary K (Kolasa 1993). In North America and many parts
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in adults for potassium (K) is 4000 to 4700 mg, and this is much
of the world, potatoes are consumed more than any other veg-
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higher than for other minerals such as calcium (Ca), magnesium
etable. Annual consumption of potatoes in the United States is
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(Mg), iron (Fe), and zinc (Zn), where the recommended amounts
approximately 61 kg per person (http://www.ers.usda.gov/Data/
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are 1000 to 1600, 310 to 500, 18 to 21, and 8 to 14 mg/d, respec-
FoodConsumption). In parts of South America, potato is a staple
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tively (Anonymous 2005). Potassium is required in relatively large
crop and consumption can be as high as 800 g per person each
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amounts because it functions as an important electrolyte in the
day (Burgos and others 2007). As a percentage of fresh weight, the
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nervous system. Potassium is used to regulate heartbeat, conduct
K content of potato tubers is relatively high (approximately 0.49%)
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nerve impulses, and contract muscles. Potassium also plays a role
and compares favorably with other sources of K. Consumption of
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in osmoregulation. High levels of potassium can help to control
potatoes may benefit healthy persons by adding K and other min-
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high blood pressure (Adrogue and Madias 2007) and may decrease
erals to their diet, but potato consumption by persons with chronic
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the risk of stroke (Ascherio and others 1998; Ding and Mozaffar-
renal failure may be harmful. The recommended potassium intake
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ian 2006). Despite its importance for good health, many Americans,
in kidney dialysis patients is 2 to 4 g/d (Beto 1995). For these per-
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especially individuals with hypertension, do not get sufficient K in
sons, leaching cut potatoes in water prior to cooking is advised as a
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their diet (Anonymous 2005).
way to reduce K content (Tsaltas 1969; Cost 1975; Gillit and others
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A significant portion of the K consumed each day is excreted
1987). The recommended procedures for leaching vary, however,
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through normal kidney function. This prevents K from accumu-
lating to toxic concentrations, but necessitates constant replenish- and little information on the effectiveness of leaching is available.
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The method used to prepare food influences its nutritional con-
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ment of K through diet. The complications from elevated K include
tent in general and the content of K and other minerals in partic-
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weakness, numbness, and tingling, and in more severe cases, irreg-
ular heartbeat or heart attack (Beto 1995; Beto and Bansal 2004). ular. Boiling is a common method used to prepare potatoes, and
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utrition,
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boiling reduces the K content of peeled whole potatoes by 16%
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Elevated K is of special concern for patients with chronic kidney
compared to baked potatoes (Anonymous 2005). Data describing
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disease. In one study of kidney failure patients, elevated K from
Health,
how well K and other minerals are retained in potatoes cut to var-
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the diet was found to increase the chance of nerve damage in legs
H:
ious sizes before leaching and boiling are lacking. In this study, we
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present a comprehensive examination the effect of leaching and
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MS 20070969
Submitted
12/26/2007,
Accepted
3/12/2008.
Authors
are with USDA-ARS and Dept. of Horticulture, Univ. of Wisconsin-
boiling on the mineral content of potatoes. These data will provide
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Madison, Madison, WI, U.S.A. Direct inquiries to author Jansky (E-mail:
guidance to nutritionists developing diets that contain appropri-
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shelley.jansky@ars.usda.gov).
ate amounts of K and other minerals, and to clinicians who advise
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No claim to original US government works
C 2008 Institute of Food Technologists
Vol. 00, Nr. 0, 2008—JOURNAL OF FOOD SCIENCE
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doi: 10.1111/j.1750-3841.2008.00782.x
Further reproduction without permission is prohibited

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Potassium in potatoes . . .
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patients with compromised renal function on ways to reduce K differences among cultivars as well (Figure 2A and 2B), and these
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consumption.
differences were often statistically significant at P = 0.05.
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Materials and Methods
The effect of leaching and boiling on K content
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Leaching and boiling of potato pieces are 2 treatments that are
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Two trials were carried out with potato tubers produced using
conventional cultural practices at the Univ. of Wisconsin-Lelah the most likely to have an impact on mineral content of the con-
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Starks Potato Research Station near Rhinelander, Wis., U.S.A. They sumed product. We analyzed mineral content of potatoes cut into
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were harvested in September 2006, and stored at 2.2 ?C until the cubes (trials 1 and 2) or shredded into strips (trial 2). We also
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spring of 2007, when they were used for this study.
determined the mineral content of these samples after leaching
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In the 1st trial, Tubers of each of 6 cultivars (“Russet Norko- overnight in cold water (trial 1 and 2), after boiling until tender (trial
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tah,” “Ranger Russet,” “Red Norland”, “Yukon Gold,” “Superior,” 2), and after both leaching and boiling (trial 1 and 2). The data from
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and “Kennebec”) were washed, peeled, and diced into 1-cm cubes. these treatments in trial 2 are presented in Figure 3 to 5. Trial 1 pro-
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The cubes were divided into 30-g samples. Three samples (repli- duced similar results.
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cations) were prepared for each treatment. The leaching treatment
The amount of K remaining in the potato samples was strongly
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consisted of soaking each sample in 300-mL distilled water at 5.6 ?C dependent on the preparation method. Leaching of cubed samples
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for 20 h. Distilled water was used to eliminate the possibility that had little effect on K content, and the average amount of K remain-
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tuber pieces would accumulate minerals from the water. Twenty ing was 96% and 100% of control values in trials 1 and 2 (Figure 3),
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hours, which is the length of time from 10 PM to 6 PM the next day, respectively. Shredded samples retained less of their K after leach-
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was selected as the longest time that individuals are likely to leach ing, but K amounts after overnight leaching were still 83% to 98% of
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potato pieces. It was assumed that leaching would take place in a control values (Figure 3).
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refrigerator at 5.6 ?C, to minimize microbial growth. The water was
Boiling samples immediately after cubing or shredding resulted
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then decanted off and the sample was lyophilized prior to mineral in a much greater loss of K. Boiling cubed potato pieces reduced the
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analysis. The leaching and boiling treatment consisted of leaching, amount of K remaining in the samples to 50% of control values (Fig-
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decanting the water off and replacing it with 300-mL fresh distilled ure 3). Boiling shredded samples resulted in an even larger loss of K.
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water, bringing the water to a boil, boiling for 10 min, decanting Total K content in the shredded and boiled samples was only 25%
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the water off, and then lyophilizing the sample. The control con- to 31% of that in control samples (Figure 3). Boiling after overnight
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sisted of lyophilized raw tuber samples. One freeze-dried sample of leaching did not result in a greater loss of K compared to boiling
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each replication of each treatment was ground into a powder using alone (Figure 3), except for shredded samples from “Red Norland,”
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a mortar and pestle. For each sample, 500 mg of dried tuber tissue where K content was reduced to 31% of controls in boiled samples
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and 5 mL of concentrated nitric acid were added to a 50-mL Folin and 24% in samples leached overnight and then boiled, although
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digestion tube. The mixture was heated to 120 to 130 ?C for 14 to these values were not statistically different at the level of P = 0.05.
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16 h and then treated with hydrogen peroxide. After digestion, the
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sample was diluted to 50 mL. This solution was analyzed for miner- The effect of leaching and boiling
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als by inductively coupled plasma optical emission spectrometry.
on minerals other than K
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The 2nd trial was identical to the 1st trial, except that only cul-
Several other minerals are found in potato tubers, and their rel-
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tivars “Russet Norkotah,” “Red Norland,” and “Superior” were in- ative amounts after leaching and boiling followed the same general
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cluded. An additional tuber treatment was added. Shredded tubers
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(2 × 5 mm × tuber length) were evaluated along with cubes. An ad-
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ditional processing treatment, boiling, was also added. The boiling
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treatment consisted of placing a freshly prepared sample in 300-
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mL distilled water, bringing the water to a boil, and then boiling
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for 10 min. The water was then decanted off and the sample was
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lyophilized.
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Statistical analyses included analysis of variance (ANOVA) using
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the general linear model in SAS and means separation using a pro-
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tected least significant difference test at P = 0.05.
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Results
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Variation in mineral content among potato cultivars
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Figure 1 and 2 present mineral levels in the 6 cultivars evalu-
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ated in the 1st trial. Potassium was the most abundant mineral,
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Food
as expected, and varied from an average of 1.98% of dry weight
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in “Ranger Russet” to 2.31% of dry weight in “Superior” (Figure 1).
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“Red Norland” and “Superior” were significantly higher in K than
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“Russet Norkotah,” “Ranger Russet,” and “Yukon Gold” (P = 0.05).
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Values of K near 2% of dry weight agree with the value of 0.49% K
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in whole (skin and flesh), raw potatoes used for the USDA food la-
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bel, given that typical dry weights are 15% to 20% of fresh weight.
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They also agree with other published studies on potato tuber nutri-
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ent levels (True and others 1978; Klein and Mondy 1981; Randhawa Figure 1 --- Potassium content in tubers of 6 potato culti-
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and others 1984). Other minerals present in the potato samples at vars. Different letters indicate signi?cant differences be-
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moderate (P, Ca, Mg, S) or low amounts (Zn, B, Mn, Fe, Cu) exhibited tween treatments (P = 0.05).
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Potassium in potatoes . . .
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trend as K with respect to their retention (Figure 4 and 5). For each following shredding and boiling, and 25% to 30% following cubing
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of the 3 cultivars common to both trials, P, Mg, S, Zn, Mn, and Fe and boiling (Figure 5). The exception was for “Superior,” where a
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levels were significantly reduced following the leaching plus boil- 65% reduction in Mn content was observed with the shred, leach,
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ing or boiling alone treatments. Calcium, B and Cu levels did al- and boil treatment, but only a slight reduction in Mn content with
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ways follow this trend. Levels of Mg, S, Mn, and Zn were all reduced the shred and boil treatment (Figure 5).
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by an average of 50% or more in the shredding and boiling treat-
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ment. Of these, the largest percentage loss was observed for Mg.
Discussion
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Only 30% of the control Mg content was found in shredded and
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Thedatapresentedheresupport3importantfindingswithre-
boiled samples (Figure 4). Cubed and boiled samples lost 35% of
gard to the effects of processing on mineral content of potato
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their total Mg. Zn and Mn are present in low amounts, but both tubers. First, the content of K and other minerals was drastically
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showed significant, large losses of approximately 50% on average, reduced by either cubing or shredding potatoes and then boiling
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Figure 2 --- (A) Phophorus,
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calcium, magnesium, and
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sulfur levels in tubers of 6
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potato cultivars. (B) zinc,
boron, manganese, iron,
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copper, and sodium levels in
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tubers of 6 potato cultivars.
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Different letters indicate
signi?cant differences
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between treatments for the
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same mineral (P = 0.05).
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Potassium in potatoes . . .
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them (Figure 3 to 5). Reductions in K were as large as 75% of con- 2007). For individuals wishing to maximize mineral consumption
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trols for samples that were shredded prior to boiling and 45% of through diet, these data suggest that boiled potatoes should not be
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controls for samples that were cubed prior to boiling. These val- cut into small pieces as is commonly done to reduce cooking time.
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ues are much greater than published values for the K content loss
Second, leaching samples overnight had a much smaller effect
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that results from boiling whole, peeled potatoes (True and oth- on the mineral content of potatoes than boiling. Because leach-
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ers 1979). This is also in contrast to the negligible loss of Fe and ing alone was an ineffective method for reducing the K content of
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Zn when whole, unpeeled potatoes are boiled (Burgos and others potatoes, there would be little gain for renal failure patients trying
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Figure 3 --- Potassium content
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remaining in tubers of 3 potato
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cultivars following cubing or
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shredding prior to leaching and/or
boiling treatments. Different
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letters indicate signi?cant
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differences between treatments
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(P = 0.05).
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Figure 4 --- Effects of leaching and
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boiling treatments of shredded and
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cubed potatoes on phosphorus,
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calcium, magnesium, and sulfur
levels in 3 potato cultivars. Different
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letters indicate signi?cant
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differences between treatments for
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the same mineral (P = 0.05). Clear
bars = shredded samples, grey bars
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= cubed samples, C = control, L =
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leached, LB = leached and boiled,
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B = boiled.
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Potassium in potatoes . . .
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to reduce K consumption to prepare hash browns, for example, and inconvenient leaching step is unnecessary and can be
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from shredded and leached potatoes than from freshly shredded omitted.
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potatoes.
This study also analyzed the mineral content of tubers from 6
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Finally, the data presented here show that mineral loss following major potato cultivars. We chose major cultivars that represent
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boiling alone was comparable to that following leaching overnight the common market classes of potato. “Ranger Russet” and “Rus-
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and then boiling. These data suggest that the greatest benefits for set Norkotah” are russet cultivars, “Superior” and “Kennebec” are
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individuals attempting to minimize the K consumed in boiled pota- white potatoes, “Red Norland” is a red-skinned cultivar, and “Yukon
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toes come from cutting potatoes into small pieces or shredding Gold” is a popular yellow-fleshed potato. All are typically eaten as
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them, with very little additional benefit from leaching them after fresh market potatoes, although “Ranger Russet” and “Superior” are
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cutting. These 3 findings provide additional guidance to health pro- processed as well. Significant differences in mineral content among
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fessionals and consumers who are making diet choices based, in cultivars were detected in this study and a previous one (True and
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part, on the mineral content of the food.
others 1979). However, these differences were typically small and
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The original “dialysis of the vegetables in vitro” method for re- would not justify the choice of one cultivar over another for its nu-
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ducing the K content of potatoes and other vegetables was put for- tritional quality with respect to mineral content. For example, the
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ward by Tsaltas in 1969 and has been recommended for patients amount of K in the cultivar with the lowest level (“Ranger Russet”)
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with kidney disease ever since. Reductions in K content were found was 85% of that of the highest cultivar (“Superior”). Assuming that
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to be much greater when diced or sliced potatoes were leached dry weight is 20% of fresh weight, the K content in a 250-g “Ranger
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prior to boiling. In particular, K reductions for thinly sliced potatoes Russet” tuber would be 1.1 g, while that in a comparable “Supe-
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after boiling for 5 to 10 min were approximately 50%, and reduc- rior” tuber would be 1.0 g. Our study showed that cooking has a
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tions with leaching and boiling were approximately 80% (Tsaltas much more dramatic effect on mineral levels, with shredded tubers
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1969). No details were provided as to potato variety, and the data from boiling treatments containing only 25% of the K found in raw
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do not include measures of variability from the mean. The data tubers.
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presented here support the findings of Tsaltas (1969) showing that
The genetic variability among North American cultivars is low
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boiling thinly sliced potatoes resulted in greater K loss than boil- (Mendoza and Haynes 1974), so it is not surprising that the culti-
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ing diced potatoes Tsaltas (1969). They differ from previous find- vars evaluated in this trial are similar in mineral content. In con-
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ings (Tsaltas 1969) in that they do not show a significant, additional trast, mineral content varied more widely in Andean potato culti-
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benefit from leaching (Figure 3). For each of the 3 varieties tested, vars, which are more genetically diverse (Burgos and others 2007).
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as well as for diced and shredded potatoes, boiling alone was as For example, the Andean cultivar with the lowest Fe levels was 44%
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effective as leaching and boiling as determined by us (Figure 3) of that of the highest cultivar. In our trial, the lowest cultivar was
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or others (Tsaltas 1969). We conclude that the time-consuming 64% of the highest cultivar. Potato breeders are fortunate to have
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Figure 5 --- Effects of leaching and
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boiling treatments of shredded
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and cubed potatoes on zinc,
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boron, manganese, iron, and
copper levels in 3 potato
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cultivars. Different letters
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indicate signi?cant differences
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between treatments for the same
mineral (P = 0.05). Clear bars =
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shredded samples, gray bars =
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cubed samples, C = control, L =
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leached, LB = leached and boiled,
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B = boiled.
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Potassium in potatoes . . .
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access to about 200 wild relatives of potato. Wild species provide
References
4
genes for resistance to biotic and abiotic stresses, as well as tuber Adrogue MD, Madias NE. 2007. Sodium and potassium in the pathogenesis of hyper-
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quality (Hanneman 1989; Spooner and Bamberg 1994; Jansky 2000).
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Anonymous. 2005. Dietary guidelines for Americans. Dept. of HHS and USDA ed 6.
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In one study, tuber calcium levels in wild Solanum species were re-
U.S. Government Printing Office.
Q1
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ported to range from 0.016% to 0.074% dry weight, compared to Ascherio A, Rimm EB, Hernan MA, Giovannucci EL, Kawachi I, Stampfer MJ, Willett
WC. 1998. Intake of potassium, magnesium, calcium, and fiber and risk of stroke
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a range of 0.010% to 0.030% in the cultivars evaluated in our trial
among U.S. men. Circulation 98:1198–204.
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(Bamberg and others 1993). Based on previous successes with the Bamberg JB, Palta JP, Peterson LA, Martin M, Krueger AR. 1993. Screening tuber-
bearing Solanum (potato) germplasm for efficient accumulation of tuber calcium.
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introduction of valuable genetic diversity from wild potato species
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into cultivated germ plasm, breeding efforts could likely increase Beto JA. 1995. Which diet for which renal failure: making sense of the options. J Am
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tuber mineral content to levels higher than are currently found Beto JA, Bansal VK. 2004. Medical nutrition therapy in chronic kidney failure: inte-
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in potato cultivars. Consequently, in the future, it may be possi-
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Burgos G, Amoros W, Morote M, Stangoulis J, Bonierbale M. 2007. Iron and zinc con-
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Sci Food Agric 87:668–75.
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Cost JS. 1975. Dietary management of renal disease. Thorofare, N.J.: Charles B. Slack,
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als to the diet through the consumption of potatoes, then this can Hanneman RE Jr. 1989. The potato germplasm resource. Am Potato J 66:655–67
Jansky S. 2000. Breeding for disease resistance in potato. Plant Breed Rev 19:69–155.
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be done by boiling whole potatoes or potatoes that have been cut Klein LB, Mondy N. 1981. Comparison of microwave and conventional baking of pota-
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into large pieces. Alternatively, potato tubers can be microwaved,
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23
baked, or roasted without a loss of mineral nutrients (True and oth- Kolasa KM. 1993. The potato and human nutrition. Am Potato J 70:375–85.
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ers 1979). As such, potatoes are a valuable source of potassium and Krishnan AV, Phoon RKS, Pussell BA, Charlesworth JA, Kiernan MC. 2006. Sensory
nerve excitability and neuropathy in end stage kidney disease. J Neurol Neurosurg
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other mineral nutrients. If it is necessary to decrease mineral up-
Psychiatry 77:548–51.
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take while taking advantage of other nutritional qualities offered Mendoza AO, Haynes FL. 1974. Genetic relationship among potato cultivars grown in
the United States. HortScience 9:328–30.
27
by potato tubers, as is the case with individuals who have com- Randhawa KS, Sandhu KS, Kaur G, Singh D. 1984. Studies of the evaluation of different
28
promised kidney function, then boiling thinly sliced potatoes will
genotypes of potato (Solanum tuberosum L.) for yield and mineral contents. Qual
Plant Plant Foods Hum Nutr 34:239–42.
29
result in a large reduction in mineral levels. Furthermore, the data Spooner DM, Bamberg JB. 1994. Potato genetic resources: sources of resistance and
30
presented here show that it is not necessary to complicate the pro-
systematics. Am Potato J 71:325–37.
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True RH, Hogan JM, Augustin J, Johnson SR, Teitzel C, Toma RB, Orr P. 1979. Changes
cess by leaching tuber slices before boiling them.
in the nutrient composition of potatoes during home preparation. III. Minerals. Am
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33
True RH, Hogan JM, Augustin J, Johnson SR, Teitzel C, Toma RB, Shaw RL. 1978. Min-
Acknowledgments
eral composition of freshly harvested potatoes. Am Potato J 55:511–9.
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Tsaltas TT. 1969. Dietetic management of uremic patients. Am J Clinical Nutr 22:
35
The authors wish to acknowledge Krista Powell, R.D., L.D., who
490–3.
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stimulated the initiation of this study.
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