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toes have been found to contribute important amounts of essential minerals to



^Present address: School of Home Economics, University of Georgia, Athens, Georgia.



NUTRITIVE VALUE OF POTATOES 3

the diet. In Table 1 it is shown that potatoes contain approximately 1 percent
of ash or minerals. This important 1 percent may be further broken down and
analyzed as presented in Table 2.

Table 1. — Aver.a.ge Chemical Composition of Edible .Portion of the

Potato* (7)

Water 77 . 80 percent

Protein 2 . 00 percent ■

Fat 0.10 percent

Ash (minerals) . 99 percent ' '

Carbohydrate (starch, sugars, etc.) 19. 10 percent

Fuel or energy value 385 calories per pound or

85 calories per 100 grams
Approximate potential reserve alkalinity 9.0cc. normal alkali per 100 grams

♦Additional data on the composition of potatoes and other vegetables may be found in a recent
bulletin by Holland and Ritchie (20).

Table 2. — The Mineral Content of the Edible Portion of the Potato

Mineral Percent Authority

Calcium 0.013 41

Magnesium 0.027 41

Potassium 0.496 41

Sodium 0.024 41

Phosphorus 0.053 41

Chlorine 0.035 41

Sulfur 0.029 41

Iron 0.0011 41

Copper 0.000164 41

Manganese 0.000173 41

Nickel trace 4

Cobalt trace 4

Barium trace 34

Zinc 0.0004 3

Iodine 85-226 p.p.b. of 14, 18, 33

dry matter

Aluminum 0.00097 50



FACTORS INFUENCING THE COMPOSITION OF POTATOES

Fertilization, Soil, and Variety

The composition of plant material is subject to variation according to variety,
soil, and fertilization. Potatoes are no different from other plant materials in
this respect. Considerable work has been published dealing with the composition
of potatoes as influenced by variety, soil, and growing conditions. Metzger and
co-workers (35) have summarized a 'comprehensive study on the composition of
potatoes as follows:



4 MASS. EXPERIMENT STATION BULLETIN 390

Potatoes from one locality may differ significantly in starch, dry matter,
protein and ash from those of another locality. Varieties may differ
significantly in starch, dry matter, protein and ash. A significant differ-
ence in dry matter, protein and ash between years has also been obtained.
Irrigated potatoes are higher in starch and dry matter, but lower in pro-
tein than dry-land potatoes.

It has been reported by Hayne (17), McClendon, Barrett, and Canniff (33),
and Freas (14) that there is a marked variation in the iodine content of potatoes,
which appears to vary with the iodine present in the soil in which they are grown.
The iodine content of potatoes and other vegetables in relation to its presence
in the soil is of particular interest in considering the incidence of goiter.

Storage

The temperature at which potatoes are stored has a marked effect on their
relative starch and sugar content. Cool storage temperatures favor the con-
version of starch to sugar in the potato tuber, and warm storage temperatures
favor the reverse reaction. In a recent study Wright et al. (57) found that the
sugar and starch content of potatoes stored at 50°-60° F. remained practically
unchanged, whereas the sugar in potatoes stored at lower temperatures progres-
sively increased and the starch decreased. Denny and Thornton (10) reported
large differences in the amount of reducing sugar present in different varieties of
potatoes both before and after storage at different temperatures. Low storage
temperatures caused an increase in the reducing sugar.

In a discussion of the storage of potatoes Barmore (2) believes it probable
that there are only two reactions which are responsible for their sugar content:
respiration, or the oxidation of sugar to carbon dioxide and water; and second,
the transformation of starch to sugar and vice versa. When the temperature of
storage is reduced, respiration is retarded and this effects a shift in the starch-
sugar equilibrium so that the sugar concentration is increased. At higher storage
temperatures these reactions are reversed and the sugar is slowly decreased with
an increase in starch.

The question of sugar content is particularly important from the standpoint
of making potato chips from stored potatoes. The presence of reducing sugar is
responsible for objectionable brown colors in pota'-o chips (10), and when potatoes
are to be used for this purpose, storage conditions should be controlled so as to
keep the sugar content low.

THE CARBOHYDRATE, PROTEIN, AND MINERALS OF THE POTATO

Carbohydrate

As mentioned in the introduction, potatoes are generally thought of as an
important source of energy. The caloric or energy value of potatoes is due almost
entirely to their carbohydrate content. McCance and Lawrence (32) have
reported a comprehensive investigation to distinguish between the available and
unavailable carbohydrate of cooked foods. Their information is important in
that it actually evaluates the available carbohydrate content of foodstuff's as
they are served. The data for potatoes are summarized in Table 3. "Total
Reducing Sugars" includes starch, sucrose, and other carbohydrates which may
be hydrolyzed with acid to reducing sugars, as well as reducing sugars them-
selves. Practically all of the carbohydrate of the potato is available as a source
of energy.



NUTRITIVE VALUE OF POTATOES 5

Table 3. — The Carbohydrate Content of Cooked Potatoes as Served (32)

New Old

Carbohydrate Fraction Potatoes Potatoes

Percent Percent

Total reducing sugars 16.1 19.9

Pentose sugars 0.2 0.37

Non-fermentable sugars — 1.2

Available Carbohydrate 15.6 19.2



Pectin

The pectin present in the potato differs in its physical and chemical properties
from that found in most fruits. The methoxyl content is low and the viscosity
of aqueous solutions is less than that of fruit pectins. Potato pectin is useless
in jelly making.

Some investigators have suggested that mealiness in potatoes is caused by
the separation of cells brought about by the solution of pectic materials. In
studies conducted at this station, Freeman and Ritchie (15) determined the pectin
content of four varieties of potatoes and observed its change during storage and
as a result of baking. They found that raw potatoes contained from 20.0 to 26.8
milligrams of calcium pectate per gram of dry sample. No loss in pectin occurred
during six months' storage at 1.7° to 4.4° C. (35° to 40° F.). In most cases there
was a small loss of pectin as a result of baking. These authors suggest that potato
pectin can be adequately defined by two fractions:

1. A fraction that is soluble in ammonium citrate (or oxalate) and
insoluble in hot water. This fraction may be pectic acid or insoluble
salts of the acid.

2. A fraction that is easily dispersed by hot water but not by cold
water. Since this fraction is so easily removed, it is possible that it is
not the postulated cellulose-pectin complex of protopectin but rather a
lyophilic gel that is readily peptized by warm water.

They conclude:

Analyses of water-soluble pectin fractions of raw and cooked potatoes
offer additional evidence that the solution or degradation of pectic material
does not determine mealiness in potatoes.

Protein

The protein of the potato is chiefly a plant globulin known as tuberin (24).
The potato is not an important source of protein because it contains only a small
amount and even that is of relatively low biological value. Potato protein is
78 percent digestible and has a biological value of 67 and a protein value as a
food of 0.8 (36).

The White House Conference of 1932 (54) suggested that sources of protein
be grouped according to the percentage of the total calories that presumably
should be provided by them if these proteins are to be the only ones in the diet.
On this basis potatoes rate as a fair source, providing 12-20 percent of the total
calories. However, the proteins are adequate for maintenance but not for growth.

I



6 MASS. EXPERIMENT STATION BULLETIN 390

Minerals

In addition to being a good source of energy, potatoes are an important source
of certain essential minerals. The ash of potatoes is particularly rich in potassium
and contains significant amounts of many of the other minerals (Table 2). In
evaluating potatoes on the basis of minerals furnished, one must consider the
relatively large amounts of this vegetable usually included in the diet as well as
the actual composition of the ash per se.

Potatoes are considered one of the cheapest sources of iron and an economical
source of calcium (46). They also contain significant amounts of iodine in many
instances, depending upon where they are grown (17).

In order for a foodstuff to be of value as a source of minerals they must be
present not only in significant amounts, but also in a form that can be utilized
by the body. This is particularly necessary in the case of iron. "Many of the
vegetables known to contain appreciable amounts of iron cannot be considered
good sources of this element because it is not available. In potatoes the iron is
almost 100 percent available (43).

THE VITAMIN CONTENT OF POTATOES

During the past ten years many studies have been made of the vitamin content
of potatoes, particularly vitamin C. Potatoes have been found to be a good source
of vitamins Bi and C. They also contain measurable amounts of the other vi-
tamins. The vitamin content of potatoes is subject to variation according to
variety, length of storage, and growing conditions. In a general consideration of
potatoes as a source of vitamins in the diet, it is necessary to deal with average
values which have been determined by many workers. These values represent
the amount of vitamins that the consumer can expect to be present in potatoes.
In Table 4 potatoes are evaluated as to their vitamin content.

Table 4. — Vitamin Content of Potatoes per 100 grams Edible Portion

Vitamin A 30-50 international units (41)

Vitamin Bi (Thiamin) 95-165 micrograms (41)

Vitamin B2 (Riboflavin) 40-80 micrograms (41)

Vitamin C (Ascorbic Acid) 7-15 milligrams (41)

Pantothenic Acid 650 micrograms (23)

Vitamin Be (Pyridoxine) 40 rat units (40)

Vitamin K small amount (9)

Biotin small amount (25)

STUDIES OF THE VITAMIN C CONTENT OF POTATOES

The present investigation was conducted to determine the vitamin C content
of Massachusetts potatoes in comparison with potatoes grown in other sections
of the country. Differences in vitamin C content due to variety, storage, and
cooking were observed. A preliminary report on this investigation has been made
by Lyons and Fellers (29).

Difi'erences in the vitamin C content of different varieties of potatoes have been
reported by Smith and Paterson (44), Mayfield, Richardson, Davis, and Andes
(31), Ijdo (22), and Wachholder and Nehring (52). Fixsen and Roscoe (13) have
summarized the available data on varietal difference. Up to 10 percent variation



NUTRITIVE VALUE OF POTATOES 7

in ascorbic acid content between individual potatoes within the same variety
has been found by Ijdo (22). Effects of cooking on the ascorbic acid content
vary from a gain (Richardon, Davis, and Mayfield, 37), and a very slight loss
(Thiessen, 48; Armentano, 1; and Levy, 27) for potatoes boiled in the skins,
to a large loss for baked or fried potatoes (Levy, 27). Ijdo (22) reported that
the amount of ascorbic acid present is but little affected by the locality of pro-
duction, and is independent of tuber size; that there is uniform distribution
throughout ithe potato; and that there is practically no oxidized ascorbic acid
present. H^ygaard and Rasmussen (21) found that the addition of 1 percent
sodium chloride to the cooking water increased the retention of ascorbic acid.
Thiessen (48), Mayfield, et al. (31), Smith and Paterson (44), and Zilva and Barker
(58), found a decrease in ascorbic acid after storage. Woods (56) states that new
immature potatoes contain twice as much vitamin C as mature potatoes, and
that common storage of the mature potato for three to eight months does not
change the vitamin C content to any marked degree.

Methods

The method of Tillmans, Hirsch, and Hirsch (49), as modified by Bessey
and King (5) and Mack and Tressler (30), was used for the determination of
ascorbic acid in potato tubers. Twice normal sulfuric acid and 2 percent met-
aphosphoric acid were used in the extraction of the potatoes. The method of
Buck and Ritchie (6) was used to standardize the 2,6-dichlorophenolindophenol
dye solution.

At the start of this investigation the accuracy of the dye titration method for
determining vitamin C in potatoes was checked against the biological method.
Raw, baked, and boiled potatoes were assayed for vitamin C using the guinea
pig method of Sherman, LaMer, and Campbell (42) as modified by Eddy (12).
The results of the two methods were in close agreement in all cases, thus indicating
the accuracy of the chemical test for this purpose. For example, 4.5 grams of
raw potatoes or 5.3 grams of boiled potatoes had an antiscorbutic value in the
guinea pig assay of 0.5 milligrams of ascorbic acid, that is 10 international units
of vitamin C (28). When these same samples of potatoes were examined by the
indophenol titration procedure, identical values were obtained. Hence, through-
out this investigation the dye-titration method was used.

Eight varieties of potatoes, which were grown under similar conditions on
experimental plots at the Massachusetts Agricultural Experiment Station, were
used. These were compared with identical varieties obtained from several other
state agricultural experiment stations very soon after harvest. All of the Massa-
chusetts potatoes were dug at the same time the first part of October.

Dehydroascorbic Acid in Potatoes

Since an oxidized, but still physiologically active form of ascorbic acid, called
dehydroascorbic acid, is sometimes present in plant tissues, eight samples of raw
potatoes were treated with hydrogen sulfide for 30 minutes after extraction in
order to reduce any oxidized ascorbic acid which might be present. This was
followed by a two-hour treatment with carbon dioxide to remove all traces of
hydrogen sulfide. An ascorbic acid value of 0.20 milligram per gram, both
before and after this treatment, indicated that there was no dehydroascorbic
acid present in the raw potato. These findings are in agreement with those
of Ijdo (22) and Rolf (39).



8



MASS. EXPERIMENT STATION BULLETIN 390



Wolf (55), in a study of the distribution of vitamin C in potatoes, concluded
that the outer tissues are 8 to 46 percent richer in total ascorbic acid than the
inner, but that in the skin 19 to 55 percent of the total is dehydroascorbic acid.
In the inner part only to 20 percent of the total ascorbic acid was in the dehydro
form.

Table 5. — Effect of Geographical Origin of Potatoes on Their
Ascorbic Acid Content

Ascorbic Acid — mg, per gm.



Variety and Source



Average

for 8
Tubers



Average
for
Variety



Irish Cobbler

Massachusetts . 157

California 0.122

Virginia* 0.059

Kentucky 0.200

Maine 0.072

New York 0.107

Rural Russet

Massachusetts 0.161

Idaho 0.062

New York 0.108

Katahdin

Massachusetts 0.120

New York..., 0.155

Maine 0.101

Green Mountain

Massachusetts 0.132

Maine 0.095

New York 0.136

California 0.107

Chippewa

New York 0.099

Massachusetts 0.096

Triumph

Idaho 0.132

Louisiana** 0.051

White Rose

California 0.121

Virginia* 0.073

*In storage 2 months before being shipped — not considered in average.
**In storage 5 months before being shipped.



0.13J



0.110



0.125



0.117



0.09/



NUTRITIVE VALUE OF POTATOES 9

Effect of Variety and Origin

The results of the tests on the vitamin C content of different varieties of Massa-
chusetts potatoes and a comparison of these with potatoes from other states are
presented in Table 5. The ascorbic acid content of the same variety grown in
different geographical areas was not significantly different. The general average
shows that the Irish Cobbler variety was highest; Katahdin, second; Green
Mountain, third; and Chippewa lowest. In general, it cannot be said that
potatoes from any particular region were either high or low in ascorbic acid
content. In many cases, differences between varieties were less than differences
within a single variety.

Effect of Storage

Eight varieties of Massachusetts potatoes were held in storage for five months
One set of each variety was kept in cold storage at 36° F. (2.2° C). A second
set was kept in a dry, underground storage where the temperature was controlled
by climatic conditions and averaged 40°-50° F. (4.4°-10° C). This would be
similar to cellar, or common, storage on a farm. The effect of these storage
conditions on the ascorbic acid content of potatoes is presented in Tab'.e 6.

In two varieties the ascorbic acid content of the potatoes was higher in cold
storage than in common storage; in four varieties the reverse was true; and in
two varieties there was no difference. However, in every case the tubers held in
cold storage were more desirable for eating purposes.

Storage seemed to cause a leveling off of the differences between varieties in
ascorbic acid content. For instance, in October, freshly dug Houma potatoes
contained 0.104 milligrams per gram more ascorbic acid than Warba potatoes
dug at the same time. After five months of storage the difference in ascorbic
acid content of these two varieties was only 0.032 milligrams per gram.

Table 6. — Effect of Five Months in Storage on Ascorbic Acid Content

OF Eight Varieties of Mass.\chusetts-Grown Potatoes
Cold storage, 36° F. (2.2° C); common storage, 40°-50° F. (4.4°-10° C.)



Ascorbic .\cid* — mg. per gm.



Variety



October
1938



When Dug



March 1939



Cold
Storage



Common
Storage



Loss due to Storage —



percent



Cold
Storage



Common
Storage



Green Mountain 0.167 0.114 0.120 31 28

Katahdin 0.120 0.130 0.117 2.5

Chippewa 0.181 0.085 0.066 53 63

Irish Cobbler 0.157 0.078 0.107 50 32

Houma . 250 . 099 . 093 60 63

Warba 0.146 0.067 0.088 54 40

Russet 0.161 0.087 0.110 47 31.5

Golden 0.131 0.079 0.072 39.5 45.0



♦Mean value of 8 different tubers of each variety.



10



MASS/EXPERIMENT STATION BULLETIN 390



Distribution of Vitamin C in the Potato Tuber

Three separate sets of tests were made to study the distribution of ascorbic
acid within potatoes. Raw, boiled, and baked potatoes were used. The results
are presented in Table 7. There was an even distribution of ascorbic acid in the
raw tuber itself, with a lesser amount in the skin. In boiled potatoes there was a
greater amount of ascorbic acid in the area between the central and epidermal
portions. The skin and the part just beneath it were about equal in ascorbic
acid content. With baked potatoes, the skin and the pulp adhering to it were
considered the epidermal portion. The concentration of ascorbic acid was greatest
in the central portion of the baked potatoes with the amount decreasing pro-
gressively toward the skin.



Table 7.-


—Distribution


OF Ascorbic Acid in the Flesh of
.\ND Baked Potatoes


Raw, Boiled,


Ascorbic Acid — mg. per gm.


Potato

No.


Central

Portion

(18% of whole)


Medial Epidermal
Portion Portion
(55% of whole) (20% of whole)


Skin
(7% of whole)


1
2
3
4
5
6


0.129
0.134
0.120
0.136
0.147
.. 0,120


Raw Green Mountain Potatoes

0.120 0.115
0.124 0.138
0.129 0.128
0.134 0.134
0.147 0.152
0.124 0.138


0.071
0.117
0.107
0.098
0.135
0.099



Mean



Mean



0.131 0.129 0.134

Boiled Green Mountain Potatoes



0.049 0.051 0.037

Baked Irish Cobbler Potatoes



0.104



1


0.055


0.046


0.023


0.029


2


0.018


0.055


0.032


0.029


3


0.050


0.032


0.027


0.029


4


0.050


0.039


0.042


0.047


5


0.059


0.072


0.046


0.047


6


0.064


0.064


0.054


0.047



0.038



1


0.203


0.149


0.130


2


0.140


0.110


0.080


3


0.210


0.120


0.073


4


0.130


0.126


0.063


5


0.102


0.073


0.084


6


0.100


0.105


0.097


7


0.089


0.110


0.073


8


0.150


0.120


0.110



Mean



0.140



114



0.086



NUTRITIVE VALUE OF POTATOES 11

The above findings show that, whereas in the raw potato the ascorbic acid is
evenly distributed, during the cooking process something happens to change
this distribution. Possibly the outer part of the potato, which is in direct contact
with the heating medium, maintains a higher temperature throughout the cook-
ing process than the inner portion. This probably results in a greater destruction
of the ascorbic acid in the epidermal portions and greater retention in the center.
Also, in the case of boiled potatoes some ascorbic acid is lost through the leaching
action of the cooking water.

Relation of Size of Tuber to its Ascorbic Acid Content

Both large and small mature raw tubers of the Green Mountain variety grown
on experiment station plots were carefully examined for ascorbic acid content.
Marked variations existed in both small and large tubers, but there was no cor-
relation between the ascorbic acid content and the size of the tubers.

The Effect of Cooking on the Ascorbic Acid Content of Potatoes

Baking and Boiling

Baked potatoes have long been considered preferable to boiled potatoes because
of their better retention of water-soluble minerals and vitamins. As ascorbic
acid is water soluble it seemed as though baking would be the ideal method of
cooking to retain the maximum antiscorbutic value of the tuber. In order to in-
vestigate this point eight varieties of potatoes were both baked and boiled in
the customary manner. The boiled potatoes were cooked whole in the skin in
water containing 1 percent salt. From Table 8 it may be seen that, contrary
to expectations, there was in general less loss of vitamin C in boiled potatoes
than in baked potatoes. Baked potatoes showed an average loss of 49 percent

Table 8. — Effect of Variety and Cooking Methods on the Vitamin C
Content of Potatoes
(Eight tubers tested for each value)

Ascorbic .A.cid — mg. per gm. Loss — percent



Boiled Boiled

Variety Raw Baked Whole Baked Whole

in Skin* in Skin*

Katahdin 0.110 0.108 0.080 2 27

Chippewa 0.181 0.096 0.086 45 53

Houma 0.250 0.085 0.165 66 34

Irish Cobbler 0.157 0.064 0.083 59 47

Golden 0.131 0.061 0.061 53 53

Warba 0.146 0.071 0.101 51 31

Green Mountain 0.132 0.069 0.113 48 14

Russet 0.161 0.094 0.102 42 37

Average 0.158 0.081 0.099 49 37

♦Boiled in water containing 1 percent salt.



12 MASS. EXPERIMENT STATION BULLETIN 390

of their ascorbic acid as compared to a 37 percent loss for boiled potatoes. Rolf
(39) has also reported a slightly greater loss of ascorbic acid in baked than in
boiled potatoes.

Boiling in Salted and Unsalted Water

According to Lanman and Minton (26), the texture and the flavor of vegetables
are improved if salt is added to the water during the cooking process rather than
after it has been completed. In this work six potatoes were tested before and
after boiling in salted and unsalted water. After a 25-gram portion had been re-
moved horizontally across the center to determine the ascorbic acid content of
the raw potato, one-half of the tuber was placed in boiling water containing
1 percent salt and the other half in boiling unsalted water. As may be seen in
Table 9 the percentage of ascorbic acid lost varied with the individual tuber.
However, there was a 4 percent difference between the average percentage of
retention of ascorbic acid in favor of the salted water. These findings are in
agreement with those of H0ygaard and Rasmussen (21) who found that the addi-
tion of 1 percent sodium chloride to the cooking water of potatoes appeared to
aid in the retention of their ascorbic acid.

Table 9. — Losses of Ascorbic Acid Due to Boiling Green Mountain
Potatoes in Unsalted or Salted Water

Ascorbic Acid — mg. per gm. Loss — percent



Potato Raw Boiled in Boiled in Boiled in Boiled in

No. Unsalted Salted Unsalted Salted

Water Walter Water Water

1 0.126* 0.050* 0.059* 63.1 53.1

2 0.092 0.050 0.055 45.6 40.2



3


0.092


0.050


0.050


45.6


45.6


4


0.099


0.050


0.055


49.4


33.3


5


0.120


0.055


0.059


54.1


50.8


6


0.099


0.050


0.055


49.4


33.3


Mean


0.104


0.051


0.055


51.0


47.1



*The same tuber was used for each test of a series.

Various Common Methods of Cooking

Further tests were conducted on the effect of cooking on the vitamin C content
of potatoes. These tests included various methods of preparing potatoes that are



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