Copied and pasted from the pdf file. Forget about the table cuz its all messed up
164
PROCEEDINGS OF THE NUTRITION SOCIETY
Summa
y and Cmlusions
The losses of nutrients in the cooking and plate waste of
milk, eggs,
fish and meat and the losses in the storage and preparation of vegctables
have been studied. Records are presented also of waste in school and
other canteens.
Scrambling is the most
wasteful method of cooking eggs. Thermostatically controlled panv
would reduce the waste in cooking eggs.
The filleting of fish in shops would ensure more economic use of the
whole fish.
Plate waste of meat may be as high as 30 per cent. of cooked meat.
Fat should be removed from plates before vegetables are served, or it
should not be served. Imported meat has a higher plate waste than
home fed.
Plate waste in school canteens includes 7 per cent. of the protein and
7
per cent. of the fat served. In a college canteen the waste of protein
was 11 per cent. and of fat 5 to 10 per cent. In a Government canteen
6 per cent. of protein and 3 per cent. of fat served were wasted.
The wastage of vegetables is greatly increased by wilting in storage.
Marketing methods are in great need of improvement. Some system
of
local market gardens should be adopted so that consumers in large
towns could buy their vegetables fresh. Schools and institutions should
have their own gardens.
I
would like to thank the School Feeding Dcpartmcnt, Ayr, for pcrmission
to do this work, the Institutional Department in College and my
own students in College for their assistance.
REEERENGES
Andross,
M. (1939). Chem. and Ind. 58, 262.
Andross,
M. (1940). Chm. and Ind. 59, 449.
Andross,
M. (1941). Chem. and Ind. 60, 176.
McCance, R. A., Widdowson, E. M. and Shackleton, L. R. B. (1936).
Olliver,
M. (1941).
Thc waste in boiling milk is about 10 per cent.
Spec. Rep.
Ser.
med. Res. Coun., Lond.,
no. 213, p. 48.
Ckem.
a d Id. 60, 586.
Loss
of Nutrients in Cooking
Dr.
C. P. Stewart (Royal Infirmary, Edinburgh)
Introduction
Scattered through the literature arc many papers dealing with the
losses suffered by foodstuffs during cooking. Most of these deal with the
more readily estimated of the inorganic constituents
and vitamins, some
by analysis of one or two substances in a variety of foods, others 'by
detailed study of one or two closely related foods. It is difficult to h d ,
or to build up, any general view of the subject, partly because of the very
great differences in chemical and anatomical structure between the
various classes of foodstuffs, partly because the substances which are,
or
may be, lost are of such varying chemical nature, partly because of the
variety of cooking methods which involve differing physico-chemical
processes, partly because cooking lossos may be qualitative as well us
NUTRIENT
LOSSES IN PREPARING FOOD 165
quantitative, and partly because it is not easy to disentangle unavoidable
losses inherent in the process from those due to faulty cooking technique.
It
is diflicult to decide how the available data nmy best be considered
and presented. It is not claimed that the one adopted, of considering
foods in three separate classes, vegetables and fruits, meat and fish, and
cereals, would be the best under all circumstances, but it does offer
certain conveniences for the purpose in view. Nevertheless, it must be
remembered that this arrangement may well lead to the making of
statements which, though generalIy true, are subject to exceptions
which it may not always be possible to specify.
It is perhaps well,
also, to utter the reminder that cooking processes may lead to gain in
nutritive value, both quantitative and qualitative, as well as to losses,
although it is not part of the purpose of this paper to discuss these gains.
Vegetables
and Fruits
When vegetables are cooked by boiling or steaming, the changes in
quantitative composition seem to
be the resultant of three actions, the
relative importance of which may vary widely: shrinkage due to collapse
of
the cell walls and extrusion of the juices, leaching by the boiling water
or
condensed steam, and hydration.
Many vegetables, when steamed, suffer little or no loss, whether of
total weight or of water soluble constituents, and it is noteworthy that
potatoes are included among these. There is, in these cases, no extrusion
of juices, the amount of condensation is insuficient for appreciable
leaching or hydration and, in saturated water vapour, as in boiling, there
is naturally no evaporation. Other vegetables, including root vegetables
such as carrots, swedes, and parsnips, lose up to 30 per cent. or thereabouts
of their water, the amount depending on the temperature of the steam
and the time of cooking, with an approximately equal percentage loss of
their water soluble constituents. In these, shrinkage with extrusion of
juices seems to be the main factor. In still others, e.g., Brussels sprouts
and cabbage, the losses appear to be small, but cannot readily be evaluated
since the anatomical structure is such that extruded juices remain trapped
in the spaces between the leaves. Since, however, thcse juices must be
eaten with the leaves the point is only of theoretical interest.
Boiling vegetables in water obviously increases the chances of hydration
overcoming the tendency to shrinkage by extrusion or diffusion from the
dead cells, and, in fact, boiling results in little or no change in weight.
The opportunity for leaching is, however, much greater than in steaming
and, accordingly, the losses of water soluble constituents are considerable
and increase with the duration of cooking as well as, to some extent,
with the volume of water. Thus McCance, Widdowson and Shackleton
(1938),
found that potatocs, boilcd in about twice their volume of distilled
water for 25 minutes, suffered no appreciable loss of weight, lost only
1
or 2
per cent. of their starch and protein, but 16 per cent. of their
magnesium, iron, and reducing sugars, 18 per cent. of their potassium,
and 22 per cent. of their chlorine. Thc losses of water soluble salts from
some other vegetables during the normal cooking time were much
greater, for example, 70 per cent. of chlorine from runner beans, 40 per
cent. of chlorine and of potassium from carrots, and, since in these cases
also the total weight and the water insoluble constituents remained
VOL.
4, 19161
166
PROCEEDINGS OF THE NUTRITION SOCIETY
relatively unchanged, the main cause of loss appeared to be the leaching
action of the water.
It
may be objected that these results were obtained by boiling the
vegetables in distilled water. Ordinary tap water contains small amounts
of various salts and in ordinary practice it is common to add others,
e.g.,
sodium chloride and, in the case of green vegetables, sodium bicarbonate.
Several early workers have claimed that sodium chloride
rcduces the cooking losses from various vegetables (e.g., Bodinus, 1915;
Gricbel and Miermeister,
1926), though this was denied by others
(e.g.,
Masters, 1918; Lang, 1930; Clifford, 1931). McCance et al. (1938),
showed that potato, cooked in
2 per cent. NaCl solution, absorbed
sodium and chlorine, but lost potassium to the same extent as in
distilled water, a result to be expectcd on the supposition that the salt
losses during boiling are due to leaching and one which, they point out,
results in a most useful change in the K:Na ratio from 435 : 5 to 355 : 103.
The loss of calcium during the boiling of vegetables is generally much
less than that of potassium, and may be nil, because calcium so readily
forms insoluble salts which are not removed by leaching. Ziegelmaycr
(1931)
reported that whereas potatoes” lost some 25 per cent. of thcir
calcium whcn boiled in distilled water, they might even gain up to 300 per
cent. in very hard watcr. McCance et al. (1938), doubt whether such
increases ever occur in practice, but substantial increases in the
calcium content of green vegetables have been observed to occur
in practice and to be reproducible by using water of a hardness
found in some parts of the country (unpublished observations by the
author). Similar increases in the iron content of vegetables have also
been found, probably when weak acids liberated during the cooking
have dissolved iron or iron salts from the cooking vessels.
Calcium salts have been shown to delay the “cooking” (i.e,, the
softening) of vegetables (Viswanath, Row and Ayyangar, 1914-16;
Ziegelmayer,
1931) and the softening of hard water may be one cause
of thc accelerating effect of sodium bicarbonate. Sodium bicarbonate,
by raising the pH of the water, seems to increase somewhat the loss of
water soluble salts, but this effect is not great and is almost compensated
by
the shortening of the cooking time (McCance et al., 1938); by
precipitating calcium salts and so immobilizing them it tends to
decrease rather than increase the loss of this element.
Baking and frying, so far at least as potatoes are concerned, appear
to result only in loss of water, the process being one of evaporation.
It
is evident that cooking in air or fat must be regarded as the most
conservative method of cooking vcgetables so far as the inorganic constituents
are concerned, that steaming ranks next and is almost equal
in the case of, e.g., potatoes; while boiling results in considerable losses
of
all water soluble constituents. Boiling with a very small volume of
water is really partly boiling and partly “stcaming”. It is, however, worth
while, as McCance et al. (1938) have pointed out, to consider what these
losses, which seem so large when expressed as a percentage of the amounts
present in the raw vegetables, really amount to in terms of actual
*
Potatoes boiled with the skins intact suffer no appreciable loss, even of soluble
S!31t,S,
NUTRIENT
LOSSES IN PREPARING FOOD 167
quantities in relation to the daily intake. Table
1 shows the actual loss
of various substances from 150 g. potato, 50 g. carrots and 50 g. green
peas boiled for the normal time, and contrasts them with the approximate
daily intake. Since these amounts represent at least the average daily
consumption of vegetables, it is evident that the cooking losses cannot
be regarded as dietetically important.
TABLE
1
Loss
OF CERTAIN CONSTITUENTSI N TEE BOILINGO F VEGETABLCEOS MPAREDW ITH
TEIE
DA~LYI NTAKOEF THESE CONSTITUENTS
Amount
Vegetable g.
Loss
mg.
----___-
Carbo-
P
1 I hycy K I Ca 1 Na Fe
Potatoes ..
Carrots
..
Greenpeas
..
Daily intake
150
50 30
50
Total 30
Though the losses of minerals, as well as
of carbohydrate and protein,
during the cooking of vegetables appear to be of little nutritional importance,
that is merely because these foods are very minor sources of
those substances of which the percentage losses are or may be considerable.
The position is, especially in war time, very different with respect
to
certain vitamins
of which vegetables form a major source of supply.
A
survey of the scattered literature suggests that vitamin losses during
the cooking of vegetables are due to the same causes as losses of salts,
i.e.,
that they are due largely to leaching. Some ofthe vitamins, however,
are chemically unstable and subject to destruction under the conditions
of cooking, so that in these cases some part of the vitamin is actually
destroyed.
The water insoluble vitamins or provitamins appear to be little affected
by cooking; this at least has repeatedly been found to be true of carotene
(Raps, Meinke and Kemmerer, 1941; Oser, Melnick and Oser, 1943).
Water soluble vitamins appear to bc lost by soaking out and the loss of
them is, so far as this cause is concerned, affected by the same factors
which affect the loss of minerals, but because of the destructive effects of
cooking
on the less stable members of this group, losses are much more
variable than in the case of salts and are affected by factors which have
little effect on salt losses. There is, for example, ample evidence that
the volume of cooking water has a great effect on the losses of ascorbic
acid and other water soluble vitamins (Olliver, 1941; Oser et al.,
1943; Ireson and Eheart, 1944; Van Dupe, Chase, and Simpson,
1944). Losses are least, other things being equal, when the volume of
water is kept at
a minimum end therefore when steaming is substituted
for boiling. For potatoes, cooking without water, Le., roasting, baking,
VOL.
4, 10461
168
PROCEEDINGS OF THE NUTRITION SOCIETY
or
frying, gives the greatest retention of vitamin C and, when boiling is
used, retention of the skin affords considerable protection against leaching
(Esselen, Lyons and Fellers, 1942). Vitamin B, and ascorbic acid are
both subject to destruction, t.he former being heat labile especially at
high pH, the latter subject to oxidation catalysed by copper and by
certain enzymes, this also bcing more rapid at pH above 6. Accordingly,
the losses are greater if cooking is slow, and especially by the initial
raising of the temperature before enzymes have been destroyed (Allen
and Mapson, 1944). The destruction of ascorbic acid and vitamin B,
appears to continue after cooking is complete and the vegetables, exposed
to air, await serving. Thus Nagel and Harris
(1943) found that in large
scale cooking, potatoes lost 35 per cent. of their vitamin 13, and 45 pcr
cent. of their ascorbic acid; after standing on the hot plate these losses
were increased to
70 and 75 per cent'., respectively. Similarly, Kahn
and Halliday (1944) found that, although potatoes steamed in their skins
lost little or 110 ascorbic acid even after 50 minutes on the hot plate,
baked potatoes lost 20 per cent. of their ascorbic acid during cooking
and a further 39 per cent. after 43 minutes on the hot plate, while steamed
potatoes whicH had lost 39 per cent. during cooking lost ncarly all the
remainder when mashed and allowed to stand (cf. Esselen et al., 1942).
The seriousness of this from the nutritional point of view is shown by
the following data. Analyses during April of this year showed that,
given good cooking (boiling) and rapid serving,
150 g. wholo potato may
supply 9 to 10 mg. of ascorbic acid, and 50 g. of cabbage a like amount,
a total of two-fifths of the amount suggested as a reasonable daily intake.
If,
however, the potatoes are mashed and the vegetables are kept hot
for 2 to 1 hour before serving, thc total amount of ascorbic acid derived
from them may be reduced to 2 to 3 mg.
Meat
and Fish
McCance and Shipp
(1933) published the results of a careful study of
the water, salt, protein and fat losses incurred in cooking flesh foods.
These investigators found that when heat to over 60°C. is applied to
flesh foods, there is shrinkage of the fibres, which tends to be greater
the higher the temperature, and expression of juices; this occurs irrespective
of the mode of cooking, and is the most important single source of
loss
of soluble salts. When meat is boiled, there may be additional salt
loss by diffusion; when it is cooked in air or fat, water loss by evaporation
is so great that salts are left on or near the surface and so tend to be conserved.
Protein losses are very variable and dcpcnd on many factors,
the type of flesh food (the kidney, for example, loses more than muscle),
mode of cooking (more being lost in boiling than in steaming, and roasting
causing the least loss), duration and temperature of cooking, and so on.
They are not negligible, figures of the order of 7 per cent. having been reported
for boiled beef, and up to 20 per cent. for boiled or steamed kidney.
Fat losses are due simply to liquefaction and consequent leakage; they
arc, therefore, very variable, rising to 40 per cent. or more.
It
is very difficult to assess the practical importance of these losses.
The amounts of soluble salts, though they appear large when expressed
as
percentages of the amounts present in the raw food are relatively
insignificant in relation to the total daily intake. The 20 per cent. or so
NUTRIENT LOSSES IN PREPARING
FOOD 169
of iron which is lost may be more serious but there is
a suggestion in
Odham’s (1941) studies of anaemic rats that it is partly compensated by
an increased “availability” of the iron caused by cooking. The evaluation,
from the nutritional standpoint, of cooking losses is further complicated
by the facts that drippings from meat are partly or wholly eaten as
gravies or, in the case of fat, used for other cooking purposes, while the
cooking water from boiled meats is generally used for soup preparation.
The quantitative loss of protein has already been mentioned; there
exists also the possibility of a qualitative loss. Although McCance and
Shipp (1933) summarize the earlier literature by stating that “however
it is cooked, meat has been shown to be completcly digested and to lose
little, if any, of its nutritive value”, there is some evidence to the contrary.
Kapp (1937) showed that the digestibility of meat in vitro
was reduced from
86 per cent. to 60 to 64 per cent. by stewing, boiling or
grilling, and to 51 per cent. by roasting. Morgan and Kern (1934),
using both nitrogen balance and growth methods, found a decrease in
the nutritive value
of beef muscle protein boiled normally or at 15 lb.
pressure. The known effects of dry heat on cereal proteins or milk
proteins suggest that roasting or deep fiyii may well produce similar
decreases in the nutritional value of meat protein. The information
available, however, is insufficient for more than a warning that, in considering
the cooking losses of flesh foods, the possibility of qualitative
changes must be taken into account.
Meat is an important source of certain vitamins of the B group.
Fortunately most of these substances are moderately heat stable and so,
though losses occur by the same mechanism as that which brings about
losses of water soluble salts, actual destruction
is not very great, and a
considerable part of the “lost” vitamins can be found in the drippings.
It
seems to be fairly generally agreed (e.g., McIntire, Schweigert and
Elvehjem, 1943; McIntire, Schweigert, Henderson and Elvehjem, 1943;
Cover, McLaren and Pearson,
1944) that the loss of vitamin B, from beef,
veal, pork or cured ham, ranges from 20 to 50 per cent. according to the
method and duration of cooking, with a net destruction of about 20 to
25 per cent. With an average daily intake of 1OOg. of meat and an
average vitamin B, content of 100 I.U. per 100 g. raw meat there
would be a maximum daily cooking loss of about 50 I.U. of vitamin
B,;
this could only be serious if, owing to the exclusive use of low
extraction cereals, meat were promoted from a minor to a major source of
vitamin B,.
The better known members of the vitamin
B, complex are more resistant
to heat than vitamin B,; the losses of these substances appear,
as would be expected, to be smaller. Thus, according to the workers
quoted for vitamin B, losses, the loss of riboflavin ranges from 15 to
25 per cent. (0 to 13 per cent. if the drippinga are included); that of
nicotinic acid fiom 15 to 35 per cent. (0 to 10 per cent. including the
drippings) and of pantothenic acid from 10 to 25 per cent. According
to Schweigert, Nielsen, McIntire and Elvehjem (1943), 80 per cent. of
the biotin of meat survives cooking. These losses do not seem very serious,
and since they do not seem to be due to faulty technique, they must be
regarded as the unavoidable penalty of a preference for cooked rather
than raw meat.
VOL.
4, 19461
170
PROCEEDINGS OF THE NUTRITION SOCIETY
C e r d
The ways in which cereal foods are prepared and cooked are not such
as to lead
to losses of inorganic comtituents or of protein, fat and carbohydrate,
though, as in other cases, the application of dry heat may somctimes
change the nutritive value of the proteins.
Cereal foods are important sources of certain vitamins of the B group;
some of these are stable to heat, but others are not and are therefore
susceptible to destruction in baking. Thus riboflavin is heat stable and
losses in cooking are said to be negligible (Andrews, Boyd and Terry,
1942)
though, since the substance is destroyed by light, exposure of cut
slices of bread may result in some loss. Vitamin B,, on the other hand,
is more readily destroyed by heat, especially at high pH. Dawson and
Martin
(1942) state that white bread, made with a vitamin rich yeast,
loses about 8 per cent. of its vitamin B, in baking, bread from 85 per cent.
extraction flour about 27 per cent., and wholemeal bread about 35 per
cent.; Schultz, Atkin and Frey (1942) give the average baking loss as
20
per cent. under normal large scale conditions and add that the loss is
greater in the crust than in the crumb. There is, it appears, increasing
loss with increasing temperature and various workers have called attention
to the extra loss in toasting bread (e.g., Downs and Meckel, 1943)
and in biscuit making
(e.g., Barackman, 1942). The effect of pH on
loss of vitamin B, is shown by the greater loss involved in the use of
baking powders than in that of yeast. Thus, Wilson (1942) reported
that yeast bread contained 2.1 I.U. vitamin B, per g., whereas comparable
bread made with baking powder, with cream of tartar as the
acid constituent, contained only 1.3 I.U. per g. Barackman (1942)
found that when “self raising” flours were used
for biscuit making,with
water mixing, the pH was 8-5 and the vitamin B, loss 51 per cent.; at
pH
6-7, however, the loss was only 9 per cent. Decreased loss, with
lowered pH, was obtainable by using milk for mixing, or by adding acid
calcium phosphate to the flour. It seems that, as one would expect from
the known behaviour of pure vitamin B,, the loss in cereal cooking is
minimized by preserving as low a pH as possible and by avoiding excessive
heating. This probably applies also to certain members of the
vitamin B, complex, the stability of which resembles that of vitamin B,.
REFEREN~ES
Allen,
J. R. L. and Mapson. L. W. (1944).
Andrews, J. S., Boyd, H. M. and Terry, D. E. (1942).
Barackman, R. A. (1942).
Bodinus (1915).
Clifford, W.
M. (1931).
Cover, S., McLaren, B. A. and Pearson, P. B. (1944).
Dawson, E. R. and Martin, G. W. (1942).
Downs, D. E. and Meokel, R. B. (1943).
Esselen, W. B. (Jr.). Lyons, M. E. and Fellers, C. R. (1942).
Fraps, G. S., Meinke, W. W. and Kemmerer, A. A. (1941).
Griebel, C. and Miermeister. A. (19262.
Ireson, M. G. and Eheart, M. S. (1944).
Kahn, R. M. and Halliday, E. G. (1944).
Kapp,
H. (1937).
Lang, T. (1930).
J. SOC. chem.
Id., Lond., 63, 78.
Cereal Chem.
19, 55.
Cereal Chenz.
19, 121.
Phrm.
Ztg., Berl., 60,
188.
Biochem.
J. 25,
1999.
J. Nutrit.
27, 363.
J.
Soc. them. Id., Lond.,
61, 13.
Cereal Chem. 20,
352.
Bull.
Mass.
agric.
J.
Ass. off.
ayric.
Exp.
Sta.
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52, 458.
J. Home
Econ.
36, 160.
J.
Amw. diet. Ass. 20, 220.
Arch. VerdauKr.
61, 123.
Schweiz.
med. Wschr. 60,
219.
NUTRIENT LOSSES IN
PREPARING FOOD 171
McCance,
R. A. and Shipp, H. L. (1933). Spec. Rep. Ser. med. Res. Coun., Lond.,
no.
187.
McCance,
R. A., Widdowson, E. M. and Shackleton, L. R. B. (1938). Spec. Rep.
McIntire, J. M., Schweigert,
B. S. and Elvehjem, C. A. (1943). J. Xutrit. 26, 621.
McIntire,
J. M., Schweigert, B. S., Henderson, L. M. and Elvehjem, C. A. (1943).
Masters,
H. (1918).
Morgan,
A.
F. and Kern, G. E. (1934).
Nagel,
A. H. and Harris, R. S. (1943).
Odham,
H. G. (1941).
Olliver, M.
(1941). Chem. and Ind. 60, 686.
Oser, B.
L.,
Melnick, D. and Oser, M. (1943).
Schultz,
A. S., Atkin, L. and Frey, C. N. (1942).
Schweigert,
B. S., Nielsen, E., McIntire, J. M. and Elvehjem, C. A. (1943).
Van Duyns, F. O., Chase,
J. T. and Simpson, J. I. (1944). Food Res. 9, 164.
Viswanath,
B., Row, T. L. and Ayyangar, P. A. R. (191616).
Wilson,
E. C. G. (1942).
Ziegelmayer, W. (1931).
Ser. med. Res.
Goun., Lond.,
no. 213.
J. Nutrit.
25,
143.
Biochem.
J.
12, 231.
J. Nutrit.
7, 367.
J. Amer. diet.
Ass.
19, 23.
J. Nutrit. 22, 197.
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Chenz.
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Discussion
Dr.
J.
C. Thompson (Food Research Laboratory, Barkers (Contractors),
Ltd., c/o Morris Motors, Ltd., Cowley, Oxford), opener: The problem of
the loss of ascorbic acid in food is one which has aroused considerable
interest in recent years, particularly with food prepared on a large scale
as for industrial canteens and British Restaurants. This is because
there are certain processes peculiar to large scale catering which can
cause the loss of considerable amounts of ascorbic acid.
Although the emphasis is on ascorbic acid because more estimations of
the ascorbic acid content of foods have been made and more is known
of its properties, this is simply because it is the easiest of the readily lost
vitamins to estimate. As it is one of the most readily destroyed, and as
the properties which lead to its disappearance from food, i.e., by being
soluble in water, by being heat labile and by destruction by alkalis,
are equally shared by other vitamins, particularly vitamin B, and
members of the vitamin B, complex, it is safe to assume that if the
ascorbic acid content of vegetables is satisfactory, the content of the
other vitamins is assured. There are of course obvious exceptions to
this assumption, e.g., dried legumes are rich in vitamin B, but are devoid
of ascorbic acid.
The methods of cooking vegetablcs so as to conserve the greatest
amount of ascorbic acid have been widely advertised by the Ministry
of
Food. All vegetables should
be prepared, cooked and served as quickly as possible and with tho
minimum physical destruction of the vegetables, e.g., cabbage should be
coarsely shredded with a sharp knife and potatoes should not be mashed.
Expensive cooking, that is cooking involving large material waste, such
as
the discarding of the outer green leaves of cabbage and the peel of
potatoes, or high fuel consumption because of the huge quantities of
water used, produces food of inferior nutritive quality.
There are two aspects of large scale cooking
I would like to mention,
the use of bicarbonate of soda in the cooking of greens and the storage of
food in the hot plate.
VOL.
4, 19481
They may be summarized as follows
