Food Values of New Zealand Fish.
Part 9: Tinned Toheroa and Toheroa Soup.
[Read before the Otago Institute, 8th November, 1927; received by Editor, and February, 1928; issued separately, May 12th, 1928.]
The toheroa (Amphidesma ventricosum, Gray) is a bivalved molluse found in the tideway of certain long sandy beaches, especially in the northern part of the North Island (1).
Unlike the oyster its double shell does not retain sea-water long enough to allow of it being marketed at a distance, but in recent years the shell-fish has been successfully canned, and has become widely known as forming the basis of “toheroa soup.”
Since it was unlikely that the composition would prove to differ much from that of the paua, oyster, or other shellfish, no systematic analysis was made, but certain data had to be obtained for the vitamin experiments which form the main part of this paper, and these are mentioned here.
The tinned toheroa used (“Shell brand”—put up by Meredith Bros., Tikinui) was found to have 74% water, 2.57% ether extract, and 1.57% ash. The ether extract was of a rich green colour, and showed a marked spectrum of chlorophyll. The tinned soup (Tiki-Toheroa medicinal broth, Northern Canneries, Ltd., Auckland) had between 9 and 10% solids of a similar green appearance to the foregoing.
Methods: The contents of the tin were minced, and a weighed quantity, mixed with starch to form a leaven, was dried for some hours in a moderately warm oven. The weight was then made up with starch and extracted casein to that of the quantity of toheroa taken, or to some round figure. The whole was then ground to a fine meal and given as a supplementary ration to rats suffering from lack of vitamin-A. The basal diet on which these rats were otherwise fed consisted of extracted casein, rice-starch, hardened vegetable oil, “marmite” (vitamin B), salts (McCollum's mixture), and oxidized cod-liver oil (vitamin D) (2).
This method of feeding the material as a fine meal was adopted to secure uniform mixing, for the visceral part of the shell-fish is probably much richer in vitamins than the mass of the adductor muscle. The method is open to objections in that destruction of the vitamin is likely to take place during the drying in air, and, secondly, in that, when several rats occupy one cage, uneven distribution of the
combined daily ration may occur. In spite of these sources of loss or of irregularity, the results were positive and uniform as shown by the curves of growth.
In order to provide a basis for assessment of the amount of vitamin-A present in the different quantities of toheroa, the rations were given daily for a period of ten days (except in one case) and the observations on weight, eye, and other symptoms were continued till death occurred. In this way one obtains some idea of the amount of vitamin available for storage during the feeding period as well as for growth. In order to allow the rats to make the most of the material, the cages were not cleaned for some time after the dieting was begun, so that, if they chose to do so, they could eke out their stored vitamin by consuming some of their faeces. As the cages contained sawdust and fine shavings as bedding, no disagreeable odour was detectable. They were probably under more natural and more comfortable conditions than when kept in daily scrubbed metal cages without bedding and under what we would consider more hygienic conditions. It should also be mentioned that the stock rats from which the litters used were bred are kept on a diet of wheat and receive kitchen scraps twice weekly. Basal diet is begun before the young are weaned, usually about the 20th day.
Results: Litter Al, six rats, were given basal diet on the 24th day and were weaned at 28 days. They were separated into three groups, I, II, and III. Only the growth curve of Al, II, is shown (Chart 1). This group consisted of two does. By the 96th day of age their eyes were noted as beginning to be affected and their weights were falling. Line A-A on Chart 1 marks the 105th day. Both then had distinct eye-symptoms such as haemorrhagic and congested eyelids, and photophobia. As indicated on the chart they were then given toheroa meal at the 2.0 gm. level for fourteen days. After a week on this the eyes were improved, and by the 9th day were quite cured. The weights increased and continued to rise for a fortnight after the dieting ceased. Then eye-symptoms recurred in one about the 145th day and she died when 167 days old. The other showed slight eye-symptoms about the same time as the other, but lived to the 190th day—the end of her growth curve is not shown on the chart.
Group Al, I, consisted of a buck and doe. Both developed eye-symptoms earlier than Al, II. They were given toheroa at the 1.0 gm. level, but the treatment was begun too late in the case of the buck, and he died four days after the treatment began. The doe gained weight (26 gm.) and the eye-symptoms cleared up. She was shown at a medical congress and died two days later, probably from pneumonia.
Group Al, III, consisted of two does. They had a history of loss of weight and eye-trouble very like that of Al, I and II. They were given 0.5 gm. toheroa each, and showed an increase in weight (20 gms. in one, 14 gms. in the other) with cure of eye-symptoms which had been well marked in one, but only alight in the other. Unfortunately, they had to be shifted to new quarters during the experiment, and both died soon afterwards. Their weight curves resemble
that of Be, I, in Chart 1, in that the increase was not well maintained after dieting ceased.
Litter Bc consisted of ten rats. They were given basal diet when 20 days old, and were weaned on the 28th day, and divided into groups, viz.:—Bc I, four bucks; Bc II, four does; Bc III, two does.
During the ninth week after weaning, they began to show eye-symptoms and to lose weight. Line A-A (Chart 1) indicates the 90th day of age for this litter. All the groups were then at a suitable stage for commencing the toheroa diet, and in each case it was continued for ten days, to line B-B in the chart.
Bc, I: The four growth-curves of this group were so close together that, for the sake of clearness, the two lower ones on the chart have been dropped uniformly 5 grm. The dose here was equivalent to 0.5 grm. tinned toheroa to each rat. By the end of six days the eyes had greatly improved and the weights had increased. A few days after the toheroa diet was stopped they seemed perfectly normal in every way, but within another fortnight the eye-symptoms returned, the weights fell and they died at 138, 140, 143, and 145 days old.
Bc, II — received 1.0 grm each: In this case the eye-symptoms cleared up in the ten days of special dieting and the weights rose rapidly. They continued to grow or maintain their weight for about three weeks after the dieting ceased, then eye-symptoms returned while they were still apparently well, and then the weights began to decrease till they died or were killed at the following ages in days: 147, 149, 150, 152 (K).
Bc, III—received 1.5 gm. each: In these two doe rats the eye-symptoms, which were marked in one, slight in the other, improved and disappeared almost simultaneously with those of Bc II. Their weight increased more and was better maintained than in the case of Bc, II, but on the whole the difference does not seem to be proportional to the difference in dosage. This probably means that nearly the maximal amount of storage took place when 1.0 gm. was given. The rat that originally showed only slight eye-symptoms died without developing any more eye-trouble—the other had a recurrence. They died at 138 and 144 days. The tin of toheroa used for litter Al was several years old, that used for Bc was at least one year old.
Tiki-Toheroa Medicinal Broth.
This material as its name indicates is a concentrated toheroa soup. It was made into a meal and used in the same way as described for the whole toheroa, but ha order to make a comparison the composition of the meal was so arranged that it had the same amount of toheroa solids as in the other case. Only one litter (Bd) was used, the growth curves of which are shown on Chart 2.
This litter consisted of five rats and was divided into two groups: Bd, I, two bucks; and Bd, II, three does. Basal diet began on the 20th day, and they were weaned on the 32nd day. By the end of six weeks from weaning the weights were stationary and eye-symptoms had begun in the form of congested zones round the eyes with haemor-
rhagic secretion, and photophobia. The toheroa soup-ration was then given for ten days beginning at A-A in Chart 2 when the rats were 74 days old.
Bd, I, received the equivalent of 1.0 gm. toheroa, and showed rapid improvement in eyes, in weights, and in general health. In a fortnight after commencing the toheroa soup, the eyes would pass
for normal, and the animals were plump and lively, but two weeks later the eye-symptoms recurred and the weights fell. They died with typical symptoms of avitaminosis at 130 and 137 days.
Bd, II, received 1.5 gm. and showed similar recovery of eye-symptoms and growth. One of the three made quite a remarkable recovery from what seemed a hopeless condition. The effect on the
growth-curves of this group seems less than in the case of Bd, I, which received the smaller dose, but this is probably due in part to the difference in the sexes, Bd I being bucks, and Bd II, does. These rats died at 113, 130, and 137 days. Unfortunately, circumstances did not allow of a further test with smaller quantities of this material.
Discussion and Conclusions.
From the earner work on vitamins, tinned foods in general have been somewhat hastily judged to be deficient in all vitamins. In regard to vitamin-A the truth appears to be that, if the material is rich in this vitamin, the canned product still retains much—one would hesitate to say all—of the vitamin originally present. The experiments described above certainly indicate that toheroa when tinned whole, or even when made into a soup and then tinned, can supply as much vitamin-A as the rat can utilize when fed at the level of 1.0 to 2.0 gm. daily; even 0.5 gm. produces good results. While these experiments were in progress similar work was being done on Stewart Island oysters, both fresh and tinned, and, on the whole, the toheroas were the richer of the two, although both are valuable sources of vitamin-A. This may be related to the richer content in chlorophyll in the case of toheroa, and may indicate more of a phytoplankton diet as compared with a more zoöplankton diet for the oyster.
The writer again begs to acknowledge with thanks the financial aid of a grant from the New Zealand Institute: thanks are also due to Miss Earland for her services in attending to the rats.
(1) Mestayer, M. K., N.Z. Journ. of Sc. and Tech., 1921, vol. 4, p. 84.
(2) Storms, L. B., Trans. N.Z. Inst., 1927. vol. 58 p. 264. (Recent methods are referred to in this paper).