Very frequently the results of costly law cases hang on the reports of expert food examiners; every care, therefore, must be taken to avoid error. This being the case, whenever possible, chemical tests should be carried out to confirm the results of microscopic examination. When both microscopist and chemist come to the same conclusion, there is not likely to be any mistake. There are tests which the microscope cannot perform, there are some, also, which are beyond the powers of the chemist and many which are very difficult for him. A drop of milk, for example, examined under the microscope shows a number of fat globules floating in a watery liquid. However clever the microscopist and however accurate his instrument, he cannot tell if there is an excessive quantity of water, yet a simple chemical test will answer the question. This is a case in which the microscope is of little use, although it is only fair to add that microscopic examination would reveal the presence of blood, hair and dirt, to mention three common impurities, which the chemist in his test for watered milk would quite overlook. With a little care and the use of suitable stains, any bacteria which might be present would also show plainly under a powerful microscope.
Now for an example or two where the microscopist has the advantage of the chemist. Some jam makers have been known to be sufficiently unscrupulous to sell “raspberry” jam contaminated with a large percentage of some cheaper fruit, such as gooseberry. The seeds of the two fruits differ so markedly that it is really not necessary to employ a microscope to discover the fraud, but a case is on record where wooden seeds were used, so like the true seeds of the raspberry, that a very careful examination was necessary to show what had happened. In our [chapter on the Microscope in Agriculture] we have referred to this point in greater detail. Starch of various kinds is a very common food adulterant and the experienced microscopist can estimate almost precisely, the proportions of different starches in a mixture, a feat which would sorely puzzle the chemist. So in certain cases the microscope is indispensable.
Briefly the microscope is a time saver; chemical tests occupy a considerable time; microscopic examination is quick, the experienced microscopist at once recognises what he observes. Very small quantities can be examined under the microscope, relatively large quantities are required for chemical tests. Again, if only a small quantity of the material is available for examination and it is necessary to carry out chemical tests, they can be performed under the microscope and this point is considered in another chapter.
We have mentioned that starch of various kinds is a common adulterant of many foods and the budding food analyst might do worse than learn to recognise the various starch grains under the microscope. They are easily obtained and as easily observed. Each variety of starch has grains which are remarkably constant in their characteristics. A beginning may profitably be made with potato starch, for its grains are large and they possess certain well-marked features, which may or may not be present in the grains of other starches. By scraping the newly cut surface of a potato we can obtain thousands of starch grains. The surface of the potato must not be grated, just a gentle scraping with a pocket knife and a mere speck of the cloudy liquid that is obtained, added to a drop of clean water on our slide, will suffice. Cover the object with a cover glass and examine under a fairly high magnification. There are countless, oval, almost transparent bodies in our field of view, they are potato starch grains. Each one, as we shall see when we make a more careful examination, is not unlike a miniature oyster-shell. In the shell, there is a point which is its oldest part and the remainder has grown, layer by layer, round that point till the shell is fully formed. Now we magnify the starch grains as highly as possible and slowly rotate the fine adjustment to and fro, for the reason that the object is not flat and by doing so, we obtain all its parts in focus in turn. If the illumination is not too intense, we shall notice a minute dark dot corresponding to the oldest part of the oyster shell; it is, in fact, the oldest part of the starch grain. Around this point we can see as we focus up and down, ring after ring where the grain has grown larger and larger. The dark spot is called the hilum and the rings are known as striations. In the potato starch grain the hilum is not central and the striations are not circular. Wheat has large and almost round grains without a hilum or striations, those of Barley are very similar but smaller and not so uniformly round. Rye grains are frequently cracked and often have ragged edges.
A very large number of these objects may be examined, for it is useful to know their structure if one’s object be to examine various foods; from the point of view of beauty, when examined with a polariscope, they have few rivals. Maize starch, which is to be found in most houses under the name of corn flour consists of two kinds of grain. Some are many sided and angular, all of one size and without striations, they are also split at the centre; the other grains are rounded, of various sizes and are never like the angular grains grouped together. The former come from the horny part of the maize, the latter from the floury portion.
Rice starch is also many sided and angular, almost crystal like; there are, however, never any rounded forms and this serves to distinguish it from maize starch. The shape of Arrowroot starch grains varies according to the plant from which it is derived, for this substance does not all come from one kind of plant but, whether the grains be pear shaped, hammer shaped, triangular or dumbell shaped they all show striations and an x-shaped split in place of a hilum. Tapioca starch grains are usually grouped together in twos or threes; when they rest on their flat surfaces they appear circular and each hilum is surrounded by a dark ring, when on their sides they are seen to be sugar-loaf shaped.
Many more starches can be found without going far afield, Sago, Peas, Beans, Lentils and Bananas are a few common commodities containing starch. An effort should be made to study the very curious dumbell shaped starch grains of the Spurge and its relations. All these plants contain a white milky juice in which the starch grains float; by squeezing a little of this milky fluid into a drop of water on a clean slide the grains can easily be observed.
It is sometimes difficult to observe starch grains till a fair amount of experience has been gained in the use of the microscope. Should this difficulty arise, it may be overcome by adding a drop of a weak solution of iodine. This will stain the starch grains a deep blue colour and render them very easy of observation. The iodine solution must be weak, however, or the staining will be excessive and the objects rendered black and non-transparent.
Having examined many or all of the specimens we have mentioned let us turn our attention to some of the common foods, and learn some of the methods used in testing for impurities. Ordinary household bread, it is hardly necessary to state, is rich in starch and, by trying the iodine test, mentioned above its presence is easily shown. With a weak solution the deeper the blue colour produced, the greater the quantity of starch. Some parts of the bread will be stained yellow, this indicates the presence of another nourishing component of bread. Certain kinds of bread are supposed to contain no free starch, because this substance is not beneficial to some people. Iodine again will reveal whether the bread is as it is described, for, if there be no free starch there will be no blue colouration. Brown bread will show much more of the yellow colouration and less of the blue than white bread. Good, well-baked bread should keep for a considerable period without turning sour; we can easily see whether our sample is satisfactory by running a drop of litmus on to it and watching the effect under the microscope; if the litmus remains unchanged in colour the sample is not sour; if, on the other hand, the litmus turns red it shows us that acid is present and that our bread is not as it should be.
Tea is difficult to prepare for microscopic examination and most of the tests call for expert knowledge, not only in the management of the microscope but of the plant itself. The structure of the leaves can be made out clearly in specimens which have been soaked for a time in water, but this is of little interest to the ordinary microscopist. One very pretty test may, however, easily be performed. We all know that it is not good to drink tea which has been standing for a long time. Some tea-drinkers are so particular that they cannot bear to see the teapot shaken before they have poured out their cup. All this trouble arises because tea contains a poison called “theine”; it is an alkaloid, one of a large class of chemical substances which are nearly all deadly poisons—cocaine and nicotine are alkaloids. Although theine is poisonous, tea which contained none of this substance would be tasteless and the absence of this substance shows that the tea leaves have been badly prepared. Tea after being gathered should be dried at once, sometimes it is re-dried and this process drives off the theine. For our test we require, in addition to our microscope, two watch glasses, a piece of copper wire gauze and a gas burner or a spirit lamp. Place a little tea in one of the watch glasses and cover with the other watch glass; then heat gently on the wire gauze. In a few minutes drops of moisture will appear on the upper watch glass; after about ten minutes’ heating beautiful, long, needle-shaped crystals will begin to appear, with a little further heating we shall obtain a good crop of lovely crystals on the upper watch glass and they make a splendid object for examination under a low magnification. The crystals are of theine, the poisonous component of tea, and the test is used to discover whether the tea has been redried during its preparation; redried tea gives no crystals.