It is an unfortunate fact that our food is not always absolutely pure. It may be contaminated with foreign matter either by accident or by design. However careful the manufacturer may be in, say the preparation of cocoa, some dust, some waste vegetable matter, perhaps even a few stray dried insects may occur as impurities. They are out of place certainly but, at the worst, they are a sign of lack of care on the part of the manufacturer. There is another, more serious side to the question of food adulteration, where the foreign matter is added purposely, either because it is cheap, because it weighs heavily, imparts a pleasing colour or an agreeable aroma. Such adulteration is a form of fraud and the microscope is an invaluable aid in its detection.

In many respects the microscope is a better informant than the tests of the chemists; in some cases, however, it merely supplements and confirms the chemical results. Let us consider, for a moment, the advantages possessed by the microscope and also where chemistry scores.

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.

The examination of cocoa for impurities is a matter rather for the chemist than for the microscopist. It contains a vast number of starch grains, not unlike those of rice, except that they are rounded. Coffee often contains a number of impurities, the chief being chicory, various starches, ground acorns and date stones. Chicory is really an impurity, though it is one often asked for by coffee-drinkers. It is easy to detect the amount of chicory present in a sample of ground coffee, by throwing a little of the mixture on to water. The chicory sinks at once, whereas the coffee floats for a while because it is oily. In pure coffee there should be no starch and the iodine test will readily show whether we are dealing with a sample free from starch or not.

Mustard is very rarely purposely mixed with any impurities, in fact it is probably the least likely to be adulterated of any article of food. Under the microscope a large number of small objects, very similar to starch grains, can be seen. They are the cells containing mustard oil and they are not stained blue by iodine. A specimen of pure mustard contains no starch. Pepper is by no means easy to test for impurities. It contains minute starch grains, which can be recognised under the microscope after staining. It is mentioned here because of a very interesting and easily performed experiment that will appeal to every microscopist. Place a little pepper on a clean slide and moisten it with a drop of alcohol, allow it to stand for a minute or so then add a little dilute glycerine, cover the specimen with a cover glass and examine it under the microscope after the lapse of about five minutes. The sight of wonderful prismatic crystals forming one by one in rapid succession will be ample reward for the trouble taken. A drop of strong nitric acid, which must not be allowed to come in contact with any part of the microscope or with one’s hands or clothes, will turn the crystals a rich orange colour. The crystals are composed of a substance called piperine.

Everyone knows of the importance of pure water for drinking purposes but the word pure in this case is used in a very wide sense, for the only really pure water is distilled water and it would not form a very good beverage. Although ordinary tap water may contain a number of impurities it is not easy to see them without taking a little trouble. If our tap water is so contaminated that a drop, examined at random, shows us all manner of solid matter floating in the water there must be something seriously wrong. Those who are engaged in testing water under the microscope, first take a big jug full of the water and allow it to stand for twenty-four hours, covering it the while to keep out dust. At the end of this time most of the water is carefully drawn off, from the top, with a siphon and the remainder, after stirring is poured into a conical glass and allowed to stand for a further twelve hours. Then the upper portion of the water is again siphoned off and a little of the remainder, which is left in the point of the conical glass is drawn by suction into a special kind of glass tube, called a pipette. This final sample contains all the solid matter which settles to the bottom of the water after standing for thirty-six hours.

The impurities likely to be present in water make such a formidable list that we can only mention a small number. There may be various mineral substances, such as lime, sand or clay; vegetable substances, starch grains, fragments of wood, pieces of decayed leaves and the like or there may be hair, scales of fish, etc. The impurities may be living plants, of which the most likely to be found are bacteria, diatoms, desmids and Volvox, amongst plants, and rotifers, Vorticella, Paramoecium, Amoeba, also portions of tapeworms and their eggs amongst animals. These creatures are all described elsewhere so we need not dwell on their peculiarities here.

In addition to all of the above and many more not mentioned there are four metals commonly found in impure water, either in small solid particles, or in the form of one of their compounds soluble in water. The metals are iron, copper, zinc and lead. Very simple chemical tests will show whether they are present or not. To a drop of the water add a very minute quantity of hydrochloric acid and of potassium ferrocyanide solution. When iron is present the solution will turn blue; in the presence of copper it will become bronze coloured, whilst zinc turns it a milky white. To detect lead, take another drop of water and add a very small quantity of potassium chromate solution. If the suspected impurity is present the solution turns yellow. All the chemicals for these simple tests may be obtained at any chemists and there is this great advantage in testing under the microscope—only very small quantities are required.

Butter can hardly be described as an interesting object for the microscope, nevertheless, it may be of use to explain the methods of its examination. A small sample should be placed upon a clean slide, a drop of olive oil added and the whole covered with a cover slip which may be pressed firmly till the specimen forms but a thin layer. Of course the most important impurities likely to be present are bacteria but these we cannot see without special preparation and we are not dealing with bacteria in the present chapter.

If our specimen is in a film sufficiently thin to be transparent, and we should have made it so by pressure on the cover slip, we may first of all examine it carefully for starch grains which, by the way, should be absent. The amount and size of the drops of water, which every butter sample contains, are of importance in deciding its quality. In good butter there are a few scattered drops of various sizes; in milk-blended butter the water globules are all very small and all of the same size, or as nearly so as the eye can judge; in butter containing an excessive amount of water the drops appear large, much larger than in a normal sample.

If we examine various samples, we shall find that some are much more transparent than others, the transparent samples being fresh butter. The curd also in fresh samples is much more finely and evenly scattered in the field of view than in older samples. Renovated butter, that is to say rancid butter which has been melted and made palatable by forcing steam through it, should be examined by oblique light—easily arranged by tilting the mirror at an angle—when the curd appears as white patches on a dark background.

It is curious that one article of food, honey, is more likely to be pure when it contains impurities. This sounds like a bull but a great deal of honey is manufactured from various sugars but not by bees. This artificial honey contains no pollen grains, in fact any honey found to be free of pollen should be looked upon with suspicion. Starch often occurs in artificial honey, never in real bee-made honey.

To many foods adulterants are added as preservatives, the nature and quantity of such additions is settled by Act of Parliament. Many foods are preserved with small quantities of Borax or Boric Acid. The use of Formaldehyde, formerly sold under the German trade name of Formalin, is not unknown but it is very injurious. Salicylic Acid which was formerly much used is being supplanted by Benzoic Acid, for the reason that the latter is not so easily detected and therefore prosecution for excessive quantities is not so likely to follow. These preservatives are not easily detected by the microscopist unless he be a chemist also.

As we have already remarked adulterants are added for the sake of colour, either because the public demand certain colours, or to hide fraud. Milk for instance, when watered, assumes a characteristic blue colour; to hide the blue shade various dyes, anetto, turmeric or carrot juice are added or one of the aniline dyes, products of coal tar. This form of deception became so common that now the public demand yellow milk and butter. Jams made from inferior or over-ripe fruit are frequently coloured with coal tar dyes, so also are cheap sweets. Oxide of iron is added to potted meats, sauces, etc., to improve their appearance. Bottled peas, which if untreated would be of a yellowish-green colour, are made to appear bright green by the addition of the poisonous blue vitriol; fortunately this chemical unites with the chlorophyll of the peas to form a compound which is insoluble in the human body and so no great harm is done. We may compare the chlorophyll in a healthy, undoctored green pea with that in a pea which has been treated with blue vitriol; under the microscope we shall notice the striking difference between the two.

When we admire the beautiful crystals which go to the making of a piece of lump sugar we little dream that, if those crystals were pure they would be yellow. The housewife, however, demands her lump sugar white so ultramarine is added to mask the yellow colour and give the sugar its white appearance.

Most of these impurities are difficult or impossible of detection under the microscope; they are added to give the food a more pleasing appearance in the first place, for there are undoubtedly certain people who prefer to consume food which appeals to the eye, though of doubtful purity, rather than unadulterated though perfectly pure fare. When added solely for the sake of appearance it matters little, but the habit of making these additions is frequently cultivated to hide bad material and imperfections in manufacture.

Sometimes adulterants find their way by accident into our food. A good many years ago numbers of people were poisoned by drinking beer, in some cases with fatal results. Tests were made and the beer was found to contain arsenic but how it got there remained a mystery. At length the glucose, a kind of sugar used in making beer and added also to a good many of our foods, was found to contain the substance. Now in the making of glucose, sulphuric acid is used and in this particular case the impure or commercial acid had been taken. This impure acid frequently contains arsenic and, in the case we mention, the results of its use were disastrous.