Ketchup: Methods of Manufacture; Microscopic Examination

by A. W. Bitting; K. G. Bitting

MICROSCOPIC EXAMINATION.

A discussion of the microscopic appearance of ketchup in terms which can be readily understood by manufacturers is not an easy task, as it necessarily involves technical knowledge. The subject has become one of importance, owing to the attitude of many food officials in enforcing a microscopic standard for this product, and on the part of many brokers in requiring a guarantee to comply with this standard in making purchases. Many manufacturers have either assumed or found it necessary to have their finished products examined. Some employ “experts” to make the examinations in their own plants, while the majority send their samples to commercial laboratories. The total tax upon the industry for such work amounts to thousands of dollars annually. The result of the work as a whole has been beneficial, as any effort is which attracts attention to details. It has likewise been the means of causing much unpleasantness and not infrequently loss, because of lack of understanding on the part of both manufacturer and examiner as to the cause of certain findings. The manufacturers have proceeded in the usual way without sufficient knowledge of what the resultant product will be unless there is careful supervision of material and methods, while too frequently the examiner is neither experienced in technique of the examination nor in the effects of the different steps in manufacture upon the product. Furthermore, much distrust in microscopic finding is evinced when a half dozen or more samples from the same batch, sent to as many persons, result in as many different reports. It naturally causes a lack of confidence in both paid examiners and in food officials, though those who make these examinations may be absolutely honest in their findings. In order to clarify some of the points, it has become necessary to go into detail, into both the method of examination and into the effect produced by manufacture.

A scientific method of food examination is necessary for food officials in order to determine the condition of a product, but is not necessary for the manufacturer, though it may be advantageous. The latter is in a position to know what enters his factory and what changes take place in the food until it reaches the sealed package. He should have no fear of a method which correlates the findings in the finished product with that of the material used and the changes due to treatment.

Undue importance may seemingly be given to the subject of ketchup, but the principle involved applies as well to other products.

The fundamental basis for the microscopic examination of any food product must depend upon the structure of the material which enters into its composition. Any attempt to determine an abnormal condition, such as decomposition, without a knowledge of the normal, must necessarily be of little value. There is some work which can be done in a mechanical manner by almost anyone capable of looking through a microscope, and if the work is properly supervised, it may have a value, but the lines along which this can be done are very limited. Any attempt to apply such superficial methods to the general examination of food products can not properly protect the public and may be unfair to the producer. It has, therefore, been deemed advisable to incorporate a brief statement concerning the structure of the tomato before discussing the resultant products.

HISTOLOGY OF THE TOMATO AND OF THE RESULTING KETCHUP.
STRUCTURE OF THE TOMATO.

Pericarp. The tomato is a typical berry, the ovary wall, free from the calyx, forming the fleshy pericarp, which encloses chambers filled with a clear matrix, containing the seeds. The pericarp consists of an outer tough membrane, the epidermis, a more or less thick layer of parenchyma tissue, the pulp, and an inner thin, delicate membrane, the lining layer of the loculi or chambers in which are the seeds. The epidermis consists of a single layer of cells which have a very thick continuous cuticle about one-half of the diameter of the whole cell. The cuticle differs in chemical composition from the rest of the cell walls, being impervious to water, and resisting rotting longer than do the cellulose walls. As it is continuous over the whole of the fruit, the skin can be readily separated from the other tissues. Hot water facilitates the removal of the skin, as it causes the cellulose of the walls to swell more than the cuticle, producing an effect as of shrinkage of the outer wall and a consequent curling of the skin. The radial walls of the epidermis are short and irregularly thickened, leaving pits in the walls, and giving them a beaded appearance. The skin constitutes about 1.3 per cent of the tomato.

The layers of parenchyma just beneath the epidermis are closely united and flattened, with their adjoining walls irregularly thickened. On account of their position, they are called hypoderm. In the tomato the hypoderm consists of two or three layers of cells, parts of which usually separate with the epidermis. Below these cells are the thin-walled parenchyma cells, which are approximately globular, vary considerably in size, are very loosely held together, and have many intercellular spaces. These cells constitute the mass of the pulp, and with the juice constitute 96.2 per cent of the tomato.

The layer of cells which lines the chambers has the typical leaf epidermal structure, the wavy outlines, the hollows and protuberances of adjoining cells fitting one another so that they form a continuous layer. They are also flattened laterally. The structure can be understood readily when it is known that the pericarp is really a metamorphosed leaf and that the outer side of the leaf forms the inner wall of the ovary.

The chambers of the tomato are filled with a clear, slimy matrix in which the seeds are embedded. The matrix consists of parenchyma cells of various sizes and with delicate walls, and a small nucleus. The cells are massed loosely, and can be separated readily. In those cells, as well as in the wall cells, are starch grains which vary in size, being round or approximately so, and having the hilum, when visible, a straight line to one side of the center.

Coloring Matters. In the parenchyma cells are two coloring matters, one yellow, which is amorphous in structure, and the other red and of crystalline form. The sap contains a yellow color in solution which differs in its reactions from those in the pulp.

Red Color in Tomatoes. The red coloring matter in tomatoes is in the form of irregularly shaped crystal-like chromoplasts, which occur in masses of various sizes. They are present in largest amounts usually in the protoplasm which lies close to the ectoplasm and in that surrounding the nucleus. They vary from sharp, bright-colored forms to those more or less blunt in outline, and dull in color. They may be situated largely in the periderm, the soft parenchyma beneath the periderm, or through the whole mass of the parenchyma with the exception of the matrix surrounding the seeds in the loculi. In tomatoes having the color in the periderm a considerable amount is lost by adherence to the skin. The chromoplasts are not affected by rotting to the same extent as are the other constituents of the cell; they can be found floating free in the debris from rotted cells, still retaining considerable color. They lose their color gradually, in some varieties much more rapidly than in others. In stored pulp which has fermented, the color may be faded to a dull yellowish brown. In tomatoes intended for ketchup where a bright red color is desirable, care should be used in the selection of a variety having the chromoplasts bright, properly oriented, and in sufficient quantity.

Vascular Bundles. In the pulp of the tomato are found strands of vascular tissue, entering from the stem, and dividing and ramifying through the soft pulp. These consist of long tubes with thin walls, some of which have a strengthening band in spiral form on their interior walls, the associated cells being without any special marking. The strands vary in size from those having a few tubes to those having a large number.

Seeds. The seeds of the tomato are small, flattened, yellow bodies covered by a clear gelatinous membrane. Their peculiar characteristic is the out-growth of hairs of varying lengths. The seeds constitute about 2.5 per cent of the weight of the tomato.