Human Foods and Their Nutritive Value Part 8

158. Composition of Wheat Offals.--Bran and shorts are characterized by a high percentage of fiber, or cellulose. The ash, fat, and protein content of bran are all larger than of flour. The protein, however, is not in the form of gluten, but is largely alb.u.min and globulins,[16]

which are mainly in the aleurone layer of the wheat kernel, and are inclosed in branny capsules, and consequently are in a form not readily digested by man.

[Ill.u.s.tration: FIG. 42.--FLOUR AND GLUTEN.

1, flour; 2, dough; 3, moist gluten; 4, dry gluten.]

The germ is generally included in the shorts, although occasionally it is removed for special commercial purposes. It is sometimes sterilized and used in breakfast food products. The germ is rich in oil and is excluded from the flour mainly because it has a tendency to become rancid and to impart to the flour poor keeping qualities. Wheat oil has cathartic properties, and it is believed the physiological action of whole wheat and graham bread is in part due to the oil. The germ is also rich in protein, mainly in the form of globulins and proteoses. A dough cannot be made of pure germ, because it contains so little of the gliadin and glutenin.

159. Aging and Curing of Flour.--Flours well milled and made from high-grade, cleaned wheat generally improve in bread-making value when stored in clean, ventilated warehouses for periods of three to six months[9]. High-grade flour becomes drier and whiter and produces bread of slightly better quality when properly cured by storage. If the flour is in any way unsound, it deteriorates during storage, due to the action of ferment bodies. Wheat also, when properly cleaned and stored, improves in milling and bread-making value. Certain enzymic changes appear to take place which are beneficial. Wheats differ materially from year to year in bread-making value, and those produced in seasons when all the conditions for crop growth are normal do not seem to be so much improved by storing and aging, either of the wheat or the flour, as when the growing season has been unfavorable. When wheat is stored, specific changes occur in both the germ and the cells of the kernel; these changes are akin to the ripening process, and appear to be greater if, for any reason, the wheat has failed to fully mature or is abnormal in composition.

The flour yield of wheat is in general proportional to the weight per bushel of the grain, well-filled, heavy grain producing more flour than light grain.[61] The quality of the flour, however, is not necessarily proportional to the weight of the grain. It is often necessary to blend different grades and types of wheat in order to secure good flour.

160. Macaroni Flour is made from durum wheat, according to Saunders a variety of hard, spring wheat. It is best grown in regions of restricted rainfall. Durum and other varieties of hard spring wheat grown under similar conditions, differ but little in general chemical composition, except that the gluten of durum appears to have a different percentage of gliadin and glutenin, and the flour has a more decided yellow color.

Durum wheats are not generally considered as valuable for bread making as other hard wheat. They differ widely in bread-making value, some being very poor, while others produce bread of fair quality.[68]

161. Color.--The highest grades of flour are white in color, or of a slight creamy tinge. Dark-colored, slaty, and gray flours are of inferior quality, indicating a poor grade of wheat, poor milling, or a poor quality of gluten. Flours, after being on the market for a time, bleach a little and improve to a slight degree in color. Color is one of the characteristics by which the commercial value of flour is determined; the whiter the flour, the better the grade, provided other properties are equal[9]. The color, however, should be a pure or cream white. Some flours have what is called a dead white color, and, while not objectionable as far as color is concerned, they are not as valuable for bread-making and general commercial purposes. One of the princ.i.p.al trade requirements of a flour is that it possess a certain degree of whiteness and none of the objectionable shades mentioned.

To determine the color of a flour, it is compared with a standard. If it is a winter wheat flour, one of the best high-grade winter patents to be found on the market is selected, and the sample in question is compared with this; if it is a spring wheat patent flour, one of the best spring wheat patent grades is taken as the standard. In making the comparison, the flours should be placed side by side on a gla.s.s plate and smoothed with the flour trier, the comparison being made preferably by a north window. Much experience and practice are necessary in order to determine with accuracy the color value of a flour.

162. Granulation.--The best patent grades of flour contain an appreciable amount of granular middlings, which have a characteristic "feel" similar to fine, sharp sand. A flour which has no granular feeling is not usually considered of the highest grade, but is generally a soft wheat flour of poor gluten. However, a flour should not be too coa.r.s.ely granulated. The percentage amounts of the different grades of stock in a flour can be approximately determined by means of sieves and different sized bolting cloths. To test a flour, ten grams are placed in a sieve containing a No. 10 bolting cloth; with a camel's-hair brush and proper manipulation, the flour is sieved, and that which pa.s.ses through is weighed. The percentage amount remaining on the No. 10 cloth is coa.r.s.er middlings. Nearly all high-grade flours leave no residue on the No. 10 cloth. The sifted flour from the No. 10 cloth is also pa.s.sed through Nos. 11, 12, 13, and 14 cloths[63]. In this way the approximate granulation of any grade of flour may be determined, and the granulation of an unknown sample be compared with that of a standard flour. In determining the granulation of a flour, if there are any coa.r.s.e or discolored particles of bran or dust, they should be noted, as it is an indication of poor milling. When the flour is smoothed with a trier, there should be no channels formed on the surface of the flour, due to fibrous impurities caught under the edge of the trier. A hand magnifying gla.s.s is useful for detecting the presence of abnormal amounts of dirt or fibrous matter in the flour.

163. Capacity of Flour to absorb Water.--The capacity of a flour to absorb water is determined by adding water from a burette to a weighed amount of flour until a dough of standard consistency is obtained. Low absorption is due to low gluten content. A good flour should absorb from 60 to 65 per cent of its weight of water. In making the test, it is advisable to determine the absorption of a flour of known baking value at the same time that an unknown flour is being tested. Flours of low absorption do not make breads of the best quality; also there are a smaller number of loaves per barrel, and the bread dries out more readily.

164. Physical Properties of Gluten.--The percentages of wet and dry gluten in a flour are determined as outlined in Experiment No. 27.

Flours of good character should show at least 30 per cent moist gluten and from 10 to 12 per cent dry gluten. The quality of a flour is not necessarily proportional to its gluten content, although a flour with less than 10-1/2 per cent of dry gluten will not make the best quality of bread, and flours with excessive amounts are sometimes poor bread makers. The color of the gluten is also important; it should be white or creamy. The statements made in regard to color of flour apply also to color of the gluten. A dark, stringy, or putty-like gluten is of little value for bread-making purposes.[64] In making the gluten test, it is advisable to compare the gluten with that from a flour of known bread-making value. Soft wheat flours have a gluten of different character from hard wheat flours.

165. Gluten as a Factor in Bread Making.--The bread-making value of a flour is dependent upon the character of the wheat and the method of milling. It is not necessarily dependent upon the amount of gluten, as the largest volume and best quality of bread are often made from flour of average rather than maximum gluten content. But flours with low gluten do not produce high-grade breads. When a flour contains more than 12 or 13 per cent of proteids, any increase does not necessarily mean added bread-making value. The quality of the gluten, equally with the amount, determines the value for bread-making purposes.

166. Unsoundness.--A flour with more than 14 per cent of moisture is liable to become unsound. High acidity also is an indication of unsoundness or of poor keeping qualities. The odor of a sample of flour should always be carefully noted, for any suggestion of fermentation sufficient to affect the odor renders the flour unsuited for making the best bread. Any abnormal odor in flour is objectionable, as it is due to contamination of some sort, and most frequently to fermentation changes.

A musty odor is always an indication of unsoundness. Some flours which have but a slight suggestion of mustiness will, when baked into bread, have it more p.r.o.nounced; on the other hand, some odors are removed during bread making. Flours may absorb odors because of being stored in contaminated places or being shipped in cars in which oil or other ill-smelling products with strong odors have previously been shipped.

Unsoundness is often due to faulty methods in handling, as well as to poor wheat, or to lack of proper cleaning of the wheat or flour.

[Ill.u.s.tration: FIG. 43.--FUNGOUS GROWTH IN UNSOUND FLOUR.]

167. Comparative Baking Tests.--To determine the bread-making value of a flour, comparative baking tests, as outlined in Experiment No. 29, are made; the flour in question is thus compared as to bread-making value with a flour of known baking quality. In making the baking tests, the absorption of the flour, the way in which it responds in the doughing process, and the general properties of the dough, are noted. The details should be carried out with care, the comparison always being made with a similar flour of known baking value, and the bread should be baked at the same time and under the same conditions as the standard. The color of the bread, the size and weight of the loaf, and its texture and odor, are the princ.i.p.al characteristics to be noted.

[Ill.u.s.tration: FIG. 44.--COMPARATIVE BAKING TESTS.]

The quality of flour for bread-making purposes is not strictly dependent upon any one factor, but appears to be the aggregate of a number of desirable characteristics. The commercial grade of a flour can be accurately determined from the color, granulation, absorption, gluten and ash content, and the quality of the bread. Technical flour testing requires much experience and a high degree of skill.

168. Bleaching.--In the process of manufacture, flours are often subjected to air containing traces of nitrogen peroxide gas, generated by electrical action and resulting in the union of the oxygen and nitrogen of the air. This whitens and improves the color of the flour.

Bleached flours differ neither in chemical composition nor in nutritive value from unbleached flours, except that bleached flours contain a small amount (about one part to one million parts of flour) of nitrite reacting material, which is removed during the process of bread making.

The amount of nitrites produced in flour during bleaching is less than is normally present in the saliva, or is found naturally in many vegetable foods, or in smoked or cured meats, or in bread made from unbleached flour and baked in a gas oven where nitrites are produced from combustion of the gas. The bleaching of flour cannot be regarded as in any way injurious to health or as adulteration, and a bleached flour which has good gluten and bread-making qualities is entirely satisfactory. It is not possible to successfully bleach low-grade flours so they will resemble the high grades, because the bran impurities of the low grades blacken during bleaching and become more prominent.

Alway, of the Nebraska Experiment Station, has shown that there is no danger to apprehend from over-bleaching, for when excess of the bleaching reagent is used, flours become yellow in color[65]. Similar results have been obtained at the Minnesota Experiment Station. As bleaching is not injurious to health, and as it is not possible through bleaching to change low grades so as to resemble the patent grades, bleaching resolves itself entirely into the question of what color of flour the consumer desires. Pending the settlement of the status of bleaching the practice has been largely discontinued.

[Ill.u.s.tration: FIG. 45.--WHEAT HAIRS AND DeBRIS IN LOW GRADE FLOURS.]

169. Adulteration of Flour.--Flour is not easily adulterated, as the addition of any foreign material interferes with the expansion and bread-making qualities and hence is readily detected. The mixing of other cereals, as corn flour, with wheat flour has been attempted at various times when wheat commanded a high price, but this also is readily detected, by microscopic examination, as the corn starch and wheat starch grains are quite different in mechanical structure. Such flours are required to be labeled, in accord with the congressional act of 1898, when Congress pa.s.sed, in advance of the general pure food bill, an act regulating the labeling and sale of mixed and adulterated flours.

Various statements have been made in regard to the adulteration of flour with minerals, as chalk and barytes, but such adulteration does not appear to be at all general.

170. Nutritive Value of Flour.--From a nutritive point of view, wheat flour and wheat bread have a high value.[66] A larger amount of nutrients can be secured for a given sum of money in the form of flour than of any other food material except corn meal. According to statistics, the average per capita consumption of wheat in the United States is about 4-1/2 bushels, or, approximately, one barrel per year, and from recent investigations it would appear that the amount of flour used in the dietary is on the increase. According to the Bureau of Labor, flour costs the average laborer about one tenth as much as all other foods combined, although he secures from it a proportionally larger amount of nutritive material than from any other food.

CHAPTER XI

BREAD AND BREAD MAKING

171. Leavened and Unleavened Bread.--To make unleavened bread the flour is moistened and worked into a stiff dough, which is then rolled thin, cut into various shapes, and baked, forming a brittle biscuit or cracker.

The process of making raised or leavened bread consists, in brief, of mixing the flour and water in proper proportions for a stiff dough, together with some salt for seasoning, and yeast (or other agent) for leavening. The moistened gluten of the flour forms a viscid, elastic, tenacious ma.s.s, which is thoroughly kneaded to distribute the yeast. The dough is then set in a warm place and the yeast begins to grow, or "work," causing alcoholic fermentation, with the production of carbon dioxid gas, which expands the dough, or causes it to "rise," thus rendering it porous. After the yeast has grown sufficiently, the dough is baked in a hot oven, where further fermentation is stopped because of destruction of the yeast by the heat, which also causes the gas to expand the loaf and, in addition, generates steam. The gas and steam inflate the tenacious dough and finally escape into the oven. At the same time the gluten of the dough is hardened by the heat, and the ma.s.s remains porous and light, while the outer surface is darkened and formed into a crust.

When the flour is of good quality, the dough well prepared, and the bread properly baked, the loaf has certain definite characteristics. It should be well raised and have a thin, flinty crust, which is not too dark in color nor too tough, but which cracks when broken; the crumb, as the interior of the loaf is called, should be porous, elastic, and of uniform texture, without large holes, and should have good flavor, odor, and color.

Meal or flour from any of the cereals may be used for unleavened bread, but leavened bread can be made only from those that contain gluten, a mixture of vegetable proteids which when moistened with water becomes viscid, and is tenacious enough to confine the gas produced in the dough. Most cereals, as barley, rice, oats, and corn, some of which are very frequently made into forms of unleavened bread, are deficient or wholly lacking in gluten, and hence cannot be used alone for making leavened bread. For the leavened bread, wheat and rye, which contain an abundance of gluten, are best fitted, wheat being in this country by far the more commonly used.

172. Changes during Bread Making.--In bread making complex physical, chemical, and biological changes occur. Each chemical compound of the flour undergoes some change during the process. The most important changes are as follows[64]:

1. Production of carbon dioxid gas, alcohol, and soluble carbohydrates as the result of ferment action.

2. Partial rupturing of the starch grains and formation of a small amount of soluble carbohydrates due to the action of heat.

3. Production of lactic and other organic acids.

4. Formation of volatile carbon compounds, other than alcohol and carbon dioxid.

5. Change in the solubility of the gluten proteins, due to the action of the organic acids and fermentation.

6. Changes in the solubility of the proteids due to the action of heat, as coagulation of the alb.u.min and globulin.

7. Formation and liberation of a small amount of volatile, nitrogenous compounds, as ammonia and amids.

8. Partial oxidation of the fat.

173. Loss of Dry Matter during Bread Making.--As many of the compounds formed during bread making are gases resulting from fermentation action, and as these are volatile at the temperature of baking, appreciable losses necessarily take place. Experiments show about 2 per cent of loss of dry matter under ordinary conditions. These losses are not confined to the carbohydrates alone, but also extend to the proteids and other compounds. When 100 pounds of flour containing 10 per cent of water and 90 per cent of dry matter are made into bread, the bread contains about 88 pounds of dry matter. In exceptional cases, where there has been prolonged fermentation, the losses exceed 2 per cent[64].

[Ill.u.s.tration: FIG. 46.--BREWERS' YEAST.]

174. Action of Yeast.--Yeast is a monocellular plant requiring sugar and other food materials for its nourishment. Under favorable conditions it rapidly increases by budding, and as a result produces the well-known alcoholic fermentation. It requires mineral food, as do plants of a higher order, and oftentimes the fermentation process is checked for want of sufficient soluble mineral food. The yeast plant causes a number of chemical changes to take place, as conversion of starch to a soluble form and alcoholic fermentation.

C_{6}H_{10}O_{5} + H_{2}O = C_{6}H_{12}O_{6}.

C_{6}H_{12}O_{6} = 2 C_{2}H_{5}OH + 2 CO_{2}.

Alcoholic fermentation cannot occur until the starch has been converted into dextrose sugar. The yeast plant is destroyed at a temperature of 131 F. It is most active from 70 to 90 F. At a low temperature it is less active, and when it freezes the cells are ruptured. A number of different kinds of fermentation are a.s.sociated with the growth of the yeast plant, and there are many varieties of yeast, some of which are more active than others. For bread making an active yeast is desirable to prevent the formation of acid bodies. If the work proceeds quickly, the rising process is completed before the acid fermentation is far advanced. If fermentation is too prolonged, some of the products of the yeast plant impart an undesirable taste and odor to the bread, and hinder the development of the gluten and expansion of the loaf.

175. Compressed Yeast.--The yeast most commonly used in bread making is compressed yeast, a product of distilleries. The yeast floating on the surface of the wort is skimmed off and that remaining is allowed to settle to the bottom, and is obtained by running the wort into shallow tanks or settling trays. It is then washed with cold water, and the impurities are removed either by sieving through silk or wire sieves, or, during the washing, by fractional precipitation. The yeast is then pressed, cut into cakes, and wrapped in tinfoil. When fresh, it is of uniform creamy color, moist, and of a firm, even texture[18]. It should be kept cold, as it readily decomposes.