Ways To Test Seeds For Moisture

LAWRENCE ZELENY.

THE MOISTURE in a seed has a strong bearing on the length of time it remains viable. Seeds may sprout or molds may develop at high levels of moisture, and the seeds may lose viability in a few days.

At the ordinary temperatures, if the relative humidity of the air around the seeds is more than 75 percent, the seeds are likely to support the growth of molds to the extent that they should not be stored even for a short time.

The moisture content of seeds in equilibrium with this critical relative humidity varies among different kinds. For the various cereal grains, an atmospheric relative humidity of 75 percent corresponds to moisture contents in the range of 13.5 to 15 percent.

Seeds high in oil usually have a lower moisture content at this humidity. Moisture levels below those that cause actual sprouting or mold development may still be high enough to support fairly active physiological activity within the living seeds. Such activity will result in time in premature weakening and loss of viability.

Within certain limits, the lower the moisture content of any kind of seed, the greater the time it will maintain viability. The optimum moisture level for the storage of many kinds appears to be between 6 and 8 percent. Excessively low moisture may cause injury to the embryo. Complete dehydration no doubt would destroy the life of the embryo. Unless artificial drying with heat is employed, however, excessive dryness in seeds is rarely a practical problem.

THE BASIC methods for determining moisture in seeds are those by which a weighed sample is heated in an oven at a specified temperature for a specified time or until they attain constant weight. The loss in weight as a result of heating is taken to be equivalent to the moisture content of the original material. Various types of ovens are used, and various temperatures and times of heating are specified.

Actually, in biological materials it probably is not possible to drive off all moisture by heating without at the same time losing at least traces of other volatile substances or else producing weight changes in some of the constituents of the material as a result of oxidation or decomposition. It is difficult by any known method therefore to determine precisely the true moisture content of any sample of seed.

Because different "basic" methods may yield somewhat different results, for comparative purposes it is desirable to make all tests of a given kind of seed by the same method.

AIR-OVEN METHODS are used commonly for moisture determinations. The air ovens are electrically heated. The air within them is at atmospheric pressure and is circulated by convection or mechanical means. A temperature of 130 C. and a heating time of 1 hour are specified for most kinds of seeds.

The loss of weight that occurs during drying, calculated on a percentage basis, is taken to be the percentage of moisture in the seed before drying.

Large seeds, such as cereal grains, beans, and peas, must be ground before one determines moisture by this method in order to provide rapid enough penetration of heat and ready escape of moisture. Small seeds, such as those of the grasses, do not require grinding.

When this method is applied to large seeds that are too wet to be ground easily without losing moisture in the grinding process, a two-stage procedure is used. A weighed portion of the seeds is partly dried by exposing it to the air in a warm place. The loss of weight in this preliminary drying is determined. The partly dried sample is then ground, and the moisture content of it is determined in the usual manner. The moisture lost in both stages of the procedure must be considered in calculating the moisture content of the original seeds.

Seeds of a high oil content usually should not be ground for oven moisture determinations because they are difficult to grind properly and because oxidation of the oil during drying may result in a gain in weight of the oil. Errors therefore may be made in the determination of moisture. Oxidation of oil is a particularly serious consideration in seeds, such as those of flax that contain oils of high iodine number ("drying" oils).

Certain seeds contain constituents other than moisture that are volatile at 130 C. and hence cannot be subjected to that temperature in the determination of moisture without introducing errors in the determination.

The rules of the International Seed Testing Association specify that for such seeds a drying temperature of 105 C. be used and that the drying time be 16 hours. The following seeds are in this classification: Shallot (Allium ascalonicum), onion (Allium cepa), leek (Allium porrum), garlic (Allium sativum), carob (Ceratonia siliqua), soybean (Glycine max), and radish (Raphanus sativus).

The seeds of Abies (fir) and Picea (spruce) contain constituents of such high volatility that oven methods are not recommended. The toluene distillation method is recommended.

Air-oven methods are specified as the basic methods for determining moisture under the Official Grain Standards of the United States and the United States Standards for rice, beans, peas, and lentils. A 130 C., 1 - hour, air-oven method is specified for wheat, barley, oats, rye, grain sorghums, soybeans, rice, peas, and lentils. The method provides for grinding the seed before drying. A 103 C., 72-hour air-oven method is specified for corn and beans and a 103 C., 4-hours, air-oven method is specified for flax seed. These methods for corn, beans, and flax do not require grinding of the seed.

Special types of air ovens have a built-in balance for weighing the dried samples while they are still in the oven. The balances are calibrated directly in terms of percentage of moisture so that no calculation is required when the designated weight of sample is used initially.

A vacuum-oven method is one of the official methods of the Association of Official Agricultural Chemists for determining moisture in the cereal grains. A weighed portion of the finely ground grain is heated at 98 to 100 C. in an oven in which a partial vacuum is maintained at a pressure equivalent to 25 millimeters of mercury or less. Heating is continued until no appreciable further loss of weight occurs (usually about 5 hours). The moisture content is calculated from the loss of weight as in air-oven methods. The results, applied to cereal grains, are approximately the same as those obtained by the 130 C., 1 -hour, air-oven method.

DESICCANTS drying agents are used sometimes to remove moisture from materials and thus to provide a method for determining moisture. The moisture in seeds and other materials may be determined by placing a weighed amount of the finely ground material in a closed container with a relatively large amount of an efficient desiccant.

The desiccant must have a lower vapor tension than the material that is being dried. The moisture in the material will gradually be vaporized and absorbed by the desiccant. Moisture content is determined by the loss in weight of the original material after it finally attains constant weight.

Reducing and maintaining the atmospheric pressure in the container to a low level will greatly reduce the time required to complete the operation,but even when this is done the time required is too great for most practical purposes. In one of the official methods of the Association of Official Agricultural Chemists for determining moisture in grain, however, the finely ground material is held under vacuum in the presence of concentrated sulfuric acid until constant weight is attained.

One advantage of the method is that it does not involve the hazard of possible decomposition of organic material by heat. Seed of high moisture content, however, may decompose as a result of the action of molds and bacteria before the moisture is reduced enough to inhibit the growth of the organisms.

THE TOLUENE distillation method is sometimes used. A weighed portion of the finely ground seed is boiled in toluene in an apparatus that condenses the volatilized materials, collects the condensed water in a tube, and returns the condensed toluene to the boiling flask. The boiling is continued as long as any water continues to accumulate in the tube provided for that purpose, and the moisture in the seed is calculated from the volume of water condensed.