Osmosis: The Molecular Theory. Larry Howlett

content and temperature.

      Seed Data

      Seeds equilibrated under conditions of constant temperature and relative humidity will approach a certain and repeatable value for temperature and moisture content.

      Although many factors may be important in maintaining seed vitality during short and long-term storage, we limit this discussion to seed temperature and moisture content. Furthermore, we consider only constant temperature and constant seed moisture content during short or long-term storage.

      At equilibrium, there will be no heat or moisture exchange between a seed and its environment. The temperature of the seed will equal the temperature of the environment and the moisture content of the seed will be in balance with the moisture content of the environment.

      Equilibration data for yellow dent corn is shown in figure 2. The graph shows storage conditions that will maintain constant seed moisture content.

      From figure 2, for seed at 15% moisture content, equivalent storage atmospheres are:

      32 °F, 49% relative humidity;

      70 °F, 62% relative humidity;

      And 90 °F, 68% relative humidity.

      At each of these storage atmosphere conditions the seed moisture content will remain constant at 15%. The seed vitality then becomes only a function of the storage temperature. We note that all of this seed storage information is directly available from the seed equilibration curves.

      The equilibration curves were generated with the modified Henderson equation:

      Where: Mc = moisture content dry weight %; rh = relative humidity %; T=temperature ºC (ºC = (ºF-32)/1.8)

      And, the empirical constants for yellow dent corn are:

      K=8.6541x10-5; N = 1.8634; C = 49.810.

      The equilibration curve, figure 2, is actually a combination of the properties of the seed and the properties of atmosphere where the seed is stored.

      Thermodynamics of Moist Air

      The psychrometric chart, figure 3, displays some of the thermodynamic properties of moist air.

      Thermodynamics relates to energy and its transformations. When a sufficient number of thermodynamic properties of a substance are known, a unique value for the remaining properties can be determined. Thermodynamic properties are directly related to the energy of a substance or a system. Thermodynamic properties of standard air are presented in tables and on psychrometric charts as shown in Figure 3.

      In many fields, thermodynamic properties of temperature, pressure, and relative humidity are considered independent thermodynamic variables because they can be directly measured using common instruments. Some commonly used dependent thermodynamic variables are chemical potential, Gibbs free energy, enthalpy, entropy, and vapor pressure.

      For an ideal gas, the thermodynamic properties are related by the perfect gas equation, equation 2. This relationship between the thermodynamic variables has been verified by experiments on many gases and gas mixtures.

      PV = RT (2)

      Pressure, P, volume, V, and temperature, T are thermodynamic properties of a gas and R is the gas constant for a specific gas or mixture of gases.

      If we define two variables (i.e. pressure (P) & temperature (T)), the third thermodynamic variable volume (V) is uniquely determined from the perfect gas equation.

      For moist air with the gas constant determined from the mixture of gases, three thermodynamic variables are required to determine any other thermodynamic property. Typical and readily measured independent values are Pressure, Temperature, and relative humidity. Some of the unique dependent thermodynamic variables for moist air are humidity ratio, enthalpy, density, and vapor pressure. Note that vapor pressure refers to the pressure of the moisture vapor in the air.

      Moisture Vapor Pressure Model

      We select vapor pressure as an important property of moist air since it is useful for many engineering applications. A psychrometric chart for atmospheric air that includes vapor pressure is shown in figure 3. At normal pressure, there is a unique relation between temperature, relative humidity, and moisture vapor pressure of air. The dependent thermodynamic properties of moist air are relatively insensitive to small changes in atmospheric pressure.

      If we apply this thermodynamic data for air (figure 3) to the equilibration chart (figure 2), we can develop a unique thermodynamic property graph for the seed separate from the atmosphere (figure 4). Each value for temperature and relative humidity determines a specific vapor pressure value for the atmosphere. We propose that the seed must exhibit this same vapor pressure be in equilibrium with the atmosphere. We propose that the vapor pressure data shown in figure 4 represents a characteristic property of the seed that is independent of the storage atmosphere.

      We hypothesize that the seed placed in a small-evacuated container at constant temperature will lose some water to the container to produce a moisture vapor pressure in the container. Since the moisture loss from the seed to the container is small, the seed moisture content will remain approximately constant. Thus, we propose that the moisture vapor pressure in the container will correspond to the seed properties shown in figure 4.

      When a seed has equilibrated to its environment, the moisture content in the seed is in equilibrium with the seed's environment.

      We postulate that the moisture vapor pressure in the seed must equal the moisture vapor pressure in the environment as a condition of equilibrium. We propose that this equilibrated value of seed moisture content, vapor pressure, and temperature are state thermodynamic variables of the seed. Thus, these conditions represent a unique characteristic of a seed that is independent of the storage atmosphere and the process by which the seed attained the temperature and moisture content. See Figure 4 Thermodynamic properties of seed.

      Hermetic Seed Storage

      Haberlandt first discussed hermetic storage in 1873. Since that introduction, it has been well documented as a successful method for seed storage. Hermetic containers change temperature with the atmosphere that they are stored in but, do not allow gas or moisture transfer through the container walls. It is recognized that this method of seed storage is successful “largely because a low level of hydration at the time of sealing can be maintained.” (Priestlley, 1986)

      Bass, 1960 presented experimental evidence that seed moisture content remained constant, even as the temperature changed when stored in hermetic containers. His results compared several different container materials including glass, tin, and flexible materials with a layer of foil. All containers provided hermetic storage conditions for the seed as demonstrated by constant seed moisture content.

      Currently seeds are stored both in hermetic containers at temperatures ranging from ambient to cryogenic (Walters, et al 1998) and in controlled atmosphere storage facilities. With both methods, seeds are dried to specific moisture content prior to storage. For controlled atmosphere storage, both the temperature and relative humidity