The moisture content/equilibrium relative humidity relationship and oil composition of rapeseed

The moisture content/equilibrium relative humidity relationships of a British winter-sown rapeseed variety Hektor, a British spring-sown variety Gulle and a Canadian variety Tower, were found to be similar. The adsorption moisture contents at 25°C in equilibrium with 70% relative humidity, conventionally accepted as the threshold for the growth of fungi, ranged from 8.3% for Tower to 7.5% for Hektor. The three varieties have similar oil contents, but Tower has a low content of erucic acid and glucosinolates.     

The relationship between moisture content and equilibrium relative humidity of pig feed

The moisture content-equilibrium relative humidity relationships at 5, 15 and 25°C of 3 pig meals and pellets made mostly from barley meal and wheatings, with various proportions of soya meals, were very similar to each other. The moisture content in equilibrium with 70% r.h. at 25°C was about 14%, similar to that for cereal grains but these pig feeds should be stored at or below 12% mositure content at this temperature, to avoid the risk of the development of mites.     

The relationship between moisture content and equilibrium relative humidity of dried figs

A vacuum oven method is recommended for the determination of the moisture content of figs, by drying sliced quarters of whole figs at 70°C, at a pressure of less than 5 mm Hg for 24 hr. There is a greater variation between the moisture content of whole figs than between quarters of a single fig and the mean moisture content of a sample of figs can be determined with a standard error of 0.2–0.3%, by taking one quarter each from a number of whole figs. Data are presented for the moisture content/equilibrium relative humidity (e.r.h.) relationship of Turkish Lerida figs.     

The relationship between moisture content and equilibrium relative humidity of five types of wheat flour

There was little difference at 5, 15 and 25°C between the moisture content-equilibrium relative humidity relationships of three types of bread flour and a biscuit flour made from wheat. A flour made from wheat a about 18% moisture content heated to 70°C for about 15 min, had a higher equilibrium relative humidity than the other types.     

The moisture content-equilibrium relative humidity relationship of cocoa beans

The moisture content-equilibrium relative humidity relationships for cocoa beans grown in nine different countries were similar to each other, and the maximum safe storage moisture content in equilibrium with 70% relative humidity was between 6.0 and 7.9% at 25°C. There was a large variation between the moisture content of individual beans within each sample, the maximum standard deviation observed in a sample of thirty beans being 0.97, after the moisture content had been increased by adding water.     

Determination of moisture content and equilibrium relative humidity of dried fruit—Sultanas

A vacuum oven method is recommended for the determination of the moisture content of sultanas, drying to constant weight at 70°C, and at a pressure of 5 mm Hg or less, for 24 hr. It is shown that accuracy of better than 0·5 per cent cannot be guaranteed and that day to day variation, and oven position variation are encountered. A distillation method, using toluene as the entrainer, over a period of $\text{2}\text{1}\text{2}\text{hr}$, gives satisfactory estimates of moisture content when a ‘rapid’ but less accurate estimate is required. Data are presented for the moisture content/equilibrium relative humidity (e.r.h.) relationship of Turkish sultanas.     

The moisture content—Equilibrium relative humidity relationship of malt

The equilibrium relative humidity for samples of green malt removed from the kiln during drying at up to 70°C, was higher than that for samples dried in the laboratory at 35°C, the difference being attributed to these drying temperatures. The relationship between moisture content and equilibrium relative humidity of finished malt removed from the kiln after drying and damped up in the laboratory, was also determined. The equilibrium curve for finished malt was closer to the desorption curve obtained by drying in the kiln than to the curve obtained by laboratory drying.     

The moisture content-equilibrium relative humidity relationship of rice bran at different temperatures

The moisture content/equilibrium relative humidity relationships at 15, 25 and 35°C, for unextracted rice brans from Burma and Tanzania, unextracted bran from a South African parboiled rice, and a pelleted extracted rice bran from India, are presented and discussed. The relationships differed in shape and position of the curves, and in the magnitude of hysteresis between adsorption and desorption. The equilibrium moisture contents at 65% r.h. and below, of Burmese, Tanzanian and Indian brans were in reverse order of their fibre and ash contents. These moisture contents did not decrease consistently with oil content as they did for those cereal grains and oil-seeds previously investigated. The hysteresis for the parboiled bran was less, and the equilibrium moisture content was generally lower than for the other brans.     

Moisture content/relative humidity equilibrium of high-oil and confectionery type sunflower seed

Adsorption and desorption moisture equilibrium (m.c./e.r.h.) was determined for high-oil and confectionery type sunflower seeds. The seeds were exposed to various relative humidities which were controlled with environmental chambers. The equilibration data for both seed types were determined at 10 and 20°C. The two seed types yielded almost identical adsorption and desorption moisture curves at 10°C which indicated that seed physical and chemical properties had little effect on m.c./e.r.h. at this temperature. The “hysteresis effect” was evident at 10°C, desorption equilibrium moisture levels being higher than adsorption for both seed types. There was no apparent “hysteresis effect” in seed equilibrated at 20°C. At the higher temperature, the confectionery seed equilibrated to higher moisture levels than did the high-oil type, thus hull to kernel weight ratio, hull thickness, and oil content could affect m.c./e.r.h. at 20°C     

Moisture content/relative humidity equilibrium of some cereal grains at different temperatures

The concept of the equilibrium relative humidity (e.r.h.) of a product as a measure of storage potential is discussed. It is stressed that whilst moisture content gives an estimate of the amount of water in a product it does not, by itself, indicate the biological activity, or potential activity, of the product. Graphs are presented giving the e.r.h., at a given moisture content, for different temperatures of a soft wheat, barley, American yellow dent maize, flint maize grown in England, and tick beans (Vicia faba L.).     

A mean moisture content-equilibrium relative humidity relationship for nine varieties of wheat

A mean moisture content-equilibrium relative humidity relationship for nine varieties of wheat at 25°C is presented, for which the curves were fitted by eye. The mean safe storage moisture content in equilibrium with 70% relative humidity (r.h.) for all the wheat varieties adsorbing moisture was 14.7% (wet weight basis) but if varietal differences and errors in measurement are taken into account, the moisture content of stored wheat may need to be as low as 13% to be safe from attack by mites. The hysteresis between adsorption and desorption reached a maximum of 0.7% moisture content at 40–50% r.h. Hysteresis was found to be statistically significant up to 75% r.h. Equilibrium moisture contents were calculated from data transformed using the Chung and Pfost equation and compared with the measured moisture contents.     

The influence of the method used for moisture adjustment on the equilibrium relative humidity of stored products

This paper examines some possible causes of differences in the magnitude of the hysteresis effect reported by different workers when studying the moisture content/equilibrium relative humidity relationship of stored products.Three techniques of changing the moisture content during sample preparation were used on Cappelle wheat and the results compared. It was found that the position of the adsorption isotherm was not affected by these techniques, but the position of the desorption isotherm was. At 35% r.h. the moisture content was 0·8 per cent higher on the desorption isotherm by one technique than by the other two. This is equivalent to a difference of 6% r.h.The effect of a fungistat, propylene oxide, on the equilibrium relative humidity, was also included. This caused a ‘reverse’ hysteresis, in that the moisture content at a given relative humidity was lower on the desorption isotherm than on the adsorption isotherm instead of higher, as with previous techniques.     

The suitability of various linear transformations to represent the sigmoid relationship of humidity and moisture content

The relationship of the moisture content of a product and the equilibrium relative humidity is usually represented by a sigmoid curve which is not easy to draw or manipulate.Several transformations that attempt to convert this curve into a straight line have been applied to practical data using as the criterion of satisfactory fit, agreement within 0.5% moisture content between the observed data and the predicted line.For starchy material and cereals the Chung and Pfost transformation gave satisfactory predictions. For field beans, dried peas, cereal derivatives other than flour, lupins, almonds and oilseeds the Halsey transformation was the best but that of Smith was satisfactory above 45% r.h. for beans, cereals and oilseeds.With dried fruits also, the Halsey transformation gave the best fit. A satisfactory fit was achieved for figs but not for sultanas. The Smith transformation, and that of Iglesias and Chirife which was specifically proposed for dried fruit, were also satsifactory for figs.For cereals, using the Chung and Pfost transformation, the position of the curve can be satisfactorily determined using three experimental relative humidity values over the range 25–70% and can be adjusted for temperature.     

Use of polyols to measure equilibrium relative humidity

Polyols were used to estimate the equilibrium relative humidity (ERH) of foods. Glycerol, ethylene glycol and propylene glycol were found to be suitable for estimating ERH, up to an ERH of 93%, by measuring the refractive index after they had equilibrated in the headspace of a food. The ERH determined by the polyol method agreed with the ERH determined using Sina and Vaisala meters to within 1% ERH for a lithium sulphate slurry, and for foods were usually within 2%. The ERH of prunes determined using the Vaisala meter was about 6% lower than the value obtained by the other methods. Unlike most electronic techniques the polyol method was unaffected by the presence of glycerol, propylene glycol or butylene glycol and so is suitable for measuring the ERH of foods containing these humectants. The polyol method can be used in developing countries where access to sophisticated equipment is limited.     

Moisture content relative humidity equilibrium, at different temperatures, of some oilseeds of economic importance

Equilibrium relative humidity relationships at temperatures from 10° to 40°C are presented in graph form for soya beans, sunflower seed, linseed, groundnuts and copra.