RELATIONSHIP OF THE MOISTURE CONTENT OF FINNISH WHEAT FLOUR AND RELATIVE HUMIDITY Yu-Yen Linko and Pekka Linko Research and Development Laboratories, Vaasan Höyrymylly Osakeyhtiö Helsinki, Finland Received October 23, 1967 It is generally known that flour moisture content varies with the relative humidity of air. The relative humidity at which a product, at a given temperature, neither gains nor loses moisture is called equilibrium humidity. If the equilibrium humidity is lower than the atmospheric relative humidity, the moisture content of the product tends to increase, and vice versa. Although the moisture relationships of different cereal grains are similar (Coleman and Fellows 1925), the adsorption and desorption isotherms of cereal flours are not the same. Thus different samples of same flour in equilibrium with the same atmospheric relative humidity may have a different moisture content (Babbit 1949). The equilibrium moisture levels of flour are higher during desorption than during adsorption, a phenomenon called hysteresis. Therefore, the moisture loss during drying may be gained back only at a relative humidity higher than the original. The relationship of the moisture content of wheat flour and relative humidity, as obtained by several workers (Andersson 1937, Anker et al. 1942, Bailey 1920, Fairbrother 1929), is summarized below; Flour moisture % Relative humidity Bailey Fairbrother Anderson Anker et al. % 25°C 37C° 30 5.2 9.4 9.7 8.5 40 6.5 9.7 11.1 9.9 50 8.0 10.7 12.0 12.3 11.1 60 9.8 12.8 13.7 13.2 12.3 70 12.1 14.0 15.4 14.5 13.7 80 15.0 15.7 16.3 15.8 90 16.7 15 It appears that the equilibrium humidity of flour of 14 % moisture is about 65 %, that of flour of 15 % moisture about 70 %, and that of flour of 16 % moisture about 75 %. The latter corresponds to that portion of the isotherm where relatively large quantities of »free» water are believed to become available to activate various grain enzymes and to be utilized by micro-organisms (Linko 1960). Thus, in practice, cereal products in equilibrium with relative humidities of 75—80 % or higher may be expected to deteriorate rapidly at normal storage temperatures. Although the relative humidity varies considerably in different parts of Finland, the monthly averages obtained from the measurements at various air port locations during ten years indicate relatively low monthly average humidities during the warm summer months: Relative humidity % Minimum Maximum January 87 92 February 84 89 March 77 S 4 April 71 79 May 64 71 June 64 70 July 70 76 August 77 *'-' September 81 86 October 86 90 November 89 93 December 88 93 During winter, the relative humidity in warehouses and storage rooms equipped with central heating may, for long periods, stay as low as 20—30 %. It may there- fore be expected that flour of original moisture content of 15 % would tend to lose moisture during normal storage, and no special package appears necessary to prevent moisture migration. However, certain special cereal products, such as some types of prepared cake mixes, may have equilibrium humidity values as low as 30 % requiring a carefully chosen package to prevent a possible increase in moisture (Borchardt 1950). The purpose of the present study was to obtain information on the changes in moisture content of commercial Finnish wheat flour during storage at different relative humidities. Materials and methods A typical commercial Finnish wheat flour packed in standard 1000 g paper bags was chosen for the object of study, since the possible variations in flour moisture and net weight are greatest in a relatively small package. The average moisture content of flour at the time of packing was 15 %. Several samples of flour were stored in a warehouse at normal conditions, with a monthly check of the moisture content and a continuous recording of the relative humidity and temperature. Other samples were stored in air-tight glass chambers equipped with a fan and a hygrometer. The relative humidity was adjusted by means of water solutions of sulphuric acid. The 16 observed relative humidity ranges in various chambers were 32—34 %, 37—41 %, 44—46 %, 51—61 %. 63—68 %, 70—72 %, 80—87 % and 91 %. Some flour bags were occasionally transferred from one humidity chamber to another. The moisture content of flour was determined by drying for one hour at 130° C. Results and discussion Figure 1. shows the average changes in moisture content of ten flour bags stored in the warehouse for one year, starting at the end of July. The relative humidity in the storage room decreased to a minimum of 20 % at the end of January. By that time the flour moisture content had dropped to 6.7 % from the original 15 %, representing a decrease of about 9.2 % in net weight. Thereafter therelative humidity increased gradually, approaching the starting value of 65 % at the end of July. During this time, the flour moisture content increased slowly to 10.5 %. After 12 months of storage, there had been a considerable total decrease in flour moisture, corresponding to a net weight decrease of about 50 g from the original 1000 g at 15 % moisture. These results clearly show that flour tends to dry considerably during normal storage conditions in Finland, the relative humidity in the ware- house rarely reaching the equilibrium humidity value of about 70 % for flour of 15 % moisture content. This is of importance since the quality of flour of low mois- ture content is affected little even during relatively long periods of storage. Examples of changes in the flour moisture content diming storage at variable relative humidities for a total period of two months are shown in Figure 2. The Figure 1. The variation of flour moisture content and net weight during warehouse storage for one year. The corresponding relative humidities are given on the graph. The area between the thin lines represents the magnitude of variation among the ten flour bags studied. 17 results are in agreement with the observations of earlier workers (Anderson 1937, Anker et al. 1942, Bailey 1920, Fairbrother 1929), confirming the observation that the equilibrium humidity for wheat flour of 15 % moisture content is approxi- mately 70%. The results also show, however, that flour which has once dried, reaches only a slightly higher than 14 % moisture level after return to about 70 % relative humidity. A very rapid increase in moisture was observed at about 90 % relative humidity, supporting the observation of Fairbrother (1929). On the other hand, during storage of flour at relatively low relative humidities, 38—39 %, a marked drying took place and no equilibrium was reached during 60 days of storage. During this time the moisture content had decreased to about 9.3—9.5 %, repre- senting a loss of approximately 5.5 % in net weight. This is a somewhat higher loss of moisture than a decrease from 14 % to about 10.5 % at 36 % relative humi- dity, as reported by Anker et al. (1942). According to Figure 1., the equilibrium moisture content at 38—39 % relative humidity would be about 8.5—9.0 %. The results show that rapid and marked changes will take place in the moisture content of flour stored at variable relative humidities in standard 1000 g paper bags. The average storage conditions, even during most of the summer, are such that flour tends to lose moisture. During winter months this loss may be considerable, occasion- ally resulting in a flour moisture content of less than 7 %. On approaching the equilibrium humidity, the net weight of flour follows closely the changes in relative humidity, although, owing to the hysteresis effect, the original moisture level and net weigt of once dried flour may rarely be regained at the normal warehouse relative humidities in Finland. It may also be concluded that the moisture content of flour of about 15 % original moisture is not likely to increase to detrimental levels during 2 Figure 2. The variation of flour moisture content and net weight during storage at variable relative humidities for two months. The percentile relative humidities for each storage period marked V- •-•-• -V are given on the graph. 18 normal storage in Finland, even if the relative humidity would exceed the critical level of 75 —80 % for short periods. Summary Changes in the moisture content of Finnish commercial wheat flour stored at variable relative humidities, representing the conditions typical of flour storage in Finland, were investigated. It could be shown that flour of 15 % moisture at the time of packing tends to dry considerably during normal storage conditions. Owing to the hysteresis effect, the moisture content of once dried flour is not likely to reach detrimental levels during normal storage, even if the relative humidity would exceed the critical level of 75 —80 % for short periods. Minimum warehouse relative humidity was observed during Januay, at which time flour moisture had decreased to 6.7 %. The equilibrium humidity for flour of 15 % original moisture content was found to be about 70 %. REFERENCES: Anderson, J. E, 1937. Equilibrium relative humidities for hard wheat and mill stocks. Northwestern Miller 192: 20-22. Anker, C. A., Geddes, W. F. & Bailey, C. H. 1942. A study on the net weight changes and moisture content of wheat flour at various relative humidities. Cereal Chem. 19: 128—150. Babbit, J. D. 1949. Observations on the adsorption of water vapour by wheat. Can. Jour. Research 27 F; 55-72. Bailey, C. H. 1920. The hygroscopic moisture of flour exposed to atmospheres of different relative humidity. Ind. Eng. Chem. 12: 1102 1103. Borchardt, L. F. 1950. The story of Pyequick. Modern Packaging 23; 121 124, 166. Coleman, D. A. & Fellows, H. C. 1925. Hygroscopic moisture of cereals, grains and flaxseed exposed to atmospheres of different relative humidity. Cereal Chem. 2; 275 287. Fairbrother, T. H. 1929. The influence of environment on the moisture content of flour and wheat. Cereal Chem. 6: 379-395. Linko, P. 1960. Water content and metabolism of wheat during short storage and germination. Ann. Acad. Sei. Fennicae A II 98: 1—69. SELOSTUS: ILMAN SUHTEELLISEN KOSTEUDEN VAIKUTUKSESTA KOTIMAISEN VEHNÄJAUHON KOSTEUSPITOISUUTEEN Yu-Yen Linko ja Pekka Linko Tuoletutkimuslaitos, Vaasan Höyrymylly Osakeyhtiö, Helsinki Kotimaisen vehnäjauhon kosteuspitoisuuden vaihteluita ilman suhteellisen kosteuden muuttuessa Suomelle tyypillisissä varasto-olosuhteissa on tutkittu. Tutkimuksissa voitiin osoittaa, että jauho, jonka kosteus pakkaushetkellä on noin 15 %, pyrkii kuivumaan huomattavasti normaalissa varastointiolosuh- teissa. Hystereesi-ilmiöstä johtuen jauhon kosteuspitoisuus ei tyypillisissä varastointiolosuhteissa yleensä pääse kohoamaan haitallisen korkeaksi, vaikka ilman suhteellinen kosteus joksikin aikaa ylittäisikin kriitillisen rajan 75 —BO %. Varastoilman alhaisin suhteellinen kosteus, 20 %, todettiin tammikuussa. Tällöin jauhon kosteuspitoisuus oli laskenut 6,7 %;iin. Jauhon, jonka kosteus pakkaushetkellä on noin 15 %, todettiin olevan tasapainotilassa noin 70 % suhteellisessa kosteudessa.