Equilibrium Relative Humidity Research Articles

Sorption Isotherms of Tomato Slices and Onion Shreds

Experiments were conducted to determine the equilibrium moisture content of tomato (cv Roma) slices and white onion shreds at temperatures of 30, 40 and 50°C and relative humidities of 15–85% obtained from an air conditioning unit. The samples were placed on trays and the equilibrium moisture contents were determined once the samples attained the constant mass. The equilibrium moisture content–equilibrium relative humidity data were fitted to the modified Henderson, Chung-Pfost, modified Halsey, modified Oswin, and modified GAB models. The modified Henderson and modified Halsey models fitted well for the tomato slices and onion shreds, respectively, based on the coefficient of regression, mean relative percent error and standard error of moisture. The values of these parameters were, 0·985, 2·58 and 0·52, respectively, for the modified Henderson model for the sliced tomatoes and 0·991, 2·01 and 0·42, respectively, for the modified Halsey model for onion shreds.

The production of ochratoxin A by Aspergillus ochraceus in raw coffee at different equilibrium relative humidity and under alternating temperatures

Abstract The effect of alternating temperatures in the storage of coffee was studied. From the day and night values, two average temperatures (25 and 14 °C) were chosen. Such changes may occur, mainly during storage in farm barns and transport. The study was carried out under different conditions of equilibrium relative humidity (ERH): 80%, 87% and 95% for the production of ochratoxin A (OTA) by Aspergillus ochraceus in raw coffee. Temperatures were cycled at 12 h intervals. Coffee was also maintained at 25 °C under similar conditions, but without temperature cycling, at the same three values of relative humidity. Ochratoxin production was analysed after periods of 39 and 60 days after the coffee had reached the equilibrium relative humidity. The water activity and moisture content of coffee were checked and OTA production was quantified. There was little or no OTA production at 80% ERH; at 87% and 95% OTA production was high after different days of incubation. Under alternating temperatures OTA production was higher than at constant temperature, and alternating temperatures indirectly favoured OTA production due to condensation and a subsequent rapid increase in moisture content and water activity of the coffee beans.

Water vapour sorption isotherms and the caking of food powders

Water vapour sorption isotherms describe the relationship between water content and water activity. Depending on the nature of food powder (crystalline or amorphous), the shape of isotherm is different. Mostly food powders have complex structures, including potentially crystallisable solutes such as sugars, which show changes in crystallinity during the adsorption of water. Even for such an apparently simple system as crystalline sugar, numerous factors affect the adsorption of water vapour and, as a consequence, the storage stability. The presence of a thin film of saturated solution at the surface of the crystal, grain size distribution and the inclusion of mother liquor droplets in the crystal are some of the factors which perturb the equilibrium relative humidity of sugar and its aptitude to caking. These conditions were carefully studied at the level of the laboratory and in a pilot silo. Conditions of “decaking” (recovering a flowing sugar after caking) were also established. In the case of noncrystalline powders, water activity, together with glass transition temperature, is important to determine if it is necessary to interpret the origin of the formation of bridges between food powder particles and the caking phenomenon.

Sorption equilibrium and thermodynamic properties of cowpea ( Vigna unguiculata)

Equilibrium relative humidity (ERH)–moisture content data for cowpea ( Vigna unguiculata) at five different temperatures (40, 50, 60, 70 and 80 °C) and moisture content values ranging from 4.5% to 22.2% (dry basis) were determined using the vapour pressure manometric method. A non-linear least squares regression programme was used to evaluate four moisture sorption models. The modified Henderson model gave the least standard error of estimate of 5.8% for ERH and 1.3% for equilibrium moisture content (EMC), as a result, this model was considered to be the best for predicting the ERH and EMC of cowpea. The experimental data was also used to determine the thermodynamic functions of cowpea. Latent heat of vaporization decreased with increase in moisture content and approached the latent heat of pure water at the moisture content of 24% (dry basis). The spreading pressure increased with increase in relative humidity but decreased with increase in temperature more remarkably in the high range of relative humidity (RH>0.65). Net integral enthalpy decreased with moisture content and the trend became asymptotic as the moisture content of 18% was approached. Net integral entropy was negative in value and it increased with increase in moisture content in the moisture range of 4–6.4% dry basis. It thereafter, remained constant until the moisture content of 14.2% was reached, and then it continued to increase with increase in moisture content.

Effect of Hydrocolloids on the Dehydration Kinetics, Color, and Texture of Mango Leather

Tray drying of mango pulp was carried out to prepare mango leather and the dehydration behavior was studied. Different hydrocolloids like guar gum, pectin, carboxymethyl cellulose, gum acacia, pectin, and sodium alginate were also added to the mango pulp at levels of 1, 2, and 3% w/w to determine their effects on drying rate, moisture ratio, color, and equilibrium relative humidity. Texture of mango leather was studied using tensile deformation. Hydrocolloids lowered the drying rate of mango leather but only during the initial 2 h of drying. Mango leather had an extensibility, peak force to rupture, deformation modulus and energy to rupture of 14.54 mm, 0.0036 kN, 0.2285 MPa, and 0.0369 J respectively. Hydrocolloids increased the extensibility and energy to rupture of mango leather. Yellowness and redness of samples was lowered but lightness of mango leather was not significantly affected by hydrocolloid addition. The equilibrium relative humidity of mango leather increased with increasing hydrocolloid concentration. The study revealed that hydrocolloids especially guar gum or pectin can be added to modify the texture of mango leather without significantly affecting the drying rate, color, and packaging requirements.

Mutant alleles at the rugosus loci in pea affect seed moisture sorption isotherms and the relations between seed longevity and moisture content.

Pea (Pisum sativum L.) mutant near-isogenic lines (RRrbrb, rrRbRb, rrrbrb) with lower starch but higher lipid contents, brought about by lesions in the starch biosynthetic pathway, had seed moisture sorption isotherms displaced below that of the wild type (RRRbRb). The negative logarithmic relationship between seed longevity and seed storage moisture content (%, f.wt basis), determined in hermetic storage at 65 degrees C, also differed: longevity in the mutant near-isogenic lines was poorer and less sensitive to moisture content than in the wild type (i.e. C(W) was lower). The low-moisture-content limit (m(c)) to this relation also differed, being lower in the mutant near-isogenic lines (5.4-5.9%) than in the wild type (6.1%). In contrast, all four near-isogenic lines showed no difference (P >0.25) in the negative semi-logarithmic relationship between equilibrium relative humidity (ERH) and seed longevity. It is concluded that the effect of these alleles at the r and rb loci on seed longevity was largely indirect; a consequence of their effect on seed composition and hence on moisture sorption isotherms. However, this explanation could not be invoked at moisture contents below m(c) where differences in longevity remained substantial (RRRbRb double that of rrrbrb). Hence, these mutant alleles affected seed longevity directly at very low moisture contents.

Moisture sorption isotherms of pea seeds

Sorption isotherms of pea seeds with three different treatments were measured by equilibrium relative humidity (ERH) determination technique at five temperatures. Four ERH models with a temperature term and the GAB model were adopted to evaluate the goodness-of-fit of sorption isotherms. Sorption data from literature were also selected to compare the effect of varieties on the sorption isotherms of pea seeds. The result of this study indicated that pea seeds revealed a hysteresis and that treatment temperature affected the sorption isotherms. The Modified-Henderson equation was an adequate model for the sorption data of three treatments used in study. No universal or best sorption model was found for the isotherms of pea seeds from published data. The GAB model was not a good model when applied to the sorption properties of pea seeds.

PH—Pastharvest Technology: Sorption Isotherms of Sweet Potato Slices

The equilibrium relative humidity of sweet potato slices with two drying temperatures and different histories were determined at temperatures ranging from 5 to 50°C. Estimated parameters, fitting ability, and predictive performance for five equilibrium relative humidity models were evaluated. The modified Halsey and modified Oswin equations were found to be adequate models to describe the equilibrium relative humidity data. In comparing the results of this study with previously published data, it was determined that the equilibrium relative humidity properties varied with species. Models from existing literature, detailing equilibrium relative humidity/equilibrium moisture content data for high-starch products ( e.g. potato, cassava), were also examined. The Guggenheim–Anderson–de Boer (GAB) model was not an adequate model by checking residual plots. The parameters of the GAB model cannot be modified for prediction of the effect of temperature by the exponential function. For simplicity, an empirical linear model was developed to fit the equilibrium relative humidity/equilibrium moisture content data of sweet potato slices based on moisture content and temperature.

Energy production and emission reduction by utilization of pit and seam gas

The processes of desalination and sea-salt production currently produce large quantities of by-product, in the form of brine and bitterns, which is generally regarded as waste. This article explores the scope for exploiting the hygroscopic salts occurring in these by-products — such as magnesium, calcium and sodium chloride — as desiccant solutions in a greenhouse cooling system. These solutions are compared to other liquid desiccants more conventionally used in solar-driven refrigeration: namely solutions of lithium chloride, lithium bromide and zinc chloride. To establish which properties are relevant, we discuss the relation of the properties of the liquid desiccant to the attributes of the greenhouse as a whole. A property of primary importance is hygroscopicity, as quantified by the equilibrium relative humidity (ERH). Further properties reviewed include cost, availability, density, viscosity, specific heat capacity, thermal conductivity, heat of dilution, water absorption capacity, human- and ecotoxicity, and corrosivity. Calculations based on five locations (Tunis, Jiddah, Abu Dhabi, Mumbai and Bangkok) show that the liquid desiccant should have ERH ≤50% to give improved cooling compared to both direct and indirect evaporative systems. Except for sodium chloride, all six salts considered meet this requirement. Magnesium chloride is the most abundant salt in seawater bitterns and both magnesium and calcium chloride stand out as being of low toxicity. Their hygroscopic properties, though inferior to those of the lithium and zinc salts, make them suitable for cooling of greenhouses (even if not of human dwellings). We envisage an integrated desalination and agricultural system, comprising a solar desalination plant supplying freshwater (for irrigation) and bitterns (for cooling) to greenhouses, enabling efficient water use and local crop production in hot climates.

Article

Spray-drying is a rapid dehydration method allowing production of high quality dairy powders. In dehydration and subsequent powder handling and storage, however, both chemical and physical changes, such as caking, lactose crystallisation, and nonenzymatic browning, may impair powder characteristics and result in loss of powder quality. Many of these changes are related to the physical state of lactose, as rapid removal of water in spray drying results in the formation of low- moisture, amorphous, noncrystalline structures of lactose and other milk components. The amor- phous components may exist as solid-like glasses or highly supercooled, viscous liquids. The forma- tion of amorphous, glassy lactose during spray drying allows production of a free-flowing powder. High temperatures or residual water contents at the later stages of the drying process, however, may cause stickiness, caking, browning, and adhesion of the powder particles to the processing equip- ment. The glass transition of amorphous lactose occurs in the vicinity of room temperature at a water content of about 6.8 g (g × 100) -1 of lactose corresponding to an equilibrium relative humidity of 37% and 0.37 a w (water activity). At higher water contents, as the glass transition of amorphous lactose is well below storage temperature, dairy powders become sticky and the amorphous lactose may exhibit time-dependent crystallisation. Crystallisation of amorphous lactose may also release sorbed water from the amorphous material, which enhances other deteriorative changes, such as the nonenzymatic browning reaction. Amorphous lactose in dairy powders encapsulates milk fat, which, as a result of lactose crystallisation, is released and becomes susceptible for rapid oxidation. The glass transition and water activity are, therefore, important factors controlling processability, handling properties and stability of dairy powders.

The effects of temperature and moisture on after-ripening of Cenchrus ciliaris seeds

After-ripening of Cenchrus ciliaris L. spikelets was modelled during storage at various temperatures and equilibrium relative humidities (e.r.h.). At 40 and 50% e.r.h., the rate of after-ripening increased linearly with increase of storage temperature. The highest after-ripening rate was achieved by storing the seeds in 40°C and 50% e.r.h. At 70% e.r.h., however, seeds remained viable at 20 and 30°C but largely failed to after-ripen. Use of the model to predict the storage period required for decreasing dormancy to any desired level is described. The application to re-seeding degraded rangelands in arid areas is discussed.

Environmentally-induced stone decay: the cumulative effects of crystallization–hydration cycles on a Lincolnshire oopelsparite limestone

An investigation has been undertaken at St Andrew’s church, Walpole St Andrew, Norfolk, to establish the underlying causes of the observed stone decay to the upper parts of the six stone piers. The stone decay was first recorded in the early 1930s. The salt-contaminated masonry within the church has been shown to undergo severe salt decay during the summer, with little damage occurring over the winter months. The south aisle piers have been shown to decay 2.5 times faster than the north aisle piers. Although crystallization–hydration cycles have been identified, the rate of decay is at its greatest when the cycling is relatively infrequent. This was not the expected trend. Furthermore, it has been shown that during extended periods where the ambient relative humidity is less than 75%, the rate of decay reaches a maximum. It is the length of this ‘drying’ period that apparently has the greatest influence on the rate of decay and could explain the significant difference in the rate of decay between the south and north aisle piers. The results have serious implications for passive conservation, where it is often recommended to lower the ambient relative humidity to well below the equilibrium relative humidity of the salt contaminant, to avoid crystallization–hydration cycles. Since, at the time of building, the church was situated on the coast, it is possible that the sodium chloride contamination occurred during the building process (1440–1520), particularly since the area was prone to sea-flooding at this time. Alternatively, the salt could have been applied as a treatment during the general restoration of 1897. Whatever the source of the salt, it seems likely that the ambient environment was changed by the insertion of a sealed floor in 1897, which could account for the onset of the salt decay.

Salmonella spp. are affected by different levels of water activity in closed microcosms.

Controlling water activity (a(w)) can significantly impact the growth of Salmonella in poultry litter and manure - a phenomenon that was studied quantitatively using two common serotypes of Salmonella. The quantitative effect of changes in levels of a(w) on Salmonella populations was determined using inoculated, frosted glass rectangles placed in closed chambers (microcosms). Glass rectangles with known concentrations of Salmonella enteritidis and S. brandenburg were placed in microcosms maintained at an a(w) level of 0.893 for 24 h at room temperature (RT) and then transferred to other microcosms maintained at the same temperature but with higher a(w) levels (0.932 and 0.987). Salmonella populations on the slides were quantified at 4, 18, 24, and 48 h. Slightly elevated levels of a(w) (<0.1, i.e., 10% equilibrium relative humidity) for 24 h resulted in a 100-fold increase in counts of Salmonella. The data also suggested that in vitro adaptation to dry environments may occur when the organisms are exposed to alternating levels of relatively high and low (0.987 and 0.893) levels of a(w). Any increased tolerance of Salmonella to reduced levels of a(w) could be the result of physico-chemical changes in the organism due to selective environmental pressure, formation of a protective biofilm, and/or entry into a dormant state. Results from this study are compatible with those from previously reported on-farm surveys, reinforcing the contention that maintaining a(w) below 0.85 in and around litter/manure surfaces in poultry or livestock bedding areas may be a critical factor in safe production of food.

MODELLING ADSORPTION EQUILIBRIUM MOISTURE CHARACTERISTICS OF ROUGH RICE

Equilibrium Moisture Content (EMC) data for medium rough rice grain, Lido variety were obtained by equilibrating rice samples at different Equilibrium Relative Humidities (ERH) and Temperatures. The employed models to fit the data were GAB, modified Chung-Pfost, modified Halsey, Modified Henderson, and modified Oswin. The data were evaluated using the standard error of moisture content, mean relative standard error (%P) of the model and the correlation coefficient r2. The GAB model by indirect regression described the EMC the best, however its parameters lacked of physical meaning. The modified Chung-Pfost and modified Henderson equations presented a similar fit of the EMCERH relationships. The modified Chung-Pfost equation had lower residual values than the modified Henderson equation and was considered the best model to explain the EMC-ERH relationships.

Moisture desorption isotherms of medium-grain rough rice

Moisture desorption isotherms (EMC/ERH) of medium-grain rough rice (Japonica variety) were determined using a constant environment chamber for various combinations of air temperature (11.8–51.0°C) and relative humidity (37.1–89.7%). The initial moisture contents were in the range of 24.7–41.6% dry basis. A thin-layer technique was used to achieve uniform drying. Each test was continued until the moisture content change in 24 h was less than 0.1% dry basis. The final moisture content was considered as the dynamic equilibrium moisture content. Four three-parameter EMC/ERH equations, the modified Henderson, modified Chung–Pfost, modified Oswin and modified Halsey equations, were compared for their ability to fit the experimental EMC/ERH data. The residual sum of squares (RSS) and standard error of estimate (SEE) were adopted as the criteria to evaluate the fitting performance of the models. The modified Chung–Pfost equation was identified as the most appropriate equation for representing the EMC/ERH desorption isotherms of rough rice. Coefficients for equilibrium moisture content as a function of equilibrium relative humidity and temperature are given. The EMC/ERH data obtained in this study agreed well with previously published data. However the average isotherm, combining desorption and adsorption data, of ASAE does not predict the desorption EMC of rough rice accurately.

PH—Postharvest Technology: Comparison of Seven Equilibrium Moisture Content Equations for some Medicinal and Aromatic Plants

In this research, seven equilibrium moisture content (EMC) and equilibrium relative humidity (ERH) equations (modified Henderson, Chung–Pfost, Modified Halsey, Henderson, Chen–Clayton, Iglesias–Chirife and modified Oswin) are compared on their ability to fit data for some medicinal and aromatic plants. Thirteen data sets comprising 13 plants are used. Comparisons are based on standard error of estimate, coefficient of determination, residual sum of square and residual plots. Both the modified Halsey and the modified Oswin equations were found as the most versatile models to accurately describe the equilibrium moisture content and equilibrium relative humidity (EMC/ERH) relationships for medicinal and aromatic plants. The modified Henderson equation is a good model for fennel and cinnamon. The Chung–Pfost equation is suitable for muscat, coriander, ginger and cinnamon. The Iglesias–Chirife, Henderson and Chen–Clayton equations are among the least successful models for medicinal and aromatic plants, respectively.

MOISTURE MEASUREMENT OF GRAIN USING HUMIDITY SENSORS

To measure the moisture content of grain, the equilibrium relative humidity (ERH) and temperature of the air environment in equilibrium with the grain were measured with temperature and humidity sensors. The humidity sensor readings were transformed by a previously established calibration equation. The moisture content was calculated by the known EMC model. Comparing calculated values with the moisture values determined by the oven drying method, the average deviations of calculated moisture content were within 1.0% for rough rice and corn kernels, excluding the RH range above 85%. This technique could serve as a monitoring system for variations in temperature and moisture content of bulk–stored grain.

TECHNICAL NOTES: NEAR EQUILIBRIUM MOISTURE CONTENT VALUES FOR HULL–LESS OATS

Sorption characteristics of hullless oats were obtained by thin layer drying (TLD) and equilibrium relativehumidity (ERH) methods. For the TLD method, oats were at initial moisture contents of 11, 16, and 22% [dry basis (db)] andthe air was conditioned to obtain relative humidities of 27, 37, 48, 61, 68, 69, 75, 88, and 93% at temperatures from 15 to35
C. For the ERH method, low (7.6 to 8.7% db) and high (19.6 to 21.2% db) initial moisture content samples were used attemperatures from 15 to 35
C. A significant difference exists between the values of equilibrium moisture content (EMC calledapparent hysteresis) obtained by two methods. Apparent hysteresis was 0.68 to 1.06 and 0.46 to 3.62 percentage points forthe TLD and ERH methods, respectively. High apparent hysteresis in the ERH method is attributed to the fact that it failedto attain EMC in 2024 h as is suggested in the literature. Additional tests for extended time for ERH method narrowed theapparent hysteresis, indicating that true EMC values were not obtained by the ERH method.

The effects of different maturation conditions on seed dormancy and germination ofCenchrus ciliaris

AbstractSeeds ofCenchrus ciliarisL. were produced under different hydro–photo–thermal environments with and without fertilizer. Dormancy loss of spikelets and extracted caryopses was tested during dry after-ripening at 40°C and 43% equilibrium relative humidity. Caryopses had higher initial germination and lost their dormancy faster than spikelets. Dormancy of both caryopses and spikelets generally decreased with an increase of maturation temperature and fertility, whereas dormancy increased if water stress was imposed during maturation. The latter effect was smaller when the mother plants were exposed to water stress after caryopses were fully formed than when water stress cycles were applied throughout maturation. Daylength extension (to 14 or 18 h d-1) by artificial light increased dormancy of both caryopses and spikelets. The effect of long days declined when plants were exposed to natural daylight for more than 10 h d-1. The after-ripening curves were consistent with the hypothesis that dormancy periods of individual seeds are normally distributed within each seed lot. Rates of loss of dormancy were quantified by the slopes of these curves. In a given experiment, these rates were identical for caryopses but not always for spikelets that matured in diverse environments. Even for caryopses, however, the slopes varied between experiments. Therefore, the results do not support the hypothesis that a dormancy model can be applied universally to all seed lots ofCenchrus ciliaris. Methods of predicting the period of after-ripening required to achieve desired levels of dormancy for reseeding degraded rangelands are discussed.

Fungal growth and survival in building materials under fluctuating moisture and temperature conditions

Growth and viability of fungi on three building materials under moistening and drying conditions were investigated in the laboratory. The materials were taken from buildings under repair and no additional inoculation of fungi was used. The materials underwent four treatments (4–8 weeks of each): capillary absorption of water, drying in air at a relative humidity (RH) of 30%, condensation and finally drying at 50% RH. Moisture content (MC), equilibrium relative humidity (ERH), and concentrations of culturable fungi, actinomycetes and total spores were determined in the materials at 1- or 2-week intervals. The results showed that when water was absorbed by capillary action to the materials, fungal growth started fast and was abundant in the wood-based materials with MC above 20%. Such a limit value could not be defined for fungal contamination in the gypsum board because of complex behaviour of moisture between the gypsum bulk and paper covering. The condensation under the varying RH and temperature conditions caused only restrained fungal growth in the materials. The fast drying (RH 30%) seemed to decrease the viability of fungi but along with the experiment fungal flora was modified to tolerate fluctuating conditions and the drying at RH 50% had only a slight effect on the viability of fungi.