P. A. Sopade

The use of Peleg's equation to model water absorption in some cereal grains during soaking

Abstract Water absorption during soaking of maize, millet and sorghum at 10°C, 30°C and 50°C was studied using Pelegs equation. The equation gave a reasonable fit to experimental data. Peleg constants were obtained for the cereals. The constant K2 was unaffected by temperature of soaking. Temperature dependence of the reciprocal of the Peleg constant K1 was determined using an Arrhenius equation. Activation energy was in the range 13·99–16·23 MJ mol−1 compared to 19·02–19·56 MJ mol−1 obtained for soybean, cowpea and undehulled groundnut. An exponential relationship was proposed to describe the relationship between the rate of absorbed water per unit change in temperature and the activation energy.

Application of the Williams–Landel–Ferry model to the viscosity–temperature relationship of Australian honeys

The rheological behaviour of nine unprocessed Australian honeys was investigated for the applicability of the Williams-Landel-Ferry (WLF) model. The viscosity of the honeys was obtained over a range of shear rates (0.01-40 s(-1)) from 2degrees to 40 degreesC, and all the honeys exhibited Newtonian behaviour with viscosity reducing as the temperature was increased. The honeys with high moisture were of lower viscosity, The glass transition temperatures of the honeys, as measured with a differential scanning calorimeter (DSC), ranged from -40degrees to -46 degreesC, and four models (WLF. Arrhenius, Vogel-Tammann-Fulcher (VTF), and power-law) were investigated to describe the temperature dependence of the viscosity. The WLF was the most suitable and the correlation coefficient averaged 0.999 +/- 0.0013 as against 0.996 +/- 0.0042 for the Arrhenius model while the mean relative deviation modulus was 0-12% for the WLF model and 10-40% for the Arrhenius one. With the universal values for the WLF constants, the temperature dependence of the viscosity was badly predicted. From non-linear regression analysis, the constants of the WLF models for the honeys were obtained (C-1 = 13.7-21.1: C-2 = 55.9-118.7) and are different from the universal values. These WLF constants will be valuable for adequate modeling of the rheology of the honeys, and they can be used to assess the temperature sensitivity of the honeys

Characterisation of sweetpotato from Papua New Guinea and Australia: Physicochemical, pasting and gelatinisation properties

The physicochemical and functional properties of flours from 25 Papua New Guinean and Australian sweetpotato cultivars were evaluated. The cultivars (white-, orange-, cream-, and purple-fleshed, and with dry matter, from 15 to 28g/100g), were obovate, oblong, elliptic, curved, irregular in shape, and essentially thin-cortexed (1-2mm). Flour yield was less than 90g/100g solids, while starch, protein, amylose, water absorption and solubility indices, as well as total sugars, varied significantly (p<0.05). Potassium, sodium, calcium, and phosphorus were the major minerals measured, and there were differences in the pasting properties, which showed four classes of shear-thinning and shear-thickening behaviours. Differential scanning calorimetry showed single-stage gelatinisation behaviour, with cultivar-dependent temperatures (61-84°C) and enthalpies (12-27J/g dry starch). Oval-, round- and angular-shaped granules were observed with a scanning electron microscope, while X-ray diffraction revealed an A-type diffraction pattern in the cultivars, with about 30% crystallinity. This study shows a wide range of sweetpotato properties, reported for the first time.

Rheology and microstructure of sago starch from Papua New Guinea

The flow properties and microstructure of six sago starch samples from different localities (Kavieng, Kerema, Lae, Lorengau, Madang, and Wewak) in Papua New Guinea have been investigated. The rheological behaviour of 4% (w/w) paste was studied at 30 degreesC, 40 degreesC 50 degreesC, 60 degreesC and 70 degreesC, results were presented as shear stress-shear rate data and analysed using six yield stress and the power-law models. The viscosity was generally time-independent (CV = 2-6%) and irrespective of the temperature, shear stress increased with the rate of shear for all the samples (pseudoplastic). The power-law model provided a good estimation (r = 0.8739-0.9979) of the rheological behaviour with its index being 0.38 +/- 0.072 to essentially show no dependence on temperature and the source of the starch. The consistency index was temperature-dependent and E-a = 9.8-23.0 kJ mol(-1). Scanning electron microscopy revealed some damaged starch granules but granule morphology was identical for all the samples. The pH of the samples ranged from 3.7-5.1 and samples with low pH were generally of a low consistency index irrespective of the temperature

Thickened Fluids and Water Absorption in Rats and Humans

Individuals with dysphagia are commonly provided with oral fluids thickened to prevent aspiration. Most thickening agents are either gum-based (guar or xanthan) or are derived from modified starches. There is evidence, predominantly anecdotal, that dysphagic individuals are subclinically dehydrated. Dysphagia has a particular impact on elderly individuals and there is justifiable concern for dehydration in this population. It has been speculated that dehydration may, in part, be the result of the water-holding capacity of these thickening agents decreasing water absorption from the gut. The aim of this study was to determine the rate of intestinal absorption of water from thickened fluids. The method used was a laboratory tracer study in rats and humans in vivo. We found that there were no significant differences in water absorption rates between thickened fluids or pure water irrespective of thickener type (modified maize starch, guar gum, or xanthan gum). These data provide no support for the view that the addition of thickening agents, irrespective of type, to orally ingested fluids significantly alters the absorption rate of water from the gut.

Effect of added sucrose on extrusion cooking of maize starch

Abstract The effect of sucrose (20–50%, solid basis) on the extrusion cooking of maize starch was investigated. An increase in sucrose content decreased the specific power consumption, die pressure and melt temperature, and increased the tendency of the extrudate to collapse on cooling. The collapse phenomenon was associated with reductions in the starch concentration. Sucrose content increased melt elasticity and longitudinal expansion. Both starch gelatinization and sucrose dissolution were complete up to the 35% sucrose level. The presence of sucrose inhibited degradation of the starch granule. At the 50% sucrose level, the degree of starch gelatinization reduced as moisture content was increased or as barrel temperature was decreased. Sucrose dissolution at the 50% sucrose level was enhanced by an increase in temperature and was decreased on increasing the moisture content.

The influence of solid and sugar contents on rheological characteristics of akamu, a semi-liquid maize food

Abstract An investigation was carried out on the effects of solid (5–13%) and sugar (0–54%) contents on the viscosity of akamu . A rotational viscometer was used at speeds between 10 and 100 rev/min. The viscosity was measured at 30 °C, 40 °C, 50 °C, 60 °C, and 70 °C. An empirical power-law equation fitted the rheological behaviour with correlation coefficients between − 0.9932 and −1.0000. Akamu exhibited a pseudoplastic behaviour and a value of 0.32 was proposed as its power-law index. Neither temperature, solid content nor sugar content affected the pseudoplastic response. Consistency indices decreased with temperature and sugar content but increased with solid content. Exponential equations were used to describe these dependencies. Solid content affected the viscosity more than the sugar content. Sweetened akamu was more sensitive to temperature changes. Consistency index (K) was related to solid content (S) and absolute temperature (T) using an equation of the form: K = 3.03 × 10 −4 exp (0.41S + 1965.56/T) (r = 0.9796) and to sugar content (G) and temperature by an equation: K = 2.35 × 10 −3 exp (−1.55 × 10 −2 G + 2924.62/T) (r = 0.9410)

Criteria for an appropriate sorption model based on statistical analysis

Moisture sorption behaviour of food systems is valuable in analysing and predicting their dehydration characteristics. Many sorption models are in use and selected ones were listed. An appropriate model is recommended on the basis of its statistical assessment. Regression parameters (coefficient of determination, standard error and mean relative deviation modulus) from about 290 data sets were presented and related one to another. A low mean relative deviation modulus did not result from either a high coefficient of determination or a low standard error of estimate in all the cases. The residuals from the Oswin, Smith, Henderson, GAB, ChungPfost, and Halsey models on typical sorption data were discussed. No one parameter was found to solely indicate a perfect fit, and sorption models will be better assessed on more than one statistical parameter before being chosen.

Rheological characterization of akamu, a semi-liquid food made from maize, millet and sorghum

Viscosity-rotational speed (shear rate) relationships were determined for sweetened and unsweetened akamu, a cooked starchy extract prepared from fermented maize, millet and sorghum, at temperatures between 10 and 70 °C. The rotational speeds were between 10 and 100 rev/min. Akamu exhibited pseudoplastic behaviour at all temperatures. Akamu from maize had the highest viscosity and millet-akamu the lowest. A power-law equation adequately described the relationship, and correlation coefficients were from –1·0000 to –0·9915. Power-law indices were not affected by temperature, sugar and grain type, and were between 0·33 and 0·37 for all the samples. Consistency indices decreased with temperature, and the Arrhenius equation was used to describe the dependence. Activation energies varied from 3·3 to 11·6 MJ/mol. Sweetened samples had slightly lower activation energies. Sorghum-akamu showed the least sensitivity to temperature and maize-akamu the highest.

APPLICATION OF PELEG'S EQUATION IN DESORPTION STUDIES OF FOOD SYSTEMS: A CASE STUDY WITH SAGO METROXYLON SAGU ROTTB STARCH

ABSTRACT Moisture desorption is pertinent to the modelling of dehydration of food systems. The variation of moisture content with time during desorption of sago starch from six towns in Papua New Guinea was analysed using Pelegs equation. Three temperatures (30° 407deg; and 45° C) and four aw ranging from 0.103 - 0.923 were studied. The equation gave a highly significant fit to the experimental data ( r2 - 0.998 - 1.000), the two constants (Ki and K2) were obtained and both constants varied significantly (p< 0.05) with the source of the starch, temperature and aw. Kz was used to calculate the (true) equilibrium moisture content and this was unaffected by the length of the curve used in the computation. The true equilibrium moisture content approximated the moisture content that would have been obtained when weight changes were fairly constant (pseudo-equilibrium moisture content) The true equilibrium moisture content was shown to be more reliable and consequently recommended for sorption studies.