Characteristics of ground alfalfa in relation to steam conditioning

Abstract

Conditioning is an intermediate process in alfalfa pelleting designed to make alfalfa grind easier to bind. It is an important step because the quality of final pellets depends to a great extent on the quality of the conditioned mash. The mash pliability is in turn dictated by the fundamental characteristics of alfalfa grind such as its morphological attributes, thermal properties, rate of moisture diffusion, and equilibrium moisture relationship. There has so far been a lack of research pertinent to these characteristics imperative to the improvement of mash quality. This study was conceived for such a need, in which physical, morphological, thermal, moisture diffusion, and moisture equilibrium characteristics of alfalfa grind were scrutinized. The physical properties studied were the density of alfalfa grind as affected by particle size and moisture content, particle size distribution, and particle characterization. Models were developed to correlate the bulk and the solid densities to moisture content and particle size. The surface area of alfalfa grind particles based on sieving tests was much lower than that measured by nitrogen sorption. The difference was attributed to the pores, cracks and fissures in the particles that could trap nitrogen. The thermal properties in this study included specific heat capacity, thermal conductivity, and thermal diffusivity. The specific heat capacity of the alfalfa grind was determined by differential scanning calorimetry. A multiple regression model was developed to correlate the specific heat, thermal conductivity and thermal diffusivity of alfalfa grind to moisture content and temperature. Moisture diffusion characteristics of alfalfa grind were studied in terms of moisture diffusivity. Based on the thin-layer kinetics, moisture diffusivity of whole dehy alfalfa grind was $3.0310\sp{-8}$ m2/s. There was a significant difference in moisture diffusivity between the dehy and the sun-cured alfalfa grinds $(\alpha=0.05).$ From the "ring stack" diffusion tests, a mean diffusivity of $8.5510\sp{-8}$ m2/s resulted for the whole dehy grind. It was found that the relationship between the moisture diffusivity of alfalfa grind and particle undersize could be best described by the Gaussian function. Moisture equilibrium characteristics of alfalfa grind included moisture sorption isotherms and hysteresis behavior. A hypothesis was proposed to account for the origin of sorption hysteresis. Based on the hypothesis, a mathematical model was developed to quantify the magnitude of the hysteresis loops. The applicability of the developed model has been verified by the hysteresis data of alfalfa grind. Pilot-scale steam conditioning tests of alfalfa grind have been conducted in this study. Semi-empirical models have beendeveloped for describing the meal tempersture and moisture content trends in a steam conditioner. The goodness-of-fit of the models was verified with the pilot-scale steam conditioning test results.