Physicochemical properties of root fiber suspensions. A comparison between parsnip (Pastinaca sativa) and yacon (Smallanthus sonchifolius)

Sammanfattning: The physicochemical properties of two root suspensions, parsnip (Pastinaca sativa) and yacon (Smallanthus sonchifolius), were studied and compared at different concentrations and after being subjected to processes such as homogenization and heating. The study and the evaluation of the physicochemical properties of these plant materials in the form of pastes might help to increase the interest in these two roots, and possibly encourage their further industrial use. Parsnip and yacon roots are a promising source of dietary fiber, with fiber content of about 30% and 45% of the dry matter, respectively. However, the composition of their dietary fiber differs. The dietary fiber in parsnip mainly comprises cellulose, hemicellulose and pectin substances. The dietary fiber in yacon is mainly composed of inulin-type fructans, which comprise two-thirds of its total dietary fiber; the rest is cellulose, hemicellulose, and pectin. Another difference between these two roots is the amount of dietary fiber determined as soluble and insoluble. Most of the dietary fiber in parsnip is obtained as insoluble dietary fiber, whereas most of the dietary fiber in yacon is obtained as soluble dietary fiber. Parsnip and yacon suspensions showed a bimodal area-based particle size distribution i.e. two populations of small and large particles. The large particles in parsnip and yacon suspensions were mainly large cell clusters with an average diameter of about 300 µm in both cases. The small particles were mainly small cell clusters and single cells with an average diameter of about 26 µm (parsnip) and 9 µm (yacon). The size of the large cell clusters was reduced by homogenization, but the single cells remained somewhat intact. In the case of yacon suspensions, the size of the large-cell clusters was also reduced, but the small fragments started to aggregate. The aggregation was more evident in yacon suspension with a high content of yacon paste. The elastic properties dominated the viscous properties in all parsnip and yacon suspensions (Gʹ>Gʹʹ). Yacon suspensions started to develop a strong particle network at relatively low concentration of water insoluble solids (WIS) compared with parsnip suspensions. An elastic modulus of about 750 Pa was reached by a yacon suspension with only 0.87% WIS, whereas a parsnip suspension required about 3.42% WIS to reach similar high elasticity. This study suggested that the high elasticity shown by yacon suspensions at low WIS content was due to a contribution of the fructans. The fructans are one of the mayor constituents of yacon roots and they are present in the continuous phase. Our results further suggest that the ‘soluble’ fructans in fact form small particles in the continuous phase, thereby showing they are not truly ‘soluble’. The sizes of these fructan particles were in the nano-range. With regard to particle properties, large particles were shown to be primarily responsible for the network formation in parsnip suspension. Other particle properties, such as rigidity, were also important for the elastic properties of parsnip suspensions with WIS above 2.7%. This became evident when parsnip suspensions were subjected to heating that favored pectin methyl esterase (PME) activity (heating at 60°C for 40 min). A parsnip with 3.42% WIS under these conditions showed relatively high elastic modulus of about 2500 Pa. It was suggested that this was caused by the increased rigidity of particles due to cross-linked pectin in the cell walls. Conversely, a parsnip suspension with 3.42% WIS subjected to beta-elimination reactions (heating at 85°C for 2 h) showed lower elastic modulus (1500 Pa) compared to the suspension subjected to PME activity. This was possibly because the particles of the former suspensions were less rigid due to solubilization and some depolymerization of pectin from the middle lamella and cell walls.