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

Abstract: Popular Abstract in English The current interest in improving the nutritional benefits of food products and the need to recover undeveloped crops prompted us to study two plant roots: parsnip (Pastinaca sativa) and yacon (Smallanthus sonchifolius). Parsnip is a root native to Europe and Asia, and is often used for culinary purposes and to feed livestock. Parsnip was a popular vegetable in the Middle Ages but the interest in this root declined with the arrival of the potato in Europe. Yacon roots are originally from Andean countries in South America, cultivated since pre-colonial times, and it is commonly eaten raw as a fruit due to its slightly sweet taste and crunchy texture. The cultivation of yacon in Andean countries declined slowly through most of the past century, limiting access to this root and its commercialization. However, interest has been growing steadily in the past ten years due to studies showing its health benefits. The study and evaluation of the properties of these two roots might help to increase interest in these roots and their further industrial use. Parsnip and yacon roots have a dry matter content of 20% and 10%, respectively. The two roots differ in their storage polysaccharide (i.e. the plant’s energy source). The storage polysaccharide in parsnip is starch, whereas the storage polysaccharide in yacon is fructans. These fructans also have dietary fiber properties. The dietary fiber in parsnip mainly comprises the cell wall polysaccharides (cellulose, hemicellulose, and pectin substances), whereas the dietary fiber in yacon is mainly fructans and only a small proportion in the form of cellulose, hemicellulose, and pectin. Parsnip and yacon roots are both a promising source of dietary fiber with relatively high values of about 30% (parsnip) and 45% (yacon) of the dry matter. Yacon in particular is a significant source of fructans. 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 while, in yacon, most is obtained as soluble dietary fiber. Parsnip and yacon roots in the form of pastes were used to prepare aqueous suspensions by suspending the paste in distilled water. These aqueous suspensions were subjected to processes such as valve pressure homogenization and heating. These two processes are often used in industrial processing of these types of materials. Properties such as the amount of insoluble particles, the particle size distribution, and the volume fraction of the particles were measured in the suspensions, and then related to the rheological properties. Parsnip and yacon suspensions initially comprised cell clusters, but homogenization broke these cell clusters. After homogenization, parsnip suspensions had smaller cell clusters and some single cells; yacon suspensions after homogenization, besides presenting smaller cell clusters, also presented aggregation of cell fragments. Yacon cells were slightly easier to disrupt by homogenization than parsnip cells. Parsnip and yacon suspensions displayed ‘solid-like’ rheological behavior (Gʹ>Gʹʹ) because of the amount of insoluble particles in the suspension. Parsnip suspensions started to present a rather strong particle network when the concentration of insoluble particles (i.e. cell fragments) was quite high (around 2.7 g/100 g suspension). In contrast, yacon suspensions already exhibited a strong particle network at a rather low concentration of insoluble particles (around 0.7 g/100 g suspension). We should remember that yacon contains a significant amount of fructans, but these are mainly found in the soluble fraction (i.e. often not considered as particles but as soluble polysaccharides). Results from our investigation suggested that these ‘soluble’ fructans form small particles with sizes in the nano-range. Besides the amount of particles, particle properties such as rigidity are also important in the rheological properties of concentrated parsnip suspensions (above 2.7 g insoluble particles/100 g suspensions). When parsnip suspensions were heated at 60°C for 40 min, an enzyme (pectin methyl esterase) was activated. This enzyme facilitated the cross-link of pectin chains in cell walls of the particles and so the rigidity of the particles increased. Parsnip suspensions with these types of particles showed enhanced rheological properties. Conversely, when parsnip suspensions were heated at 80°C for 2h, chemical reactions occurred that solubilized pectin. The solubilization of pectin from the cell walls reduced the rigidity of the particles. Parsnip suspensions with these types of particles showed lower elastic properties than suspensions with rigid particles.