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

Abstract

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.

Abstract (Swedish)

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.

• author: Castro, Alejandra
• supervisor: Eva Tornberg
• opponent: Professor Gallegos, Crispulo, University of Huelva
• organization: Division of Food and Pharma
• publishing date: 2013
• type: Thesis
• publication status: published
• subject: Food Engineering
• keywords: physicochemical properties, parsnip, yacon, plant suspensions, particle size distribution, rheology, fructans, dietary fiber
• categories: Higher Education
• pages: 160 pages
• defense location: Lecture Hall B, Center for Chemistry and Chemical Engineering, Lund University Faculty of Engineering
• defense date: 2013-03-08 10:30:00
• ISBN: 978-91-978122-6-9
• language: English
• LU publication?: yes
• id: 0daf42c4-f0b1-482e-a8e5-eda0ea3fe182 (old id 3461505)
• date added to LUP: 2016-04-04 11:55:11
• date last changed: 2018-11-21 21:07:58