Understanding the functional properties of pectin-depleted citrus fiber as texturizing ingredient

The high production of citrus fruits worldwide, with large proportion of the harvested fruits going into the processing industry, creates a high volume of by-product which may become a burden for food companies and for the environment. On the other hand, citrus by-product is rich in cell wall material (CWM), also more commonly known as dietary fiber, which has been widely studied for its excellent techno-functionality and health-related benefits. Therefore, many past and current research efforts have focused on the valorization of citrus by-products. One of the most common valorization routes of citrus by-product is pectin extraction. However, pectin extraction utilizes only a fraction of the by-products and leaves a residue that is rich in cell wall polysaccharides, such as cellulose, hemicellulose and residual-pectin. It has been shown that this acid residue (AR) can be functionalized using high pressure homogenization (HPH), to create suspensions with excellent rheological properties.

Several aspects in the functionalization of the citrus AR after pectin extraction have not been explored yet, for example the effect of the CWM source or the role of the distinct residual cell wall polysaccharides and their structure on the resulting suspension’s rheology. Thus, further studies are needed to better understand this novel material. Several specific research questions are answered through the experimental studies in this dissertation. The first objective was to investigate the possible differences between citrus species (lemon, orange, grapefruit) and fruit parts (peel and pulp) as the source of the ARs. Second, the effect of CWM residue composition and structure was investigated through alkali extractions of the AR at different concentrations. Lastly, different methods to capture the moisture dependent molecular mobility changes of the functionalized AR and its stability during storage in dry state were studied in detail.

The first experimental study demonstrated that citrus ARs, regardless of the by-product source, can be functionalized into a texturizing ingredient. With HPH, the G’-value of all AR suspensions significantly increased. However, the peel ARs from grapefruit and lemon exhibited better rheological properties and may be considered as better sources compared to the other citrus by-products. A certain degree of pectin extraction from the CWM was favorable to functionalize the residue. Some physicochemical properties of the CWM residue, specifically the rhamnose content, the residual pectin degree of methyl-esterification and the hemicellulosic glucose content strongly correlated to the G’ of the suspension before HPH. However, these correlations ceased after HPH, indicating a large effect of HPH on the residue’s microstructural properties. After HPH, fragmentation and aggregation of the particles was observed based on the particle size analysis and microscopy visualization.

In the second experimental study, alkali extractions of the AR at different concentrations (0.1 mM – 6 M KOH) created CWM residues with different polysaccharides compositions. The use of mild alkali extraction (up to 1 mM KOH) allowed further removal of pectin from the AR (to a maximum degree of pectin extraction) while the excellent rheological property of the corresponding CWM residue suspensions was maintained. However, residues obtained from AR alkali extractions at intermediate concentrations (0.1 M – 1 M KOH) showed a significant deterioration of their suspension’s rheological property, while no significant change was observed in the polysaccharide composition compared to the residues from 1mM KOH extraction. It was suspected that a treatment with 0.1 M – 1 M KOH changed the hemicellulose structure in the CWM residue, changing the rheological property of the suspensions. Using harsh alkali extraction conditions (2 M – 6 M KOH), next to a maximum removal of pectin, hemicellulose was partially removed from the AR and the residual CWM could not be functionalized. Light microscopy and Scanning Electron Microscopy (SEM) indicated that the particles opened upon HPH when the CWM residue was functionalizable. As the concentration of the extracting alkali solution increased, the particles became denser with a closed structure, causing the CWM residue to be less functionalizable. The crystallinity index of the cellulose decreased with higher alkali concentration and HPH did not affect the cellulose crystallinity. However, no clear relation between cellulose crystallinity and rheological property was observed.

In a follow up study, the changes in microstructure and the network formed in the suspensions for several CWM residues with distinct rheological properties were examined. For CWM residues with high functionality, HPH caused a decrease in the mobility of the entrapped water in the suspension due to the formation of smaller pockets and a stronger network of CWM residue. However, when the CWM residue was treated with mild alkali (1mM KOH), despite having the highest G’ value, the suspension was more prone to sedimentation and the water mobility was higher than for CWM residues with relatively lower G’ (AR suspension). Thus, better bulk rheological properties did not always correspond to stronger CWM residue networks in the suspension. In case the CWM residue could not be functionalized (having relatively poor rheology) due to harsh alkaline treatment (≥ 1M KOH), immunolabeling results indicated changes in certain pectin structure accessibility (highly methyl-esterified HG and RG-I were less available). The CWM residue particles in this case were also suspected to be collapsed, leading to a closed structure, since the hemicellulose was not accessible to enzymatic hydrolysis.

Glass-transition analysis, explored in the last experimental study, showed the limitations of DSC to capture the changes in the moisture-plasticized molecular mobility of the AR residue, which is an important property to predict a material’s storage stability. On the other hand, the structural relaxation phenomena elucidated by the change in mechanical properties measured by Thermal Mechanical Compression Test and Dynamic Mechanical Thermal Analysis (TMCT-DMTA), specifically the tan δ curve obtained from DMTA, corresponded to the results of storage stability of the material. The onset point of the tan δ curve change determines the storage conditions (temperature and moisture content) that will ensure the stability of the CWM residue (avoiding collapse and loosing its functional properties in suspension).

From the results of all the experimental studies, the improved understanding of the functionalization of CWM residues obtained from citrus by-products can be used to tailor the CWM residue in order to obtain texturizing ingredients with the desirable characteristics. Partial pectin depletion favors the creation of a CWM residue with open structure, thus facilitating the functionalization by HPH. Collapse of the material which causes a closed structure should be avoided at all times.  This, in particular, applies to the proper storage of the dried material in order to prevent moisture plasticized molecular mobility and thus collapse/loss of functional properties of the material.