In situ pectin engineering as a tool to tailor the consistency and syneresis of carrot purée
Highlights
► In situ pectin engineering: tool to tailor consistency and syneresis of carrot purées. ► Low-temperature blanching: higher consistency and more syneresis. ► High-temperature blanching: limited syneresis due to pectin solubilisation. ► High-pressure homogenisation: lower consistency due to smaller particle size.
Introduction
In today’s food industry, a global trend towards the manufacture of healthier and more natural fruit and vegetable food products, such as soups, smoothies and sauces, is ongoing, as well as the incorporation of puréed vegetables in other food products (Blatt, Roe, & Rolls, 2011). The creation of these products involves the mechanical disruption of parenchyma-rich plant tissues. The resulting plant-food dispersions are a combination of a liquid phase (serum), containing pectic polysaccharides and other soluble substances, and a dispersed phase (pulp), containing the plant insoluble solids such as cell walls (Lopez-Sanchez et al., 2011). High-pressure homogenisation, a more intense shear treatment to further mechanically disrupt the plant material compared with conventional blending, has recently been introduced in the context of vegetable processing as a tool to further exploit the natural structuring potential of different plant sources. The deliberate application of particular thermal and mechanical processes on raw plant material makes it possible to design naturally structured/textured food products without the addition of texture-controlling agents such as starches, gums and stabilizers.
The rheological properties of plant-food dispersions are often related to parameters such as particle size, morphology and volume (Day et al., 2010, Lopez-Sanchez et al., 2011). Detailed research towards the role of pectin on the flow properties of purées on the other hand is lacking. However, it is known that in situ pectin structural modifications during food processing remarkably alter the textural/rheological properties of plant-based foods (Sila et al., 2009) and, in addition, this polysaccharide occurs in both the liquid and dispersed phase of plant-food dispersions due to its solubility characteristics (Van Buren, 1979). Hence, the role of pectin on purée consistency and syneresis (i.e. the spontaneous separation of serum and pulp) should not be neglected.
One of the most abundant building blocks of pectin is homogalacturonan (HG), a linear chain of galacturonic acid (GalA) residues in which some of the C-6 carboxyl groups are methyl-esterified. HG in general and its methyl-esterification (degree and pattern) in particular strongly determine the functionality of pectin in plant-based food products (Willats, Knox, & Mikkelsen, 2006). During processing, HG is prone to chemical and/or enzymatic conversion reactions leading to pectin depolymerisation and/or demethoxylation. In low-acid plant tissues, pectin depolymerisation occurs at high temperatures (>80 °C) through a β-elimination reaction which is favored by hydroxyl ions and methyl-esterified GalA residues (Van Buren, 1979). The depolymerisation and solubilisation of pectic polymers involved in cell–cell adhesion has been demonstrated to cause serious texture deterioration in processed plant tissues (De Roeck, Sila, Duvetter, Van Loey, & Hendrickx, 2008). Pectin demethoxylation, by the action of cell-wall-bound pectin methylesterase (PME), can improve the intercellular adhesion since the increase in free pectic carboxyl groups provides a greater opportunity for pectic polymers to be cross-linked with divalent ions such as Ca2+. Endogenous PME activity is enhanced during conventional low-temperature blanching, typically 15–45 min at 50–60 °C (Ni et al., 2005, Sila et al., 2005), whereas high-temperature blanching inactivates PME. In some fruits and vegetables however, demethoxylated pectin can act as a substrate for the action of the depolymerising enzyme polygalacturonase, which results in texture/viscosity loss (Sila et al., 2009).
Assessing the influence of processing on pectin in food matrices has predominantly been performed using ex situ analysis techniques and more specifically, via physicochemical analysis of fractionated walls and isolated polymers (Sila et al., 2009). Anti-pectin antibodies now provide a new range of opportunities as they allow the precise localisation of defined structural pectic domains in intact plant cell walls (Christiaens et al., 2011, Willats et al., 2006). Of particular interest for food technologists are a set of antibodies that bind to HG domains of pectin, including JIM5, JIM7, LM18, LM19, LM20, PAM1 and 2F4 (Knox et al., 1990, Liners et al., 1989, Verhertbruggen et al., 2009, Willats et al., 1999). Knowledge on the binding specificities of these antibodies is at hand: antibodies LM18 and LM19 need a stretch of unesterified GalA residues for recognition, while methyl-esterified residues are required for the binding of LM20 (Christiaens et al., 2011, Verhertbruggen et al., 2009). In contrast, the epitope of JIM5 contains both methyl-esterified and non-methyl-esterified GalA residues. JIM7 can be used as a general anti-pectin probe as it recognises HG with very diverse degrees and patterns of methyl-esterification (Christiaens, Van Buggenhout, Ngouemazong, et al., 2011). PAM1, on the other hand, is a much more ‘specific’ antibody since it only binds to long blocks of approximately 30 non-esterified GalA residues (Willats et al., 1999). Finally, localisation of Ca2+-cross-linked pectin, important in cell–cell adhesion, is possible with monoclonal antibody 2F4 (Liners et al., 1989).
The objective of the current study was to explore the opportunities of in situ pectin engineering in tailoring the consistency and syneresis of vegetable purées. Carrot (Daucus carota), in which PME-induced pectin changes play an important role in determining the structural/textural properties of the tissue, was selected as a commercially important plant material. The effect of low-temperature and high-temperature blanching, as well as the effect of two types of mechanical disruption, blending and high-pressure homogenisation, on the flow properties and pectin structure of carrot purée was investigated. Pectin was examined, not only via the traditional physicochemical analysis of fractionated walls and isolated polymers, but also via anti-HG antibodies entailing in situ (microscopy) and ex situ (immuno-dot assays) analyses.
Section snippets
Materials and methods
A schematic overview of the experimental set-up is presented in Fig. 1.
Influence of pretreatments and high-pressure homogenisation on the macroscopic properties of carrot purée
The flow behaviour of the differently prepared carrot purées was empirically tested using a Bostwick consistometer. Fig. 2 shows the Bostwick consistency index of the different samples with a distinction between the pulp and serum fraction. Purée prepared by blending non-pretreated carrots showed a rather low Bostwick consistency index for the pulp fraction, i.e. a rather high consistency, and pronounced syneresis. LTB of carrots further increased the consistency of the purée and resulted in an
Conclusion
In conclusion, it can be stated that in situ pectin engineering is a helpful tool in tailoring the consistency and syneresis of carrot purées. Purée prepared by blending non-pretreated carrots showed a rather high consistency and pronounced syneresis. Treatments that solubilise pectin, such as high-pressure homogenisation and, in particular, high-temperature blanching, limited syneresis phenomena. The thermosolubilisation and β-eliminative depolymerisation of pectin in high-temperature blanched
Acknowledgements
This research has been carried out with financial support from the KULeuven Industrial Research Fund (KP/08/004), the Belgium Federal Government (IAP VI Program), the Flemisch Government (Long term structural funding: Methusalem Program) and the Hercules Foundation (HER/08/21). S. Christiaens, K. Moelants and C. David are Ph.D. Fellows and S. Van Buggenhout a Postdoctoral Researcher, funded by the Research Foundation Flanders (FWO).
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