In-depth study of the structure-function relation of pectin as emulsifier

Pectin is considered a group of heteropolymers rich in galacturonic acid present in the cell wall of higher plants. It is one of the most complex polymers occurring in nature. This complexity results in a multifunctional character of pectin polymers both in planta as well as in situ in food systems and in extracted form. Extracted pectin has been studied for many years and has found industrial application, among others, for its gelling capacity in the production of jams and jellies. More recently, the emulsifying and emulsion stabilizing potential of pectin has raised the interests of food scientists. Investigating and tailoring pectin structure, its use to stabilize food emulsions could be initiated, resulting in additional industrial applications of pectin as a food ingredient. In this context, pectin could replace synthetic emulsifying compounds in food products. At the same time, this results in value addition as pectin is produced through extraction from industrial side streams. Although most pectin commercially available today is extracted from lemon peel and apple pomace, other fruit and vegetable based industrial side streams could be used as a pectin source as well. The main challenge is that the structural complexity of pectin makes it difficult to predict how a certain pectin structure will or will not be able to stabilize emulsions. In other words, it remains difficult to elaborate in detail on the relation between pectin structure and its function as an emulsifying and/or emulsion stabilizing compound. The structural complexity and diversity of pectin polymers play a dual role in this context. While it complicates obtaining unambiguous insights into the structure-function relation, it is at the same time responsible for the promising potential of pectin as emulsifying and emulsion stabilizing compound.

This doctoral research aimed to investigate the emulsifying and emulsion stabilizing potential of specific pectin structures obtained after chemical extraction. By analyzing pectin extracts from various fruit and vegetable sources, the influence of botanical origin on pectin structure and functionality could be explored. In the later stages of this doctoral research, specific structure modifying techniques (i.e. chemical modification and fractionation using ultracentrifugation) were applied to pectin samples with promising emulsion stabilizing potential. The isolation of specific (sub)structures (i.e. linear versus branched parts of pectin) was hypothesized to deliver further insight into the overall potential of pectin to stabilize emulsions and moreover, to possibly explain the promising potential of the original samples in more detail. The research was performed using a multimethod approach, combining extensive structural characterization techniques, multiple parameters determining the functional potential and detailed storage stability studies of emulsions stabilized by pectin samples.

Initially, multiple chemical extraction methods were evaluated starting from blanched and frozen carrot dices and pectin extracts were structurally characterized. In this early stage of the research, the goal was to obtain a pectin rich extract with desirable and promising structural characteristics aiming at the emulsifying and emulsion stabilizing potential. The influence of extraction conditions was apparent and nitric acid extraction resulted in a pectin extract characterized by high galacturonic acid content, high pectin related neutral sugar content and a high degree of methylesterification, structural features which were desirable in the context of this work. Hence, nitric acid extraction was selected and performed to extract pectin from apple, onion and tomato next to the already obtained carrot pectin extract. From a comparative study of the four pectin rich samples of different botanical origin, it was concluded that both onion pectin and carrot pectin displayed promising emulsion stabilizing capacities. During refrigerated storage for 14 days at 4 °C, no visual creaming was observed in emulsions stabilized by these two samples. Moreover, the oil droplet size changed only slightly during the storage period and no flocculation was observed. It was concluded that these emulsions stabilized by carrot pectin and onion pectin were the most resistant against emulsion destabilization. The promising potential of the carrot and onion pectin samples observed, served as the starting point for further in-depth research.

For carrot pectin, a linear homogalacturonan rich and a branched rhamnogalacturonan rich pectin sample were isolated. To the best of our knowledge, this specific research approach is the first to elucidate in-depth the relation between different carrot pectin subdomains and its emulsifying and emulsion stabilizing potential. Structural analyses clearly indicated the enrichment of the desired subdomain in the respective samples as well as an expected molar mass reduction of the polymers. The stability of emulsions produced with these isolates was diverse, although all of them were susceptible to creaming. Interestingly, the homogalacturonan rich sample was able to stabilize oil droplets at pH 2.5 while extensive flocculation was observed at pH 6.0. Similarly, the emulsions stabilized by the rhamnogalacturonan rich pectin sample displayed extensive flocculation at pH 6.0. Besides, these emulsions were characterized by larger oil droplet sizes at pH 2.5 compared to the emulsions stabilized by the homogalacturonan rich sample at pH 2.5. Comparing these results with the emulsions stabilized by the original carrot pectin suggested that the presence of both linear and branched regions in the pectin structure is beneficial for its emulsifying and emulsion stabilizing potential. The molar mass reduction during the isolation procedures might certainly play an important role in the observed functionality of both isolates.

As for the onion pectin rich extract, this sample was characterized by a bimodal molar mass distribution and therefore, the objective of the final experimental study was to separate both fractions by ultracentrifugation and assess the influence of the fractionation on the emulsion functionality. It was hypothesized that the presence of both fractions increased the emulsion stabilizing capacity of the onion pectin sample. Consequently, the functionality of the individual fractions could further assist in understanding the promising potential of onion pectin. Results indicated that the high molar mass fraction was still able to stabilize emulsions after fractionation in contrast to the low molar mass fraction which was found to consist of mainly short chain galactans. The viscosity increasing effect of the high molar mass fraction was clearly evidenced during rheological measurements. The low molar mass fraction did not have a significant effect on viscosity.

Overall, this work indicated the importance of the pectin molar mass for its potential to stabilize emulsions. The viscosity increasing effect in the continuous phase of free pectin polymers as well as the combination of steric and electrostatic effects of pectin polymers adsorbed at the oil-water interface might determine emulsion stability. Further, the addition of pectin as an emulsion stabilizer resulted in the creation of smaller oil droplets at pH 2.5 during emulsion production compared to pH 6.0. Finally, the versatile research strategy combining structural and physicochemical characterization and storage stability studies of oil-in-water emulsions proved promising and powerful for studying the potential of the pectin samples.