Sugar beet leaves: from biorefinery to techno-functionality

Sugar beet leaves (SBL), which are a side stream of the sugar beets cultivation, are currently left unexploited after sugar beets have been harvested. The general aim of this thesis was to study the biorefinery of SBL, with a special focus on the isolation of proteins. To reach this aim the research was divided into three sub-aims: 1) to determine whether there is variability in the chemical composition of the leaves due to pre-harvest conditions (plant age), 2) to evaluate the variability of the techno-functionality of leaf soluble protein concentrate (LSPC) due to system conditions and 3) to extend current product and process synthesis approaches to enable the design of biorefining process. To address the first aim, SBL collected at different time points were used. Despite a small variation in the chemical composition of the leaves of different plant ages, a large effect of the plant age on the quality of LSPC was observed. In particular, LSPC from old plants was brown (indicative of polyphenol oxidase - PPO - activity), whereas LSPC from young plants was yellow. Based on these data, samples extracted with sodium disulfite (to inhibit PPO-mediated browning) were used for further experiments. The obtained LSPC consisted mainly of protein (69.3% w/w db (N∙5.23)) and carbohydrates (5.1% w/w db; half of which was charged carbohydrates). The main protein present in LSPC was Rubisco. The emulsion and foam properties of LSPC were studied as a function of protein concentration (Cp), pH and ionic strength (I). The minimal Cp of LSPC needed to form a stable emulsion (Ccr) was comparable to that of other widely used plant proteins, such as soy protein isolate. A critical ζ-potential (ζcr ~ 11 mV) was identified, below which flocculation occurs. At pH 8.0 and high I (0.5 M) the Ccr was higher than at low I (0.01 M), which relates to a higher protein adsorbed amount at the interface (Γmax). The foam ability (FA) of LSPC increased with Cp at all conditions tested. The FA was related to the soluble and not to the total Cp in the bulk. Interestingly, the minimal Cp; i.e.CcrFA needed to reach highest FA was constant as a function of pH. At high I (0.5 M) LSPC had higher FA than at low I (0.01 M), which was related to the faster adsorption of proteins at the interface. A minimum Cp was required to form stable foams. At pH 3.0 and 5.0 the foam stability of LSPC was higher than at pH 8.0. This was postulated to be due to formation of aggregates (between proteins or between proteins and charged carbohydrates). From these data it was shown that the techno-functional properties of LSPC could be linked to the molecular and interfacial properties of the dominant proteins in the concentrates. Thus, predictions for the techno-functional properties of impure systems, such as LSPC, can be made using only the known molecular properties of the dominant proteins and a small set of experiments. The knowledge acquired through the previous studies was used to adapt an existing methodology; namely the product-driven-process synthesis (PDPS) methodology, to extend its use in biorefinery. The adapted PDPS contained 4 novel steps, which facilitated its use in biorefinery. To illustrate how this new approach can be used in practice, a case study of a sugar beet leaves biorefinery was presented.