Processing concepts for the use of green leaves as raw materials for the food industry

Large-scale processing of leaves for food applications requires quick processing or stabilisation to avoid perishability, due to the high moisture content in this biomass. Leaf perishability is compounded by the seasonal availability of crops, like sugar beet plants, of which the leaves are regarded as a potential protein source. This study evaluates the resource efficiency of a hypothetical sugar beet leaf processing chain by comparing supply chain options. First, two options consider leaf processing with and without stabilising the leaves by freezing. Then, these two options are considered in a centralised and decentralised process configuration. The latter places leaf freezing and pressing at the farm and further processes occur at a central facility. Energy usage and exergy consumption were used to quantify the thermodynamic performance of the processing options. Freezing has negligible effect on the process-ability of the leaves in terms of protein content and protein yield. The overall resource efficiency of the process was dominated by the amount of leaf material effectively used, which stresses the importance of full use of all (side-)streams. This outcome also explains the limited additional energy requirements for freezing. Exergetic indicators were affected by variations on the dry matter content of the starting biomass, compared to a negligible effect of other parameters (equipment scale, efficiency or energy use). Transportation load and soil quality were also discussed for the centralised and decentralised configurations. On-farm processing of the leaves (decentralised chain) clearly reduces the transportation load due to the large difference in bulk densities of leaves (73 kg/m3) and leaf juice (1000 kg/m3). Additionally, decentralised scenarios enable direct returning of the leaf pulp to the soil and thereby improving soil quality (i.e. nutrient retention and fertility). Soil quality is required to fully assess the use of biomass that is currently regarded as waste, but that actually plays a role in soil fertility. Therefore, the preferred chain configuration would be a decentralised system where the leaves are directly pressed at the farm, the pulp is used to fertilise the soil, and the leaf juice is chilled transported to a centralised factory.