Development and validation of a pilot juice extractor technique to process green sugarcane with brown leaf

Abstract

Annually, approximately 90% of the sugarcane planted in South Africa is burnt prior to harvest. The burning of sugarcane is a pre-harvesting technique that is well known to the sugar industry and one that has proven to be efficient. Due to the numerous associated disadvantages that exist, such as the public nuisance of soot and smoke, soil damage and its contributions to air pollution, it is foreseen that government legislation will become more stringent, prohibiting the burning of sugarcane, thereby forcing the industry to consider other alternatives. Processing green sugarcane with brown leaf is one of the sustainable alternatives to be considered which would introduce a beneficial option for both the sugar industry and the environment. For this alternative to be recommended as a viable option, an investigation was required to determine the effects of processing green sugarcane with brown leaf in a sugar factory specifically in alignment with their current juice extraction systems (diffusers).

Conducting such experiments on a commercial diffuser presented several challenges. A novel approach was undertaken to develop a pilot juice extraction technique to access and quantify the effects of processing green sugarcane with varying quantities of brown leaf in a sugarcane diffuser on a more controlled scale. Efforts were made to simulate conditions in a diffuser and thereby produce a juice that would closely represent the quality of juice extracted from a sugarcane diffuser.

A pilot juice extractor technique was designed, fabricated and its performance verified before determining suitable operating conditions for further experimental work. Experimental juice extraction systems and the applicability of the system to the proposed work was evaluated. The outcome of an extensive review of the literature revealed that the pilot juice extractor design had to be based on an upward forced-flow, submerged column with a steam jacket and electrical heating option. The pilot extractor showed good ability in differentiating between juices extracted from burnt and green sugarcane with and without brown leaf. The extracted juice quality was assessed and compared on the key analytes present in the extracted juice such as gravity purity (sucrose/brix), colour, conductivity ash, reducing sugars (fructose and glucose) and non-sucrose content. An experimental design allowed for key operating conditions of time (30, 45 and 60 minutes) and temperature (75 °C, 0 °C and 85 °C) to be tested. Suitable operating conditions for the pilot juice extractor, which emanated from the experiments, included a temperature of 80 °C and a retention time of 30 minutes. In addition, the juice concentrations in the pilot extractor were found to be different (higher or lower) to the concentrations of analytes present in the juice extracted from two established methods, namely cold digestion and press methods, for most juice quality parameters.

The pilot juice extractor performance was subsequently validated against a South African commercial diffuser for 16 different consignments of sugarcane of different varieties and included both green and burnt sugarcane. The diffuser draft juice was compared to the juice obtained from the pilot extractor, cold digestion and press processes. Due to a lack of green sugarcane samples tested at the factory, the correlations between draft juice and the extractor were derived for burnt sugarcane only. The pilot extractor juice quality for burnt sugarcane compared more favourably, in terms of the concentrations of the analytes, with the draft juice quality rather than the quality of the juice extracted from the cold digestion and pressing methods. The investigation of the effects of varying quantities of brown leaf on the quality of juice extracted and the effect on the sugarcane density and percolation rate was carried out using the pilot juice extractor. The tests considered included four different sugarcane varieties (N12, N16, N47 and N39) to obtain a good representation of the different types of sugarcane that is processed in sugar factories. Results showed that an increase in brown leaf content adds to colour, conductivity ash and non-sucrose content and reduces purity, sugarcane density and percolation rates across all tested varieties. The pilot juice extractor presents a suitable method that can be utilised in future studies to assess factory specific combinations of sugarcane varieties, type (burnt or green) and the effects of adding brown leaf in a diffuser, in an effort to understand any potential factory processing impacts. This will aid factories in preparation for how best to handle the situation should they be required to process green harvested sugarcane with brown leaf in the near future.