Characterisation of a low value dairy co-product and evaluation of the production of bioplastic films

Abstract

The processing and handling of milk by dairy factories and the cleaning of dairy plants results in the production of large volumes of milk protein-rich wastewater. The protein component of milk, primarily casein, imparts a high organic load on the wastewater which can be a major issue for water treatment facilities. Dissolved air flotation (DAF) technology has been applied in dairy processing plants to harvest solid material from dairy wastewater in New Zealand. This produces a clarified water component which can now be more readily treated in addition to a large quantity of casein-rich sludge as a co-product. A further process has been developed to convert the sludge into a stabilised powder which is referred to in this thesis as ‘DAF-casein powder’. This has attracted considerable interest as to the potential for development of value-added products.

Milk is a rich composite comprised of casein proteins, whey proteins, lipids, minerals, sugars and ash. While it has been demonstrated previously that fractionated caseins can be processed into various biomaterials such as plastics, fibres, emulsions and adhesives, there is very limited research on the production of a biomaterial from either a dairy waste stream or a crude milk derived material. Hence the aim of this research project was to investigate the fundamental properties of the DAF-casein powder and evaluate the processing and production of bioplastic films involving DAF-casein and other suitable biopolymers.

The first part of this research investigated the fundamental properties of reconstituted DAF-casein powder compared to that of reconstituted skim milk powder and fresh milk. This utilised one dimensional- and two-dimensional-polyacrylamide gel electrophoresis (1D- and 2D-PAGE respectively), reversed phase-high performance liquid chromatography (RP-HPLC), rheological studies, rennet coagulation and lipid extraction methods. These studies showed that DAF-casein has an altered hydrophobic behaviour, increased viscosity and impaired rennet coagulation compared to skim milk powder, which is likely to be due to the alteration of the casein micelle structure during the generation of the milk containing wastewater.

The second part of this project investigated the film forming capability of reconstituted DAF-casein powder in combination with other suitable biopolymers. This enabled the analysis of the physical and mechanical properties of the films in the presence and absence of plasticisers and crosslinkers. Produced films were also evaluated for their stability in water and gas permeability properties. It was initially found that reconstituted DAF-casein powder successfully formed a cohesive film under ambient conditions, however it possessed poor mechanical properties, with a low tensile strength and elongation at break. Blending DAF-casein with other suitable biopolymers such as κ-carrageenan, gelatin and sodium carboxymethylcellulose (NaCMC) created composite films with enhanced physical and mechanical properties. Modifying the films with plasticisers and crosslinkers was found to increase the durability, stability and flexibility of the films. From preliminary experiments, four composite formulations were selected for further examination on the basis of their physical, morphological and mechanical properties. These films exhibited varying degrees of stability in water, water vapour permeability and resistance to enzymatic and chemical degradation.

Overall this study has provided an initial characterisation of the DAF-casein powder, evaluated the production of DAF-casein based bioplastic films and demonstrated the potential for the development of value-added biomaterials from DAF-casein.