Executive Summary: Dyes from horticultural wastes

Summary of Project and Key Findings:
New Zealand’s horticulture and viticulture sector is efficient and substantial ($9.5 billion in 2019, including $6.2 billion of exports). Where processing takes place onshore, by-products are produced, such as grape marc from wine making, which are incorporated into circular processes such as composting. Arguably, these resources are not fully utilised, as they contain compounds with functional properties such as strong colours, antimicrobial, and antioxidant activities, and these compounds could be extracted before composting. The extracts from grape skins alone, in dried form, have a projected value of $5 million per annum in the local and international markets for dyes and antimicrobials.

This small, proof-of-concept, aimed to enhance the value proposition of locally produced natural dyes, by investigating the extraction of water-soluble coloured compounds (natural dyes) from horticultural and viticultural by-products, and how they could be applied to New Zealand’s most significant natural fibres, wool and harakeke.

Working with the skins of onions, grapes, tomatoes, and kiwifruit, kindly donated by local growers, AgResearch scientists developed methods to extract coloured compounds (anthocyanins, flavonols, tannins etc) that could be performed on the large-scale. Methods were developed to apply the extracts to wool and harakeke, without using metal mordants. The extracts imparted a range of purple, pink and brown shades, which were very uniform. Strong colours were imparted to wool, but only pale shades could be achieved on harakeke. Antioxidant activity was detected in fibres treated by extracts from the four types of skins, which could be expected to give antimicrobial and anti-odour effects.

This work led to a BPA Student Internship at AgResearch’s Research Centre in Lincoln, to investigate a complementary area to the main study, i.e., the extraction of water-insoluble coloured compounds such as carotenoids (which give carrots their colour), from tomato and grape skins using bio-solvents. The student achieved excellent results and gained experience with several techniques in a busy research laboratory.

The results of this work were shared with some local companies who have demonstrated a commitment to sustainability and innovation, and they expressed a desire to work with AgResearch to further this technology. Thus, we can report that the work was completed in full.