Sources for natural dyes results of a screening for sources in food processing

From a dyer's point of view the coloration technology itself may be more important than the source of the dyestuff,3 particularly when total costs are at comparable levels. One main benefit of synthetically produced dyes is their tinctorial strength. For an average colour depth of 2%, 2 g of synthetic dyestuff is used per 100 g cotton, while for a similar shade 100-300 g of dry plant material is required.3'7 Besides their different ecological profiles, natural dyes also have to compete with synthetic dyes with regard

Table 19.6 Energy balances for handling, transportation, extraction and concentration steps during dyestuff production from 100 kg wet plant material (including the dyeing process for 100 kg goods)

Production of a concentrated Energy consumption dyestuff for 100 kg of bark

Extraction of crop in dyehouse

Energy consumption for 100 kg of bark

Table 19.6 Energy balances for handling, transportation, extraction and concentration steps during dyestuff production from 100 kg wet plant material (including the dyeing process for 100 kg goods)

Formation of commercial

Extraction

E =

157 MJ

Drying for stabilisation

E =

180 MJ

product

(LR 1 : 5, temperature

95 °C)

Concentration

(final volume 10 L product)

Evaporation from 400 L to

E =

421 MJ

10 L (membrane

concentration to 10 L)

E =

140 MJ

Waste (wet extracted

material, approx.

mass = 200 kg)

Transportation (<100 km)

10 kg of concentrated extract

E =

0.7 MJ

50 kg of dried material

E =

3.5 MJ

(or 100 kg of wet

E =

7 MJ

material)

Dyeing

Preparation of dyebath

E =

314 MJ

Preparation of dyebath

E =

314 MJ

100 kg goods,

and extraction of bark

LR 1 : 10, 95 °C

Waste: wet extracted

material approx.

mass = 200 kg

Total energy consumption

Conc. by evaporation

E =

892.7 MJ

Stabilisation by drying

E =

497.5 MJ

Conc. by membrane process

E =

611.7 MJ

Without drying

E =

Conc., concentration; LR, liquid ratio (mass of solids per volume of liquid).

OP w

Ul Ul

Conc., concentration; LR, liquid ratio (mass of solids per volume of liquid).

to the costs required to achieve a certain colour depth. Production of dyestuff-containing plants by direct farming results in very high specific costs. Growing and harvesting of traditional dye plants like madder, weld and woad is expensive and time consuming,41 although recently considerable research work on the cultivation of plants for plant dyes has been carried out.5'40'42-45 In addition to the high price, the required farmland is not available.3'41'46'47

To improve cost effectiveness' alternative sources of dye-containing plant material have been studied. Instead of cultivation of dye plants' some plants could be gathered from the wild' like dyer's greenweed (Genista tinctoria L.) and Canada goldenrod (Solidago canadiensis L.).7'41 From the literature' not only traditional dye plants' but also berries' fruits' vegetables' peels' roots and barks are known as potential sources for dyestuff extrac-tion.2 In an extensive evaluation of new sources for colour-containing plant material' the food and beverage industries were found to release considerable amounts of cheap vegetable material as wastes.6'7'19'48 These sources for natural dyes are less effective in terms of dyestuff content compared with the raw material' nevertheless remarkable amounts of natural dyes can be extracted.19 Wastes from fruit and vegetable processing exhibit several advantages:22

• sizeable amounts are available (however they are seasonally dependent)'

• non-hazardous material' defined hygienic standards'

• high quality standards of released material.

Another important source for natural dyes was identified in the wood processing industry which releases enormous amounts of bark.11'18'48 Based on these different sources for plant material' the calculated price for vegetable dyes can be expected to be comparable with synthetically produced colorants.7 Screening studies showed that not all colour-containing wastes are suitable for textile dyeing. Handling and processing' application facilities' dyeability' available colour and fastness properties have to be investigated to identify suitable plant material.

While a range of brown to olive shades can be obtained from barks released as wastes from the timber industry' there is still a need for brilliant red and blue colours.49 Brilliant red dyes could be extracted from pressed berries and grape pomace from the beverage industry.19 For green' blue and black shades only a few sources are available - e.g. natural indigo' privet berries' hollyhock buds' extracts from barks - and further research is required. A broad study to evaluate the potential of wastes released from the food and beverage industries has led to the identification of some of the relevant aspects to be considered:

• Storage and standardisation of material differ and are dependent on the source. Stabilisation of material by drying and freezing (energy consumption!) forms a basis to overcome problems due to the seasonal release of waste and changes in product quality.

• Enormous amounts of plant material have to be handled. Due to the low content of dyestuff, approximately the same amount of plant material as textile good has to be extracted.

• The extracted material will appear after extraction with at least the same mass of waste; in the case of dried raw material, the water content will almost double the mass of waste.

A brief overview of the outcome of the screening study is given in Tables 19.7 to 19.9. A separation into three groups was performed:

• wastes from berry and grape processing (juices, wine, strong liquor, beverages);

• wastes from vegetable processing (food industry);

• wastes from tea, fruit and nut processing (beverage and food industry, and others).

Only selected representatives of the experimental results are shown in the tables. Laboratory dyeings were characterised by shade, CIELab-coordinates and selected fastness properties. The results of all experiments were summarised in a pass/fail decision, which identifies sources that exhibit sufficient potential and could serve as a source for natural dye extraction.

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