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Peel away the technology for waste from a breakfast staple

Citrus fruits including oranges, lemons, limes, grapefruits and tangerines are commodity products like coffee and tea in terms of international trade

In 2012-13 the major citrus producing countries, such as Brazil, China, India, US, EU27, Mexico, Egypt, Turkey and South Africa, produced around 160 million tonnes of such fruit, of which 60% were oranges.

Worldwide, it is estimated that more than 30% of citrus fruit produced – or 40% in the case of oranges – is processed by the food industry each year. This generates 19 million tonnes of waste peel from juicing and canning operations, which has the potential to be a bio-refinery raw material.

Major components of dry citrus peel waste include cellulose (37.08%), pectin (23.02%) and up to 3.8% D-limonene. This is mainly used as a flavour and fragrance compound as well as in the production of adhesive terpene resins via polymerisation, and it can also be used as a renewable bio-solvent as an alternative to environmentally unacceptable halocarbon solvents.

The extraction of D-limonene by steam distillation or microwave- assisted steam diffusion has already been reported, and is currently performed on-site in large-scale citrus juicing operations in Brazil and Florida.

Pectin, one of the most important food additives, is a complex structural heteropolysaccharide found in non-woody plant tissues. It is used as a gelling agent and a thickener. Citrus fruits contain roughly 20–30% extractable pectin, traditionally extracted by acidic hydrolysis.

A single methodology to combine all these extractions in an integrated process would be advantageous. As a result, the Orange Peel Exploitation Company (Opec) project was born with the aim of bringing together technology developed at the University of York so that fuels and chemicals can be derived from waste orange peel (WOP).

The Green Chemistry Centre of Excellence’s research in the area of microwave activation of biomass, under the supervision of Professor James H Clark, has led to the application of a hydrothermal low-temperature microwave process to WOP that produces marketable chemicals.

The superiority of the microwave protocol compared with other techniques lies in its ability to simultaneously produce valuable products in situ using a clean, flexible, green and transportable technology without the need for acidic additive. Microwave technology is adaptable for continuous processes, is easily scalable and can be applied directly to the desired biomass without need for pre-treatment such as drying. This is a key factor for future industrial-scale applications, given the importance of WOP drying costs.

Results in the laboratory show that it is possible to treat fresh WOP through three steps, for which we are able to extract:

  • D-limonene using microwave energy and no additional solvent
  • Sugars and flavonoids, a group of natural compounds that plays an important role in a number of biological functions, after washing the residual WOP with hot ethano
  • pectin under acid-free conditions. Such samples show comparable characteristics such as colour, viscosity and gel properties to those extracted under acid conditions.

Citrus juicers operating above 50,000 tonnes of fruit are equipped to manage the citrus peel produced efficiently. It is assumed that the microwave plant will be located on the same site as the juicing plant and that it will be running 24/7 for six months, which is the average orange harvest season.

According to the product yields (dry basis), industries should yield 152 and 1,080 tonnes a year of Dlimonene and pectin, respectively. The value generated by selling those products would result in more than £11m for pectin and around £10,000 for D-limonene. After processing costs, it could generate an overall profit of more than £7m a year.

These profits are based on the actual market price for the products which may vary and do not allow for repayment of the capex required (notably the microwave processor). But it does seem that a WOP biorefinery could be cost-effective.

Supported by the outcomes at bench-scale and considering the feasibility study of industrial- scale production, D-limonene and pectin extraction parameters are now being tested on a larger scale of 20 litres. Good results would not only confirm the utility of the microwave technology as a viable alternative to other technologies, but also demonstrate the possibility of using the method at an industrial scale.

Dr Julen Bustamante is Opec project manager at the Green Chemistry Centre of Excellence in the University of York. Further information: julen.bustamante@york.ac.uk

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