Aircraft leave vapour trails across the sky above Frankfurt, Germany.
Over the past decade, planes have been powered by fuel squeezed from plant seeds, wood chips, nicotine-free tobacco, algae and recycled oil from deep fryers.
The range of feedstock is vast, and in the four years to 2015, more than 20 airlines have made over 2500 commercial passenger flights with blends of up to 50 per cent biofuel.
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The quest for a replacement for the traditional kerosene-based jet fuel took flight in February, 2008, when Virgin Atlantic became the first airline to fly a commercial airliner with biofuel. The airline used a mixture of coconut and babassu oil in one of the four main fuel tanks of a jumbo jet.
Later that year Air New Zealand flew a Boeing 747 with a 50/50 mix of jatropha biofuel and conventional jet fuel for two hours, and successfully completed a series of performance tests.
Since then, both airlines have started the long chase for new sources of alternative fuel.
But commercial scale jet biofuel remains a work in progress - timetables for its introduction tend to blow out by years.
According to International Air Transport Association figures, as of last year there were five production pathways technically certified and another 16 on the drawing board.
But so far, the quantity of biofuel produced is only a drop in the bucket, compared with the 1.5 billion barrels of jet fuel used every year.
Producing commercial quantities of biofuel - and importantly, being able to pump it through existing infrastructure - is becoming more pressing.
Most of the airline industry has signed up to ambitious targets. From 2020, any increase in airline CO2 emissions will need to be offset by activities such as tree planting, to soak up the carbon dioxide aircraft pump into the atmosphere at the rate of close to 800,000 tonnes a year.
The United Nations' International Civil Aviation Organisation estimates the agreement would cost airlines between US$5.3 billion ($7.5b) and US$23.9 billion annually by 2035. Emissions from international flights - the main source of such pollution - account for about 2 per cent of global greenhouse gases and are forecast to triple by 2050.
Biofuels have been shown to reduce the carbon footprint of aviation fuel by up to 80 per cent over their full life cycle. If commercial aviation were to get just 6 per cent of its fuel supply from biofuel by 2020, this would reduce its overall carbon footprint by 5 per cent.
Last year, more than 60 nations representing over 80 per cent of aviation traffic pledged to voluntarily participate in the system when it begins in 2020, including the United States, China and most of Europe.
The deal, which has drawn criticism from India and Russia, becomes mandatory for most nations in 2027.
Despite the cost, companies said a single international standard would be cheaper and easier to follow than a jumble of programmes implemented by different countries.
Fuel is the biggest single cost for airlines, and while the latest generation planes are up to 20 per cent more efficient than the aircraft they replace, and optimal flight paths can play a big role, biofuel is seen as a the only energy solution to mitigate the industry's emissions growth in the medium term.
Unlike ground transport, which is rapidly shifting to battery technology, aviation has no alternative to liquid fuels. IATA estimates that electric commercial aircraft are unlikely before 2040, when battery weight and safety issues could be solved.
"The main challenges to a wide deployment of alternative jet fuels are not technical but commercial and political," says IATA. "Currently a number of alternative jet fuel production pathways are more expensive than fossil JetA/A1."
Relatively depressed oil prices over the past 18 months are also working against biofuel development.
It is estimated that ethanol made from corn, one of the cheapest of the alternatives, becomes cost competitive when oil prices rise past US$65 a barrel. They've been around US$50 or less for much of the past year, hitting the higher mark only for a short time in 2015.
And biofuel has hit other structural roadblocks.
Air New Zealand was not the only airline interested in jatropha, a plant which grows in tropical and subtropical regions. Investors piled in around 2007 and 2008, investing in huge plantations.
But it was found that, though the plant could grow in arid conditions, it needed more fertile soils and better irrigation to produce the oil-rich seeds required for biofuel, therefore pitching it into competition with crops grown for large populations in those regions.
Food crops such as corn, sugarcane, soybeans, palm oil and sugarbeet have also been used for biofuel - mainly used in ground transport - and likewise have competed with food production.
Air New Zealand, with Virgin Australia, is now searching for alternatives and the airlines are upbeat about the prospects of producing aviation biofuel locally, following their year-long joint investigation.
The airlines have completed an extensive review of more than 30 responses received from organisations in this country, Australia, the US, Canada and Europe.
They will now work with short-listed companies on strengthening the commercial case for investment following the request for information.
Air New Zealand's head of sustainability, Lisa Daniell, says the airlines achieved their key objectives in testing market readiness and gaining a better understanding of potential supply opportunities.
"The [request for information] has helped stimulate industry dialogue on the production of sustainable aviation fuel in the Australasian region. Importantly, the process has also greatly expanded our understanding of the technologies and processes involved and the potential timeframes to scale up to the volumes required," she says.
In this country, Z Energy is developing a plant in Wiri that makes biodiesel from animal fat and is leading the pack here, having produced a large test batch before Christmas.
David Binnie, general manager of supply and distribution at Z Energy, says alternative aviation fuel will be a long game for those who want to get into the field.
He says some of the building blocks of biodiesel technology are transferable.
"If we chose to adapt the biodiesel plant to something else, there's still some unknowns about that."
The growth in flying - at a time when battery power will increasingly replace liquid fuel in ground transport - means there is big scope in the long term.
"It's really high potential in terms of the scale of it," says Binnie. "On the jet stuff, we're just embarking on that journey but in the next six months to a year we'll make some choices on that."
Another New Zealand company, LanzaTech, is at the forefront of a pioneering technology in partnership with Virgin Atlantic.
It was founded in New Zealand but is now based in the United States and has a production facility in China where it has already produced a batch of 5700 litres of jet fuel derived from waste industrial gases from steel mills, using a fermentation process.
The airline hopes to run its first proving flight this year following work with Boeing and a range of other industry players.
The summer expansion sees two new aircraft and a demand for more Aussie destinations.