IT MAY not help feed the starving, but at last there鈥檚 a use for the obscene quantities of food rich countries throw away every year.
快猫短视频s in Hawaii have developed a biological reactor that converts a slurry of food waste into a biodegradable plastic. The polymer could be used to make greener packaging, disposable products such as bottles, or even pills that dissolve slowly to release drugs in the body.
More than 22 million tonnes of waste food are ditched each year in the US alone. The waste makes landfills stink and releases the greenhouse gas methane. And the reeking contents can also leach into groundwater.
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But biochemical engineer Jian Yu and his colleagues at the Hawaii Natural Energy Institute in Honolulu reckoned that not only could food scraps be put to good use, they could cut the cost of making biodegradable plastics into the bargain. The idea is not completely new. The British chemicals company ICI started making the biodegradable polymer (PHB) about 10 years ago. Although popular, PHB was 10 times as expensive as standard polymers because it was made using pure sugar and an organic acid.
Now Yu reckons he鈥檚 found a cheaper way to make PHB. He collected food scraps from a local restaurant, mixed them with water and blended the result to create a pretty disgusting slurry. He then stored some of this slop in a warm, airtight container. After a few weeks, strains of bacteria that can thrive without oxygen had multiplied in the slop. Such bacteria break down the organic molecules in the food, releasing acids as by-products 鈥� most commonly lactic and butyric acid.
Fresh slurry was added to the container, while a pump siphoned off the acidic product and fed it into a sack-like sheath dangling inside a second vessel (see Graphic). This container held an aerated suspension of the bacterium Ralstonia eutropha in a nutrient broth containing phosphates and sulphates.
He found that the acid molecules in the slurry gradually diffused through the sheath and into the bacterial suspension on the other side, while particulates and other large molecules remained trapped. The bacteria absorbed these acids and synthesise them into large polymers which they store as a source of carbon 鈥� not unlike the way animals store fat.
Yu discovered that when he used a silicone rubber sheath, the bacteria converted the acids into biodegradable PHB. But using a polyester sheath altered the ratio of acids available to the bacteria, and they converted them instead into a tougher biodegradable polymer PHBV. 鈥淏y controlling the properties of the barrier, you can control which molecules get through,鈥� says Yu.
Yu鈥檚 team, which reports the technique in Environmental Science & Technology (DOI: 10.1021/es011110o), says the bioplastic polymer makes up around 70 per cent of the bacterial mass harvested from the second container. And his team can produce between 22 to 25 kilograms of polymer for every 100 kilograms of slurry.
Waste-management companies are expected to harness the technology to turn a profit from the food thrown out by schools and corporate canteens, for example, instead of burying it in landfills. And an Asian company already has plans to use the new process.