Sharks haven’t turned up in Hawaii for the toffee

Looks like an oil slick, but the similarity ends there. Badagnani

Swimmers in Hawaii have been warned out of the water for fear of shark attacks after around 1,400 tonnes of molasses - treacle - leaked from a pipe into the sea while being transferred to a tanker from a sugar plantation.

Although molasses is superficially similar to oil – both are viscous and dark brown - it’s far less toxic to wildlife and less damaging to the environment. Yet there are reports of many fish dying around Hawaii, with surviving fish apparently gasping for air at the sea surface.

To understand what has happened, and what the longer term consequences might be, we need to understand that molasses is the result of the first processing of sugar cane (or sugar beet). Juice from the canes is boiled, which results in some of the sugars forming gooey, dark-coloured solids (by caramelization, a process well known to cooks). A sugary, water-soluble product, molasses will dissolve in water – unlike oil – although the sticky caramel-like material dissolves much more slowly that the sugar crystals themselves.

Sugars are excellent substrates food for bacteria, and when the molasses spilled into the sea the millions of bacteria present in every litre of sea water will have begun to degrade the sugars. Most of these bacteria require oxygen to do so, as we do, and quickly the sheer number of bacteria in the water will have used up more and more of the oxygen dissolved in sea water for this purpose. This would have caused sea water around the vicinity of the spillage to become rapidly de-oxygenated - hence why the fish around the spill were gasping for air.

Molasses itself is not toxic, and is far more quickly degraded than mineral oil, and that means this will be a relatively short-term environmental issue - unlike the results of oil spillages. Local authorities expect the problem to persist for several weeks, though it could be shorter. They have also expressed concern about increased growth of algae resulting from the spillage, but this is highly unlikely as the majority of algae, unlike bacteria, do not grow on sugars and are stimulated more by the accidental release of nutrients such as nitrogen or phosphorus (such as from fertilisers).

Interestingly, when the oxygen in sea water has been used up, bacteria (and yeasts) can continue to break down the sugars in treacle. This time they use the fermentation process which produces a number of by-products, including some organic acids, such as acetic acid, or vinegar, and of course ethanol. These products in turn are used by other bacteria in sea water that make hydrogen sulfide, which causes a instantly recognisable rotten egg smell. So the sea around Hawaii - smelly, sticky, brown, and full of dead fish - will be pretty horrible for the next few weeks.

The sharks, by the way, are there not for the sugar but for all the dead fish that make an easy meal.