杏吧原创

Honey monster

How can an unopened jar of runny, clear honey suddenly begin to turn into a hardened block of sugar with no obvious external stimulus? Jars that have remained clear for years can, over the space of a couple of weeks, change into solid sugar while the jar remains motionless on its shelf. Temperature does not seem to be a factor 鈥� the process can occur in winter or summer.

鈥� Bee-keepers argue about this, as honeys from different sources behave differently. Honey is a supersaturated solution of various proportions of sugars (mainly glucose and fructose), and is full of insect scales, pollen grains and organic molecules that encourage or interfere with crystallisation. Glucose crystallises readily, while fructose stubbornly stays in solution. Honeys like aloe honey, which is rich in glucose and nucleating particles, go grainy, while some kinds of eucalyptus honey stay sweet and liquid for years.

Unpredictably delayed crystallisation means a nucleation centre has formed by microbes, local drying, oxidation or other chemical reactions. Crystallisation can also be purely spontaneous, starting whenever enough molecules meet and form a seed crystal. Some sugars do this easily, others very rarely.

By seeding honey with crystals, or violently stirring air into it, you can force crystallisation. Products made this way are sold as 鈥渃reamed鈥� honey. The syrup between the sludge crystals is runnier and less sweet than the original honey, because its sugar is locked into crystals. Gently warm some creamed honey in a microwave until it dissolves, compare the taste of the syrup with the sludge 鈥� you will be astonished.

Jon Richfield

Somerset West, South Africa

鈥� I have seen this happen many times. The time before crystallisation starts seems to depend on the source of the nectar the honey is made from. Oilseed rape honey will crystallise within a week or two of the bees making it. Heather honey never seems to crystallise. Fuschia honey is extremely runny and, unlike any other I have seen, seems prone to fermentation, even when all the extracted honey comes from cells capped by the bees for storage. Even this crystallises after a year or two.

Pat Doncaster

Cork, Ireland

Gurgle time

Does liquid pouring from an inverted bottle flow faster at the beginning and end of its expulsion or when it reaches the 鈥済lug-glug鈥� point somewhere in the middle? And, whatever the case, what is the explanation for the different velocities?

鈥� Water flowing from an inverted bottle has no free surface. So the water coming out has to be replaced by something else because liquids do not expand or contract very much when the pressure changes. In the case of a thin-walled plastic bottle, the volume can be replaced by the walls of the bottle being pushed in by air pressure as the water runs out, so this is what happens first.

Once this has happened (or in the case of a glass bottle, immediately) another replacement mechanism is needed and bubbles of air have to enter through the neck of the bottle. Essentially, the bubbles and the water escaping have to take turns, coming in and out, respectively, giving rise to the glug-glug effect.

Two other important factors affect the rate of flow. Firstly, if there is a significant amount of gas inside the bottle above the liquid, this can expand to replace the volume of water lost. This process goes on until the reduced pressure of the gas is just sufficient to support the height of liquid above the exit. Then glug-glug starts again.

The second factor is swirl. If the bottle is handled so that the liquid swirls the water moves to the outside of the neck and allows a column of air relatively free passage up the centre. By actually moving the bottle in small circles before releasing the liquid, you can get a very effective tornado in a bottle.

These effects are important in industrial separating devices called hydrocyclones, which are shaped something like an inverted milk bottle. It is possible to tell how effectively these devices are working simply by viewing the pattern of discharge from the bottom, described as rope, cone or spray.

Martin Pitt

Department of Chemical and Process Engineering University of Sheffield, UK

鈥� Note to bar staff in a hurry: A little experimentation in the New 杏吧原创 laboratory confirms that whichever angle the bottle is held at, flow is fastest when the bottle is full, because this is when there is greatest pressure on the liquid at the mouth of the bottle.

To pour out the contents of a bottle quickly, it is far more effective to keep it at an angle than simply turn it upside down. This is because the angled pour avoids the gurgling that slows the passage of liquid through the neck of the bottle. For an extra edge, follow Martin Pitt鈥檚 advice: spin the bottle and then invert it while continuing to rotate it rapidly about its axis.

We found that a 750-millilitre wine bottle emptied in 9.9 seconds if inverted, but in 8.1 seconds if held at 45 degrees. Swirling the bottle so that a little tornado forms in its neck, allowing air to enter continuously and replace the liquid, brings pouring time down to just 7.7 seconds.

In all cases, the rate of emission slows as the head of water above the neck falls. Dividing the volume of water in the bottle into equal thirds, the first, second and third volumes left the inverted bottle in 2.5, 3.5 and 3.8 seconds; the angled bottle in 2.0, 2.4 and 3.7 seconds, and the swirling bottle in 2.0, 2.3 and 3.3 seconds. The swirling technique, although very smooth once perfected, is not recommended for high-speed pouring of beer or any drink containing gas 鈥� Ed

This week鈥檚 questions

Ice art

On some cold mornings the frost on windows and cars makes patterns that look just like leaves, ferns and branches. How does this happen?

Bob Clarke

New Minas, Nova Scotia, Canada

Fast work

My computer is a well-maintained PC with a 20 gigabyte hard drive. When I need to look for a file containing a particular phrase, searching the whole hard drive can take several minutes. So why do I get search results in a matter of seconds when I plug a phrase into a search engine like Google, which searches millions of websites?

Joel Smith

West Carrollton, Ohio, US