Each time you open a soft drinks bottle, all the gas escapes. When you put the top back on, the drink goes flat until the escaped gas is in equilibrium with gas dissolved in the drink. After a while, the lemonade goes completely flat.

I'm suggesting a lid with a pressure tap on it. To get a drink, turn the bottle upside down (carefully) so the lid is totally submerged. Push a button on the cap to let lemonade out. During this process, the gas is floating above the lemonade, so it doesn't get out. The drink stays fizzy for longer.

The pressurised gas above the drink would expand to fill the space left by the drink you've just poured out. If you didn't pour too much out, then the pressure of the gas in the headspace would remain above atmospheric, and the drink would be fizzy for longer. If you poured out enough that the pressure in the headspace fell below atmospheric, a pressure relief valve could allow air into the bottle.

Net result: Bottle is either at atmospheric pressure, or above it, and your lemonade/soda pop takes longer to go flat.

Edit (13:00 GMT 21st Jul 2004) have removed posts discussing US/UK terms: Lemonade in the UK is like soda pop in the US. No offence intended by removing posts, just kinda wanted to keep the annos on the subject.

Edit (17:00 GMT) ...and look how much it helped :-)
-- david_scothern, Jul 21 2004

Google is your friend http://www.prairiem...z/clastalsodsi.html
First link in search for "seltzer bottle" [Freefall, Oct 04 2004, last modified Oct 05 2004]

Fizz Keeper http://www.marketex...206_m1_1s9_2s37.htm
Pump it to keep stuff fizzy. [oneoffdave, Oct 04 2004, last modified Oct 05 2004]

[david] many soft drink companies (such as the company I work for), inject a gas into the headspace of a can/bottle to maintain the quality of the drink.

(it's also used to give rigidity to plastic bottles of still drinks - if they are not rigid, it means that it's harder to pack and it's impossible to vend).

Some companies, for cheaper manufacturing, don't flush the headspace and allow the carbonated drink to come into equilibrium with the headspace. This means that you have to over carbonate the drink during manufacturing to ensure that when the customer gets to drink it, carb levels are perfect.

So I'm thinking that some kind of mechanism for re-filling the headspace is the easiest way to ensure fizzy drinks at all times.
-- jonthegeologist, Jul 21 2004


How does the lemonade get out, without letting air in? The pressure tap bit is a red herring. You can recreate this effect by pouring out the fizzy drink, then squeezing the bottle so that there is no air gap, then tightening it again. However, leave it a few hours in the fridge, and the pressure of the gas escaping will have filled the original gap with CO2. I've yet seen any research which can tell me whether this is better or worse than just leaving the air in in the first place. I suspect it's worse.
As far as I can see, you would have the same effect. So drink doesn't stay fizzy any longer at all.
I think Jon's right - you need a cap which reinjects gas (Nitrogen say) into the headspace to reequalise the pressure.
-- goff, Jul 21 2004


Ah. So-called lemonade in UK is sweetened fizzy water, flavoured with artificial citric horrors. I think Dr Pepper and 7-Up are examples, but they may be different in US / Canada.
-- angel, Jul 21 2004


[goff] definitely worse. It'd be like having a vacuum in the space above the drink, rather than air pressure. Much more of the fizz would boil off to re-pressurise it.
-- david_scothern, Jul 21 2004


Lemonade is lemon soda - but clear, just like seven up really (but without the lime element). //child of the seventies comment//Also referred to simply as "pop", or "fizzy pop" - though that's a bit old fashioned really //child of the seventies comment//. Principle would apply to all carbonated drinks though.

but [david] how would the drink come out, without any air going in to replace it? If you stop the air coming in, the liquid will simply refuse to flow out (exactly beacuse of the vaccuum effect as you describe)
-- goff, Jul 21 2004


You have pressurised gas in the headspace above the drink; thus if you turn the bottle upside down, drink will come out as the gas expands.
-- david_scothern, Jul 21 2004


[goff] a nitrogen cap is what we use for non-carbonated products in the UK.

[angel] Dr. Pepper and 7-Up are the same across the globe I think.
-- jonthegeologist, Jul 21 2004


Dr Pepper is more like cherry coke here in Canada. And pop refers to carbonated drinks with artificial colour and flavour, whereas lemonade is still real lemon juice, water and sugar, no fizz.

Perhaps drinks should be sold flat, and refrigerators equipped with a carbonator. Right next to the ice dispenser. Think of the fun you could have with it.
-- lintkeeper2, Jul 21 2004


// Perhaps drinks should be sold flat, and refrigerators equipped with a carbonator // very similar to a soda stream then! Nice.

Dr Pepper and Cherry Coke are different products but many people think they taste similar.
-- jonthegeologist, Jul 21 2004


//refrigerators equipped with a carbonator//
I have a carbonator (brand name "Sodastream") - are they available in US?
-- angel, Jul 21 2004


I've only ever seen them in restaurants. Tell me, [angel], do you carbonate things you should not? Meatloaf, for instance? Do you frequently win belching contests and no-one has guessed your secret?
-- lintkeeper2, Jul 21 2004


I confess, I have yet to try carbonating meatloaf. I cannot imagine that this situation will change. I have many secrets, but this is not one of them.
-- angel, Jul 21 2004


Mmmm, fizzy gravy. Tingly peanut butter on your tongue. Jelly/Jam donuts with a fun fizzy filling. Bubbly butter on a bagel. Prefizzed shampoo, take the 'effort' out of washing your hair. Endless pointless possibilities.
-- WYBloke, Jul 21 2004


David_scothern, what you describe is essentially a carbonator bottle. These are baked. Though they're not in wide use anymore now that soda (or pop, or whatever you call your fizzy drinks) comes pre-carbonated in cheap disposable plastic bottles, they are definitely still easily available.
-- Freefall, Jul 21 2004


Freefall... what everyone else is describing is a carbonator bottle / sodastream etc. I was looking for a relatively simple and inexpensive way to keep drinks fizzy. Sure, you can create them on the spot, yes, there's a sodastream in my attic somewhere, but you can't recarbonate them later - if they have any reasonable viscosity (read - if they have sugar / syrup in) they bubble over as soon as the pressure is removed.

I'm not looking for an old-style soda syphon here, essentially I'm just after a different bottle lid.
-- david_scothern, Jul 22 2004



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