Bath pasteurisation of kegged cider

[Henry]

Good morning all.

I have checked the archives on this and whilst the indications are not favourable, I want to raise the question again, in case there is any new experience.
I am interested in knowing if there is any way to bath pasteurise cider in standard DIN 30 Litre capacity kegs.  The kegs fit nicely in my home made bath pasteuriser.   The cider has 8-14 g/L sugar content and is mildly carbonated.  I can unscrew the central spear and insert a thermometer into a spare keg to check temperature in the centre.  If I do this 2 more kegs would fit into the bath.  I understand one of the main issues would be temperature diferential from the outside where heat transfer takes place and homogenisation of the heating cider within the keg, but if it is done slowly (how slow?) it seems that it could work in theory.

I want to avoid artificial sweeteners if I can.

Henry

[Wes Cherry]

I would be worried about the pressure buildup in the keg.  The German mades ones I have are rated at a working pressure of 3 bar (~45 psi), though the burst disc is rated to blow at 35 +/- 5 bar.

Maybe attach a pressure relief valve on the gas port of a keg coupler while pasteurizing?

I suspect convection currents in the keg would enable a fairly quick, relatively even, heat transfer to the cider.   It might even be on the order of the same time as botttles because the thinner stainless will transfer heat faster than the thicker glass of a bottle.

-Wes

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[winters...@gmail.com]

Why not pasteurize in the keg before you carbonate the cider to avoid high pressure?  You could even let it breath a bit from the top while checking the temperature of the actual keg you have in the water bath.

[Nat West]

We pasteurize one cider in bath, in kegs. It is non-carbonated and takes an hour. Compare that to 7-9 minutes in bottles. We put a corny keg in the bath with the lid open and use a LOOOONG stem thermometer to check temp. There is a large variance in temperature between the top of the keg (hot) and bottom (cold). We do not see any convection, just stratification.

While pasteurizing these kegs, each one is tapped with a sankey head with the bits taken out so it vents freely to the air to avoid pressure buildup.

You could re-carbonate in the keg after pasteurization but you'd need to ensure your lines were sterile and use medical-grade CO2 or a sterile filter. Standard warnings apply for the difficulty in achieving this in practice. We don't carbonate this special pasteurized kegged cider.

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[winters...@gmail.com]

Nat, Why do you need to carb with "medical grade" CO2 at this point?  Presumeably because "regular" CO2 is not sterile, but is that really necessary?  Was not aware of the risk factor of problems from "regular" CO2.

[Henry]

Thanks to all for comments.  I have tried carbonation in-keg and it takes a lot of effort and some time, and its hard to get consistent carbonation levels.  These kegs are for my fledgling commercial activity and I would like to offer an alternative to dry cider.  My cider is carbonated to about 0.8 bar at 20oC.  So according to Andrew's table the pressure at 65oC will increase to about 2.4 bar so it's within the 3 bar limit mentioned by Wes, and stamped on to my kegs, so I think I will give it a try.
Noted Nat's comments on the temperature stratification and the need to procure a long thermometer.  I wonder if there's a way to hoist the kegs out of the bath pasteuriser at e.g. 50-55oC, shake them a bit and put them back, without endangering life and limb.  Will think about that.

[Henry]

OK so I tried it with three water-filled kegs, one of which was open topped for temperature measurement.  When the internal temperature of 50oC was reached we hoisted out the "control keg", fitted the spear and shook it around to homogenise the temperature.  My observations are:

1. the heat transfer seemed to be pretty efficient with a temperature lag from bath to keg centre of 5-6oC.  I was not able to measure temperature profile or layering due to the length of the thermometer stem, but there was a 1.0 oC difference from the very top o 7-8 centres into the keg (the length of th temperature probe).  When we hoisted the temperature-measurement keg out at 50oC, shook it and replaced it. the temperature remained pretty well the same after shaking.  So it appears, somewhat empirically,  that from the heat transfer standpoint, including hoisting and shaking,  it works.

2. The kegs were completely filled with 30 kg of water.  This surprised me as I expected there to be some headspace room.  The expansion of the volume for 30L of water on heating from 10oC to 65oC is about 0.6 litres.  This is obviously too much from the pressure increase standpoint.  If I try it with cider, which I am considering, I will fill the kegs to 28 or 29 litres to leave some headspace.

best wishes

Henry

On Friday, August 7, 2015 at 7:01:23 AM UTC+3, Henry wrote:

[Benjamin Weaver]

Nat, 

How deep is your bath?  I don't understand the stratification you describe.  That you would measure a higher temperature on top seems to violate the laws of physics.  Your corny keg probably has a rubber bumper on the bottom that would temporarily slow heat transfer from underneath, but that doesn't go up the walls and you've still got the highly heat conductive sides.  If anything, I would expect any vertical stratification to have the opposite effect, making the top cooler.  Maybe you can help me out.

Benjamin

[Chris Schmidt]

I notice the same effect with my bath pasteurization, and that's because warm water (cider) rises. There can be a 4 degree difference between the top of a 500ml bottle and the bottom. If you stick a temperature probe into the bottle, or keg, at various depths, you'll notice the stratification layer that Nat is talking about.

So I shoot for an average temp, so that I don't cook the cider at the top levels, but make sure the bottom layer is properly pasteurized. 

[Nat West]

Our water bath is about half the height of a full sixth barrel keg, and completely higher than a 500 ml bottle but what Chris says is right. Hot rises.

On Sun, Aug 30, 2015 at 2:10 PM, Chris Schmidt <ch...@todcreekcider.com> wrote:

I notice the same effect with my bath pasteurization, and that's because warm water (cider) rises. There can be a 4 degree difference between the top of a 500ml bottle and the bottom. If you stick a temperature probe into the bottle, or keg, at various depths, you'll notice the stratification layer that Nat is talking about.

So I shoot for an average temp, so that I don't cook the cider at the top levels, but make sure the bottom layer is properly pasteurized. 

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[Benjamin Weaver]

Hmm..  in a closed system, you might expect this kind of stratification.  But in a bath pasteurizer, you essentially have a heat source and a heat sink.  The hot water acting as the heat source and the ambient air as the heat sink.  Your bath being on the bottom and ambient air temperature on top, this defies any kind of thermal equilibrium, unless the bath itself is also stratified.  Maybe I'm thinking of this wrong.  Is your hot water not recirculating?  

Thanks for the reply.

[vince wakefield]

But is the heat sink taking away the heat away quicker than the heat source is adding the heat?

 

Vince

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[Benjamin Weaver]

Vince, 

The short answer is that it doesn't matter at what rate the heat transfer occurs.  It will reach some kind of steady state within the bottle or keg whose heat gradient is only dependent on the external source and sink.  I think I see where my confusion was.  I was thinking of the top as the heat sink, but with the lid on the bath pasteurizer, the hot water and steam rising is trapped.  There's your external heat gradient with heat source on top, leading to the same conditions in the bottle. I was thinking of an open rather than a closed system and hence my confusion. 

You've probably already thought of this, but if you're recirculating during pasteurization and you could somehow put your inlet at the top of the water level and outlet at the bottom (or vice versa) you should get rid of this stratification.  But I imagine it's not too much of a problem to begin with.

[Michael Holzer]

Hi Nat,

I agree bath pasteurization is not a great option for kegs. But how do you ensure that you kegged cider stays healthy? Can you shortly elaborate on your practice of kegging cider?

[Nat West]

Sorry that I nearly missed this email. We don’t ensure that it stays healthy. We keg it, backsweetened with fermentables, but having cross flowed it, the cider is quite low on yeast load. A touch of sulfites on some ciders, keep them cold and drink them fresh means we are generally very happy with 6 months shelf life. We work through the beer industry, which generally has a lot of appreciation for stock rotation and cold chain, which works to our benefit for kegged ciders.

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