Heat Pasteurisation and Carbonated Cider

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Chalkyt

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There seems to have been quite a few questions lately about heat pasteurising and sweet carbonated cider. So, one of my Covid10 lockdown “projects” has been to gather together the various bit of information out there. So, read on if you wish.

Since the great Australian drought and bushfires trashed our small apple crop this year, I have just started making this year’s cider with juice sourced from a commercial orchard. The plan is to bottle some medium sweet carbonated cider with this juice without the worry of losing my own precious juice if things go wrong.

First of all I must declare that none of the following is particularly new or unique, rather I have been able to “stand on the shoulders of giants” (Pappers and all those others who have posted and advised about heat pasteurisation on this forum plus some stuff from “Dr Google”, Andrew Lea and Claude Jolicoeur).

Having done this, I thought it might be worthwhile to post it all in one place here on HBT.

Everyone has different ideas about what carbonation and sweetness they want, but a good starting point suggested by Andrew Lea (Craft Cider Making) is to bottle when the cider is still sweet (say 1.010 or whatever you like), let it carbonate to around 2 atm (which involves letting fermentation continue for about another 0.004), then stop fermentation by heat pasteurisation to end up with a carbonated medium sweet cider.

To get some insight into what happens to the cider, and when, I set up some bottles with carbonated water, pressurised to 2 atm at 20C. (once again, I didn’t want to waste cider if something went wrong!)

We all get concerned about the potential for bottle bombs. So, I used Grolsch type bottles rather than “standard” bottles. Like many, I bottle my cider in 333 ml “standard” bottles as they are a convenient size and easy to get. It was interesting to note that Grolsch bottles are about 20% heavier (typically, 245g vs 195g) so possibly 20% stronger and hopefully able to handle 20% more pressure and thus a bigger “bottle bomb” safety margin.

So, in summary, this is what I found out (much of this, we already know but it might help people just starting out) …

- Bottle internal pressure is a function of pasteurisation temperature which releases CO2 from the cider into the bottle space. The higher the temperature, the higher the pressure and more chance of bottle bombs.

- At the temperature (65C/149F) currently recommended for pasteurising cider, bottles carbonated to 2 atm at room temperature can generate up to 6atm (90psi) of internal pressure (this is exactly what Andrew Lea says, so no surprises here). This is about the practical limit for “standard” beer bottles, although there is some suggestion that they are tested to 100psi before being used in volume production.

- Grolsch type or champagne bottles probably have a 20% or more safety margin over standard beer bottles.

- You can heat pasteurise at anything above 60C (140F) if you leave it for a long enough time. Pasteurisation is unlikely to reliably happen below this temperature. A few degrees make a big difference to how much and how quickly pasteurisation takes place.

- The recommended amount of pasteurisation for cider is defined as 50 pasteurisation units (PUs) which is a function of temperature and time

- Pasteurisation heating time can be in the order of minutes because pasteurisation also continues when the bottles are cooling down.

- The “cooler pasteurisation” method (see posts by JimRausch) is close to the perfect process for home brew cider.

- Controlling the pasteurisation bath temperature with a sous vide heater makes life easy and helps avoid temperatures that might result in bottle bombs.

The details and fine print behind all of this are …

Bottle Pressure vs Temperature

Beer bottles are reputed to be “tested” to 100psi (i.e a bit over 6 atmospheres). So the first step was to heat a bottle carbonated to 2 atm at room temperature and see what pressure is generated at 65C (149F) which is the current “magic number” for pasteurisation.

Wow, at this temperature the pressure got up to 6 atmospheres (90 psi) which is exactly what Andrew Lea (Craft Cider Making) says will happen. This is where I stopped (partly because that is as far as the gauge went and partly because it was getting close to the supposed limit of “standard” bottles, after all I did have a bottle bomb last year when the temperature got away from me).

On this basis it looks as though heat pasteurising in “standard” bottles should be kept at or below starting pressure of 2 atm and pasteurised at 65C or lower. If higher levels of carbonation are required, then using Grolsch or Champagne bottles would be a wise move.

How much pasteurisation do you need?

A bit of background and reference stuff first…

The amount of pasteurisation needed is a bit like “how long is a piece of string”. Commercial operations work on the basis of Pasteurisation Units (PUs) and different operators seem to aim for numbers anywhere between 15 and 45 PUs, either with high temperatures for a short time or lower temperatures for a long time. It seems to depend a bit on what product is being pasteurised and what the yeast or microorganism load might be (e.g. Cider and Beer are different to Fruit Juice.)

Pasteurisation Units are based on work by Del Veccio and others in 1951, who determined that the minimum effective temperature for pasteurisation is 60C, and

1 PU = 60C exposure for 1 minute.

Currently, the accepted amount of pasteurisation for amateur or craft cider makers is in the region of 50 PUs. This comes from a few sources including Andrew Lea, and indirectly from Jolicoeur (149F for 10 minutes). Bembel also has a very detailed post on the subject on this forum (11 May 2016). Therefore 50 PUs seems like a good number to work towards, although some sources claim that 30 PUs is enough.

So, how do you get to the target 50 PUs?

There are time and temperature formulas for calculating the PUs per minute, but if you aren’t a fan of maths here are the PUs per minute for the temperature range that we are concerned with.

(Sorry, just noticed that the table is a bit confusing. In the original version which I wrote in MS Word then pasted here, the PUs line up nicely under the temperatures but when it is saved all the spacing goes away and editing doesn't correct it. You will just have to do the maths or count along the rows, e.g. the fourth temperature 63C lines up with the fourth PUs per minute 2.7 etc.)

Temp C 60 61 62 63 64 65 66 67 68 69 70

PUs per minute 1.0 1.4 1.9 2.7 3.8 5.2 7.3 10.2 14.2 14.2 19.7

(For anyone interested, the commonly used formula is, PU= t x 1.93 raised to the power of (T-60). I also found an article by Andrew Lea which uses the slightly different PU= t x 10 raised to the power of (T-60)/7. In both cases, T = temperature C, and t = time at temperature T in minutes, and both formulae give the same results).

So, what does this mean? To get 50 PUs you simply have to achieve a combination of the PUs at temperatures and times (e.g. 60C for 50 minutes or 70C for 2.5 minutes, or any other combination) that equals 50.

You will see there is a rapid increase in the amount of pasteurisation achieved per minute as temperature increases (but unfortunately for us so does the pressure inside the bottle). This increase in pasteurisation with temperature means that volume processors can use “flash pasteurisation” where the finished product is exposed to a high temperature for a short time during the production process.

The time and temperature combinations that we could safely work with also explain why JimRausch’s “Cooler Pasteurising” method is ideal for those of us without a CO2 setup who are wanting to make sweet carbonated cider while minimising the risk of bottle bombs. Attached are a couple of charts of my results. They both approximate what will happen with a “Cooler Pasteurisation” approach.

The charts take into account the pasteurisation that continues as the bottles cool down towards 60C after they are removed from the bath, so heat doesn’t need to be applied for a long time.
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The 65 C Chart shows what happens if the temperature is kept at or below 65C to minimise the chance of bottle bombs (i.e. the pressure should remain below 6 atm). Starting with room temperature (around 18-20C) bottles are pre heated with hot tap water at 50C-60C then the water is heated to 65C. The bottles get up to temperature in 5 minutes. Holding at that temperature for another five minutes results in around 34 PUs. If the hot bottles are removed at this stage (which is around the 10 minute mark) they hold their temperature quite well, taking another 5 minutes to cool down to near 60C. During this time they generate another 15PUs, resulting in just under 50PUs. For me, this was near enough as further cooling will generate a few more PUs.

The 70 C Chart follows the same process but pushes the temperature envelope a bit, minimising the amount of time that bottles need to stay in the bath, but possibly exposing them to a pressure above 6 atm for a short time. This is probably useful if you have a lot of bottles to pasteurise as they only need to stay above 65C for about two minutes. I took the bottles out as soon as their internal temperature looked like reaching 67C (around the 6 minute mark). At his point they had 24 PUs and cooling down generated another 29 PUs resulting in just over 50PUs. (Edit note... the first post had some of these numbers reversed. They have now been corrected... sorry!)

In theory it is possible that a very conservative approach of pasteurising at 62C or 63C for around 25 minutes could also result in good pasteurisation although this is in the region where PUs per minute are quite low, thus opening the process up to a large margin of error and unreliable results.

I used an inexpensive sous vide cooking stick from eBay to control the bath temperature, but simply adding boiling water would work as well (the bath was a fermenting bucket, but a cooler would probably be better). A cooking thermometer in a cork stuck into a bottle of water was used to measure the bottle temperatures, and a pressure gauge attached to a Grolsch cap and carbonated bottle monitored the pressure as temperature increased. I normally use a small plastic drink bottle as well to let me know when carbonation reaches 2 atm (i.e at that pressure the bottle is hard).

I guess you don’t need all of those bits and pieces (but they help you know just what is going on). Just the sous vide heater and a timer allows you to you monitor the process to get good pasteurisation while minimising the chances of bottle bombs.

When starting out making cider it seems that lots of people (me included) are surprised when they don’t produce something like much of the commercial cider (sweet and carbonated). So I hope this exercise has usefully pulled together the forum’s “body of knowledge” about how to produce something that is both sweet and carbonated.

Putting it together certainly kept me entertained for a while.

Cheers!


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Thank you so much Chalkyt.

One thing I've heard conflicting info on is the gravity of your cider and how that may affect pasteurization. It makes sense to me that there could be a difference between pasteurizing cider that was back sweetened to 1.002 vs 1.012.

Have your heard anything about this?
 
Regarding pasteurising ciders with different FGs. I didn't look into this specifically because there doesn't seem to be much information about other beverages, but Andrew Lea refers to the different PUs needed for cloudy apple juice (500 PUs and SG something like 1.050) vs fermented cider (50 PUs and something like maybe SG 1.005 or less). He puts this down to cider's alcohol which provides a constraint on yeast growth so less heat energy is required to inactivate the already stressed yeasts. He also suggests that cloudy juice provides a physical protection for the yeasts. I guess that different gravity beverages could be subject to different pasteurisation needs but we are talking about using 50 PUs which is a "one size fits all" number that could well be overcatering and covers the range of cider FGs that we might want to pasteurise.

As I mentioned above, Breweries seem to work on anything from 15 to 45 PUs using different time and temperature techniques so I don't think it is an exact science, rather something that works, although I imagine that minimising energy costs for them is a big driver of the methods used.

Sorry, I can't add any more than that, but others might be able to enlighten.
 
Counterpoint: A Preliminary Evaluation to Establish Bath Pasteurization Guidelines for Hard Cider

4. Conclusions
The results of this work indicate that as little as one pasteurization unit is sufficient to reduce even high populations of spoilage yeast, though further research will be necessary to evaluate the possible effects of pH, ethanol concentration, and added preservatives on cider pasteurization recommendations. The efficacy of heat pasteurization on other microorganisms, including additional S. cerevisiae strains and Zygosaccharomyces species should be evaluated. Relatively heat resistant bacterial species, such as Lactobacillus species, should also be evaluated. Though there is yet to be enough research to establish a pasteurization standard for the cider industry, it is likely that recommendations of 50–60 PUs are much too high and result in energy and labor over-expenditures. Regardless, it is important for each individual process to be validated with microbial testing to ensure sufficient heat processing and resulting reduction of microbial viability.

Not definitive research, but quite interesting nonetheless. Some numbers for MLF and Lacto and Brett would be great.
 
Thanks to ChalkyT, this is a great thread.

how is everyone monitoring temps?

like the pic above, I get an empty bottle of same size and fill with water that was sitting in keezer so it matches the temp of cider. I have a rubber bung w a hole drilled Into it. Then I check with an instant read thermometer.

Because the bottle w water isn’t capped, I’ve always wondered if there is a significant temp difference. In addition, I wonder if the capped cider’s pressure affects the temp as well.
 
Thanks for that Wasully. It is a great research paper. It supports other opinions on Dr Google that less than 50 PUs are needed for cider, but I couldn't find any other definitive research. I guess that the yeast load in cider is about the same as for beer and breweries seem to work on a lower PU target than 50. Not that it matters too much for us, as I suspect that 50 PUs is probably overcatering as is pasteurising temperature above 70C, but it works and gives us a number with a wide margin of error to aim for, and reduces the chances of bottle bombs (I know from experience that they can be quite exciting and give no warning, just BOOM!).

As time goes by, we seem to learn more but any proper research is certainly well beyond my resources (and I suspect ditto for other "craft" cider makers). Although much less than 50 PUs probably do the job, I think that the 50 PU target and temperatures fit nicely into a 10 minute or so time frame which is a nice leisurely pace consistent with pasteurising manually without having to get too fussed about absolute times and temperatures.

The follownig is a somewhat light hearted look my first methodology, but some of the lessons from my experience are probably worthwhile sharing...

Azor, as far as temp measurement is concerned, I did the same as you. I haven't considered any temperature/pressure differentials between sealed and unsealed bottles. My first attempts last year involved a bottle of water at the same temperature as the cider with a cooking thermometer in it. I didn't monitor the temperature of the bath.

Pressure was monitored with a pressure gauge in the top of a plastic drink bottle which had been sealed when the cider was capped. This had already shown that when the bottle was hard, the pressure was about 2 atm so the carbonation was about where I wanted it. So, everything into the water bath (at that time it was SWMBO's electric preserving/canning pot) with temperature controlled by turning it on and off.

What can go wrong?... the seal on the top of the drink bottle started to leak (probably around 3 atm or 45psi) so the pressure in the capped cider bottles was actually higher than the gauge said, so I was quite relaxed at that stage until I realised that the temperature was going up but the pressure wasn't... then the pressure pushed the drink bottle bottom out, so it falls over in the very hot water... I am now busier than a one armed paper hanger while trying to retrieve the gauge and forget to turn off the heat, and have lost track of the time... and everything has been in the now very hot water longer than planned... then BOOM! That brings you back down to earth while trying to retrieve the other bottles before they go boom as well.

Nevertheless it all worked, with the result being slightly sweet petillant carbonated cider which hasn't blown up in the meanwhile. Success (sort of), hence the search for a more controllable process this year.

This year I found that the cheap sous vide heater made life very much easier. You can set the temperature that you want and monitor water temperature progress on one device. I still used the thermometer in a bottle to indicate the temperature in the bottles and note that on the way up there is a temperature differential of 3-5 degrees between the bottles and bath which more or less equalises to within a degrees in a minute or so once the bath heat reaches the desired temperature.
 
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I like your idea of the sous vide added to my cooler. What is the watts of the one you use. I see them all the way from 800w to 1500w.
 
Mine is 1000W Inkbird Brand. At 1000W it does take some time to heat up the bath water from cold so I usually start with hot tap water in the bath and throw in a jug or two of boiling water to get somewhere close to 60C and go from there. Having said that, the 1000W unit maintains the bath temperature very well, although I note that there is sometimes up to a degree C difference between the souse vide temperature display and the electronic thermometer. The Inkbird beeps when the bath temperature is getting close to the set point, so this is a useful feature. Both units are "el-cheapos" that I found on eBay and given the application, a degree difference between the two units isn't a bother to me, especially as I don't know which one is correct (if any).

Merry Xmas... it is 10am Xmas Day here and SWMBO has just announced "the turkey is in the oven!".
 
Merry Christmas to you guys and thanks for all your help. I will get one ordered.
 
Happy to help, I don't think that the brand matters as we are not precisely cooking exotic souse vide meals, just heating water. I found that everyone on the forum was happy to point me in the right direction when I started making (terrible!) cider. I have learned a lot and I am sure you will too.
 
My Apple cinnamon cider was a hit at the NYE get together and I couldn't have done it without everyone's help. I have some blackberries frozen in the freezer so my wife wants me to make a apple=blackberry cider next so does anyone one have a good recipe for adding fruit. I think I will add it in the secondary after my original recipe for apple cider ferments out. I will be starting the new batch in a couple of weeks.
Thanks
 
I expect you will get similar replies to this. I haven't used blackberries but imagine they would work the same as any other fruit.

When I do apple/cherry cider I add about 10%-20% cherries from my trees that have been frozen, so that on thawing they are a bit mushy and give up their juice and flavour quite readily (as well as being reasonably easy to "pour" into the secondary carboy). They are usually pitted although when making cherry liqueur (steeped in brandy and vodka for about six months... yum!) I use cut cherries with the pits in but the flesh cut to expose them (the slightly bitter taste from the pits permeates the final product). Might try this with the cider this year as we have a bumper crop of cherries as well as apples.

I add the frozen cherries to secondary at around 1.015 - 1.020 and leave it for about a month by which time the SG is down towards 1.000 (or 1.003 if I have used SO4). Then, just proceed as normal to bottle, back sweeten, pasteurise or whatever. It certainly improves with age, developing a quite complex flavour and it would be interesting to try using the cherries for "sweetness" and bottle at say, 1.005 for carbonation.
 
Thanks for the feedback about NYE. You might find you get a better response and attract more attention if you start a new thread asking about adding fruit to cider. This is a bit "off topic" for the original post which is now getting a bit long.
 
Thanks for the advice and I will start a new one...you have been a awesome resource of information.
 
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