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Heat Pasteurizing and Carbonation... more stuff!

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Chalkyt

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Regular readers of the forum will know that I have had a “bee in my bonnet” about heat pasteurizing for sweetness and carbonation control as an alternative to chemical pasteurization. The aim is to bottle cider well above SG 1.000, allow fermentation to continue and generate carbonation to a desired level, then stop fermentation at a point where residual sugar remains… all without creating bottle bombs!

So, thanks to plenty of Covid restriction spare time, and following yet another post asking about heat pasteurization I set up some experimental trials looking at different approaches to heat pasteurizing and recorded what, when and how things happen. It was an interesting exercise, so I thought it worthwhile posting what I have found out even though it repeats some stuff that I have posted before. None of the approaches are new as they are based on previous work by Pappers, JimRausch, Bembel and others, but they each give a different insight into the process.

For craft cidermakers without access to special equipment such as low micron filters or “flash pasteurizers”, heat pasteurizing of bottled cider in a heated waterbath is the easiest and a very effective method of producing sweet, carbonated cider.

The short version of how to heat pasteurize reasonably safely is…

Heat pasteurize bottled cider which is carbonated to no more than 3 Volumes of CO2

Do this at no more than 65C-70C for 10-12 minutes then let the bottles cool down

The key issues are:

- Pasteurization occurs at temperatures above 60C and is measured in Pasteurization Units (PUs).

-One PU is the level of pathogens, yeast, etc, destroyed in one minute at 60C for the beverage being pasteurized. So, two minutes exposure will result in twice the PUs.

- The recommended target for heat pasteurizing cider is 50 PUs. The target for other beverages depends on their pathogen and yeast load.

- Pasteurization results from temperature x time. So, lower temperature for a longer time produces similar results to a higher temperature for a shorter time.

- There are opinions that 30 PUs are sufficient to pasteurize cider so in practice, achieving something approaching 50 PUs should result in effective pasteurization.

- The number of PUs generated per minute increases exponentially with temperature so pasteurizing above 65C will produce high levels of pasteurization per minute, resulting in the target of 50 PUs being reached or exceeded in a short period of time.

- Higher temperatures also produce higher bottle pressure which increases exponentially with temperature as CO2 is driven out of solution.

- Depending on the carbonation level, pasteurization temperatures between 65C and 70C can generate bottle pressure in the range 100-160 psi.

- “Standard” bottles in good condition should handle over 160 psi for the short period of time involved in heat pasteurization.

- Bottle pressure generated during pasteurization returns to “normal” (20 psi-40 psi) at room temperature.

- Effective pasteurization can be achieved at temperatures below 70C with a bottle heating time in the order of 10 minutes. Increasing heating time increases total PUs.

- Substantial pasteurization takes place during cooldown as the bottles drop in temperature towards 60C after they have been heated to the pasteurization temperature.

- Achieving well above 50 PUs doesn’t appear to cause flavor changes or pressure problems if excessive temperature isn’t involved.

Two similar approaches are commonly used for waterbath heat pasteurizing.

“High Heat” Waterbath
This is the approach outlined by Pappers (see the sticky at the top of the forum) and JimRausch (see a post on 16 April 2018) where bottles are put in a waterbath which has been heated to 80C or more, then the heat source is removed. When the bottles are put in the bath, they heat up and the water cools down. Generally, equilibrium is reached around 70C (i.e. the bottles and bath reach the same temperature), so bottles don’t reach the initial bath temperature and don’t go above equilibrium.

As pasteurizsation level is significantly influenced by both temperature and time, optional pre-heating of the bottles allows them to reach pasteurizing temperature quickly. This gives the option of short times in the waterbath which in turn minimizes the time the bottles are exposed to high pressure.

Waterbath
Temperature
Minutes bottles are in the waterbath​
Bottle temperature reached​
Level of Pasteurization Achieved​
Calculated bottle Pressure during Pasteurization
@ 3 vols of CO2​
80C (176F)​
10​
73C (163F)*​
672 PUs​
11.0 Bar (160 psi)​
82C (180F)​
10​
71C (160F)​
295 PUs​
10.6 Bar (153 psi)​
80C (176F)​
5​
70C (158F)*​
175 PUs​
10.3 Bar (152 psi)​
82C (180F)​
6​
66C (151F)​
59 PUs​
9.4 Bar (137 psi)​
75C (157F)​
10​
66C (151F)​
49 PUs​
9.4 Bar (137 psi)​
* Bottles pre-heated to 50C​

“Low Heat” Waterbath
This is the approach outlined by Bembel (post on 11 May 2016) where bottles are put in a waterbath heated to 65C- 70C (or even less) and maintained at that level by stovetop or sous vide heating. The maximum that bottles can reach is limited by the bath temperature.

Waterbath Temperature​
Minutes bottles are in the waterbath​
Bottle temperature reached​
Level of Pasteurization Achieved​
Calculated bottle Pressure during Pasteurization
@ 3 vols of CO2​
70C (158F)​
10​
67C (153F)​
72 PUs​
9.7 Bar (140 psi)​
67C (153F)​
12​
65C (149F)​
42 PUs​
9.2 Bar (134 psi)​
65C (149F)​
12​
64C (147F)​
40 PUs​
9.0 Bar (131 psi)​


A file (Cider Heat Pasteurising and Carbonation.pdf) is attached with more background information and detailed results from the trials. Sorry if it is a bit like a College or University assignment. It takes a bit of reading, but does deal with a lot of information for those who want to explore heat pasteurization options.

The idea was to put together enough information so forum members can make decisions about the pasteurization approach that is appropriate for their situation.
 

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SiblingCider

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Great write-up! Exactly what I've been looking for. Sweet, carbonated cider via pasteurization is, for some reason, the hill I've chosen to die on this holiday season. This should help make it less bloody (metaphorically speaking, of course; proper precautions will prevent any actual blood).
 
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Chalkyt

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Good luck... as you can see, heat pasteurisation isn't all that scary once you know "the rules".

I plan to update the information with some stuff that I have found about what pressure bottles can handle. But as a preview (especially for you!) a bit more research has shown that normal (12 fl oz) bottles tend to be batch tested during production to something like 200-250psi. However batch testing can miss underspec bottles which in the worst case potentially could fail at 20% below the batch test figure. Nevertheless this means that the "worst" bottles rattling about out there should at least handle over 150 psi.

If the carbonation is kept to 2.5 volumes of CO2 and pasteurising temperature is 70C or below, the worst case pressure generated will only be in the order of 125psi which is still below the likely failure point of worst case bottles. If the bottles are removed and allowed to cool down once the target temperature is reached, this pressure starts to drop, thus reducing the likelihood of "bottle bombs" as glass can handle quite high stresses until they develop and concentrate at weak points.

I don't know what type of seals you are using (hopefully crown caps) but another thing to consider is that Grolsch type seals can start to leak at around 75-80psi (i.e. around 90psi) which can be a bit of a trap for carbonation if heat pasteurising bottles with this type of seal.
 

SiblingCider

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Crown caps. Going to set and maintain a low (<70C) temp with a Joule sous vide and a probed beer bottle of water and track cumulative PUs on a spreadsheet. Shooting for 50 PUs, will likely take them out at 30 and keep tracking temps so I can adjust for next time. Probe bottle goes in last and comes out first.
 
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Chalkyt

Chalkyt

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Sounds great... that is exactly what I do and get great results.

You will probably find (as I did), that it is almost impossible to "underpasteurise" as cooling down from 70C typically generates 20+ PUs, so that plus a bit of resting time at 70C soon gets you to over 50PUs. There are some views that anything from 30PUs upwards is fine for cider, and I have found that a bit of "overpasteurisation" doesn't seem to have any effect on flavour or quality. I can't detect any difference between cider pasteurised at 50PUs compared with150PUs.
 

MtnGoatJoe

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I heat pasteurized my juice (not cider) this year, and it went ok. I slowly heated the juice to 160 degrees (no boiling), and then poured into sanitized 50 oz bottles. I think I should have waited 10 minutes, but I didn't, and I did have 2 bottles that developed some funk. So either my sanitation or pasteurization wasn't as complete as it should have been.

Next year, I'm hoping to get a large double-boiler pot and pasteurizing in my odd shaped bottles as you suggest.
 

tlc

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Very nice write up Chalkyt. I'm going to pasteurize when I to that point so I will be studying it in the meantime.
 

Jaypkk

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Agreed. Great post. I'll add a link if I can to some interesting research out of WSU that suggests that a mere 1 PU is enough with other appropriate microbial controls to destroy two regularly used yeast strains. Below is the abstract.

"Abstract--A Preliminary Evaluation to Establish Bath Pasteurization Guidelines for Hard Cider
Though in-package water bath heat pasteurization for hard cider production is commonly employed to improve product safety and stability, there is a considerable lack of research-based guidelines to inform industry practices. In this study, fermented cider was bottled and inoculated with high populations of Saccharomyces cerevisiae and Zygosaccharomyces rouxii yeast. The bottles were then subjected to water bath pasteurization 60 °C at varying lengths of time. For both yeast species, populations were reduced to undetectable levels after just 1 min of processing time. Though validation of each individual process is recommended, cider producers may be able to sufficiently reduce the risks of spoilage organisms with minimal water bath pasteurization, especially when combined with other methods to reduce the presence of spoilage organisms."
--
Beverages 2020, 6(2), 24; A Preliminary Evaluation to Establish Bath Pasteurization Guidelines for Hard Cider
Received: 14 February 2020 / Revised: 11 March 2020 / Accepted: 30 March 2020 / Published: 13 April 2020
 
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Chalkyt

Chalkyt

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Hi Jaypkk
Good find! I read the paper and it is great food for thought. I don't know where the "50 PUs for Cider" came from originally except both Andrew Lea and Claude Jolicoeur refer to it in their books as the recommended level. I have seen other comments that 30 PUs is enough for cider but also without any references to back it up.

I guess for us amateur craft cidermakers, heating energy costs aren't a big deal but I can certainly see that it is important in a commercial environment. What is important from this research is that WSU worked with 60C, and pasteurising at this temperature gets us even further away from bottle bomb territory. At 60C with 2.5 volumes of carbonation, the maximum bottle pressure would be less than 100psi. Wow!

I use a sous vide heater so temperature control is very good. I did try 65C for 10 minutes then cool down in air to 60C and it generated 40 PUs which on the basis of the WSU paper may well be enough.

For those without a sous vide heater, or just wanting to heat up with their stove top, the "Pappers Method" works quite well as long as the temperatures and ratio of bath water volume to bottles volume is such that equilibrium between the bath and bottles is reached at about 65-70C. I found that 80C bath and 20C bottles at a ratio of 7:1 (15 litre waterbath to 2.1 litres of bottles and contents) gets this sort of result.

Meanwhile we await further research with much interest.
 
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Jaypkk

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I too read Lea's and Jolicoeur's books and remember the 50 PU reference, but there have long been voices for lower numbers; see for example, this beer pasteurization study. I make cider professionally on a nano scale and use a small commercial pasteurizer bath for those ciders that need to be pasteurized (about 10% of what we make). It has pretty good temperature control and I now typically set it to cycle between 140-143F (60-62C) because that's near the upper limit that my bottles are rated for. I hold that range for 12 minutes. When I first got the pasteurizer I had many bottles where the caps blew off and some burst altogether when I pasteurized between 143-146F. After the sales rep for the bottles told me the bottles were only rated to 145F, I dialed it back to current settings. I also worked with a local microbiologist to test for viable yeast after pasteurization and was happy to learn there were none. I estimated PUs at 18-24, depending on how long it takes for the cider in the bottles to cool down below 60C after removal from the bath. We typically have a fair amount of back stock in our warehouse so I'm able to check older bottles to see what, if anything, has changed over the months or years since bottling. I'm interested in taste differences over that time, as well as any changes in subjective carbonation levels that might be due to yeast reactivation in those back-sweetened bottles. So far, so good.

This new research is at least suggestive if not conclusive that fewer PUs are needed than we've long thought. It also depends on your goals for pasteurizing: is it just to kill any viable yeast since you've got really clean fruit/juice and excellent sanitation practices in your cider operation? Or are you ALSO trying to clean up other than yeast organisms that may be present? "The results from the present study indicated that a >8.7 log reduction in the cell numbers of the selected organisms was achieved at just 1.59 PU. It has to be borne in mind that this study only focused on the vegetative forms of bacteria. The most heat resistant morphological forms of bacteria and yeast are spores, which were not investigated in this study. Further studies should focus on the heat inactivation of heat resistant yeast ascospores which are potential beer spoilers."
 

tlc

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been reading the post on pasteurization and I'm confused on how you know the levels of 2.5 or 3 volumes of co2 and how to calculate it
 
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Chalkyt

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The simple rule of thumb is that fermenting one gravity point of sugar will generate 0.5 volumes of CO2 (and 0.65% ABV as the sugar is converted into CO2 and alcohol).

The CO2 gas generated is taken up by the cider under pressure when it is in a sealed container (bottle). When the bottle is opened, the CO2 in the liquid is released as gas, hence the bubbles.

As a guide, 2.5 volumes of CO2 is about the carbonation level of beer or soft drink and generally this is what we aim for with cider. Of course tastes vary so the target can be anything between 1.5 and 3 volumes. The CO2 level an be monitored by bottling some cider in a plastic soda bottle and doing the squeeze test (i.e when it feels like a normal bottle of soda the carbonation is about 2.5 volumes of CO2). Alternatively a test bottle fitted with a pressure gauge will show what is happening with 1 volume being a pressure of about 15 psi).

So, to generate 2.5 volumes of CO2 you need to ferment a SG change of 0.005 (i.e. 5 gravity points of sugar fermented x 0.5 volumes of CO2 per gravity point = 2.5 volumes of CO2).

For example, if you want a dry carbonated cider, bottle at 1.005 and simply let it ferment down to 1.000. For a sweeter than dry cider, bottle at 1.010 and pasteurise at 1.005 so you get 2.5 volumes of CO2 carbonation and retain 0.005 gravity points worth of sugar etc, etc.
 

Jaypkk

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If you are force carbonating, refer to a chart, like the one posted below, and carbonate to your preferred volume. 1.95 volumes of CO2 (3.8 grams/liter) produces a bright bubbly cider. It is slightly more complicated than just temp and pressure, but this is a good place to start.
 

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