The boil

After a spate of problems struck brewing bloggers I dug out some of my revision notes from when I was a brewing student to remind myself exactly what the boil does during the brewing process. Soon after I'd read them a wine maker visited the brewery I work at and asked if we boiled the wort to sterilise it. As I'd freshly read up on it he got more than he bargained for."Oh no, it's much more than that" I said and taking a deep breath I burbled on until I saw his eyes start to glaze over.  

Here's the notes for those of you that are interested. There are a few things that confuse even me though. Why is there a question mark after saying the optimum pH for trub formation is 5.2? And what exactly did I mean by saying a wedge disrupts bacterial cell walls at low pH? I may have to investigate further...

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Briefly descirbe the various processes that occur during wort boiling:

 Extraction and transformation of hop compounds – extracts bitterness, aroma and limited antibacterial activity.
 Formation of protein-polyphenol complexes 
 Wort sterilisation
 Destruction of enzymic activity
 Increase in colour – maillard reactions
 Decrease in pH
 Production and removal of unwanted flavour compound (sulphur compound)
 Concentration – evaporation of water and volatiles.

Wort boiling aims to facilitate:

Flavour, aroma and colour development
Stabilisation (physical and biological)
Concentration

 Extraction and transformation of hop compounds – extracts bitterness, aroma and limited antibacterial activity.

- Alpha acids are essentially insoluble in wort but are isomerised during the boil to iso-α-acids (approx. 30-35% efficiency, less in high gravity brews). The iso-α-acids have the bittering and antibacterial properties. A long boil increases utilization but drives off aromas. Humulones are converted to cis and trans iso-humulones.
- Hop oils are also extracted but are very volatile so need to be added late or specific aroma oils towards the end of the boil.
- Hop polyphenols are also extracted and play an essential role in break formation, also give some limited bitterness.

 Formation of polyphenol complexes

- Polyphenols come from the hops and the malt
- Malt polyphenols appear to be more actively involved in trub formation.
- Protein-polyphenol complexes form due to interaction between negatively charged proteins and positively charged polyphenols.
- Heat increases this reaction and causes co-precipitation of the proteins and polyphenols due to the coagulation of the proteins. The longer and more vigorous the boil the more mixing and precipitation.
- This complexes form the hot break, cold break on chilling and chill hazes later on.
- Trub can be removed in a whirlpool, centrifuge or hop back.
Optimal pH for trub formation is 5.2 (?)

 Wort sterilisation.
- Heat kills all living organisms in the wort.

 Destruction of enzymic activity.
- Heat denatures all the enzymes and stabilises the protein and carbohydrate profile of the wort.

 Increase in colour – maillard reactions
- Wort becomes darker due to oxidation of polyphenols and production of melanoidins.
- Melanoidins are formed by the reaction between amine groups in amino acids and carbonyl groups in sugars.

 Decrease in pH
- This is mainly due to the production of melanoidins.
- Lower pH helps trub precipitation, increases wort colour, give better and cleaner hop bitterness, decreases hop utilisation and helps inhibit bacteria as low pH causes intracellular pH to drop and the cell wall is disrupted by a wedge causing leakage.

 Production and removal of unwanted flavour compound (sulphur compounds)
- DMS produced but also evaporated by the high temperature. The precursor is SMM from barley which converts to DMS on heating.
- DMS can end up in beer if there is poor whirlpooling, too high fermentation temperature (so SMM is still being converted into DMS) or too low fermentation temperature so not enough DMS is driven off. Or bacterial contamination.
- SMM from the barley can be converted by kilning and boiling to DMSO which will survive through to the fermentation at which point the yeast will convert it to DMS but a vigorous fermentation will drive it off. Too hot and too much SMM to DMSO though, too low and not enough DMS driven off.

 Concentration – evaporation of water and volatiles.
- Want minimum of 7% boil off per hour. Good rolling boil for trub formation and driving off of volatiles. High pressure boiling will save time and energy but produce off flavours and hazes as the volatiles are not driven off.