I think you're on the mark. a minimum of 50ppm is considered beneficial, while some water sources have in excess of 250ppm with no problems, such as Burton (295ppm) and Vienna (250ppm) water. But since you want to reduce the amount of pH reduction, your plan to add only 50ppm in the mash and put the rest in the boil is a good one.
Calcium plays a role in many areas - the mash, in sparge water and in the boil, and in the fermentor, so as long as some of your calcium gets to reach all these areas, then I think you're ok.
Beer Brewing, The Art and Science, lists beer styles ranging from 50-200 ppm in calcium. It also describes how calcium is used from the mash through to packaging:
Of the ions required for brewing, calcium is by far the most
important. This is because of the acidifying effect that calcium has
on the wort. [...] A combination of the presence of calcium ions and
the decrease in pH has a number of effects on the brewing process:
The lower pH improves ß-amylase activity and thus wort
fermentability and extract. The optimum pH for ß-amylase activity is
about 4·7. Wort produced from liquor containing no calcium has a pH in
the order of 5·8 - 6·0, compared to values in the range of 5·3 - 5·5
for worts produced from treated brewing liquor. The activity of the
ß-amylase then is greatly enhanced by the addition of calcium, this
enzyme increasing the production of maltose from Amylose, and thus
making worts more fermentable.
Calcium has a beneficial effect on
the precipitation of wort proteins, both during mashing and during the
boil.
Protein-H + Ca2+ (r) Protein-Ca ¯ + 2H+
The hydrogen ions released further reduce the pH which encourages
further precipitation of proteins.
Proteins are also degraded, that is converted to simpler substances by
proteolytic enzymes called proteases. These are found in the malt, and
have optimum activity at pH values of about 4·5 - 5·0. The reduction
in pH then caused by the presence of calcium encourages proteolysis,
further reducing protein levels and increasing wort Free Amino
Nitrogen levels (FAN).
FAN compounds are utilised by the yeast during fermentation for the
manufacture of amino acids, and an increase in FAN levels in the wort
improves the health and vigour of the yeast.
High protein levels in beers also have negative effects, making beer
more difficult to fine and encouraging formation of hazes, in
particular chill hazes. Product shelf life can also be adversely
affected.
- Calcium ions protect the enzyme a-amylase from inhibition by heat.
a-amylase is an endo enzyme, cleaving the internal 1,4 glucosidic
links of amylopectin resulting in a rapid reduction in wort viscosity.
The optimum temperature range for
a-amylase activity is 65°C - 68°C, but the enzyme is rapidly destroyed
at these temperatures. Calcium stabilises a-amylase to 70 - 75°C.
It can be seen then that the presence of calcium has positive effects
on the activity of a-amylase, ß-amylase and Proteases, some of the
most important enzymes in the brewing process.
The drop in pH encouraged by Calcium ions in the mash and copper
helps afford the wort and subsequent beer produced a greater
resistance to microbiological infection.
The reduced pH of the sparge liquor reduces extraction of
undesirable silicates, tannins and polyphenols from the mash bed. The
extraction of such materials is encouraged by alkaline sparge liquor.
These materials are very undesirable, contributing to harsh flavours,
hazes in the finished beer and decreased beer stability.
Calcium precipitates oxalates as insoluble calcium oxalate.
This again occurs in both the mash tun and the copper. If oxalates are
not removed they can cause hazes in finished beers and also contribute
to the formation of beerstone in FV's, CT's and casks. Oxalates are
also thought to promote gushing in certain beers, although this is not
generally a problem to the micro brewer.
Your low water pH probably means few dissolved solids, particularly (bi)carbonates, which is a great starting point for brewing, as you are now in control over much of the water chemistry, compared to folks with water pH in the 7-8 range with larger amounts of dissolved solids, which can make brewing pale beers tricky.
As well as the points you mention, Calcium is also good for long term colloidal stability, which helps reduce permanent haze.
EDIT: Confirmation of 50ppm
For the best results, your brewing water should have less than 50 ppm
carbonates and around 50–75 ppm calcium ions.
BYO, american pilsner profile
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