How much acid to use for stabilizing meringue? After reading up on the proper books, I started adding some acid when beating meringues. At home, I use cream of tartar, but when I am baking at somebody else's kitchen, I am lucky if they have at least some citric acid; cream of tartar is a huge rarity here. 

The trouble is that the books don't specify a ratio. They speak of throwing in "a pinch", suggesting that the exact amount is not that important. In my experience, this is not true. 

I have had meringues fail despite cream of tartar. They went from a runny mess to an overtightened stiff layer floating on water without a noticeable sweet spot inbetween. I admit that I may have used the wrong whipping speed (this happens especially often when I whip by hand), but 1) I can't attribute it to speed alone, and 2) this is exactly the kind of problem I hope the acid works against. 

Then again, I have whipped beautiful meringues with acid. Cream of tartar helps there too, but citric acid frequently gives me a great texture from a single pinch (maybe 1.5 g) per 2-3 eggwhites. The meringue gets smooth and glossy, with robust little bubbles, and holds without deflating when folded into other ingredients, even during a not-so-gentle macaronage. The problem: at this concentration, the sour taste is already very noticeable. In some very sweet applications, this still works, but most times it adulterates the taste. 

So, what is the minimal ratio at which acid works (given a sensible technique - forget hand whipping for now?) And what is the maximum ratio at which acid is still undetectable in taste? I am interested in both cream of tartar and citric acid here.
As there are multiple types of meringues, I imagine the core of this question is really the effect of acid on egg white foaming.

According to KATERYNA LOMAKINA and KAMILA MÍKOVÁ writing in the Czeck Journal of Food Science, there is a moderately complex relationship between pH of the egg whites, and its foaming capacity (overrun) and the stability of the foam:


  By the addition of a small amount
  of 1N H2 SO4  or NaOH to  the liquid egg white (pH values: 9.5, 8.6,
  6.3, 4.7, 
  3.1, 1.0), NAKAMURA and SATO (1964b) obtained  a great foaming capacity at the neutral and acidic  pHs except at the exceedingly
  acidic pH (pH 1.0).  The foam stability was high at pH 8.6, the pH of 
  the natural egg white, and decreased with changing pH. 
  
  With an aqueous
  egg albumen solution, HAMMERSHØJ and LARSEN (1999) established that 
  the foam overrun was the highest at pH 4.8 and  the lowest at pH 10.7.
  The foam stability against  drainage was the best at pH 7.0 after 30
  min, but  on a long-term scale the foam at pH 4.8 was the  most
  resistant to drainage. This is the result of the  more rigid behaviour
  of the surface at pH 4.8 and  the formation of small bubbles,
  therefore a slow  drainage of liquid from the foam, lower dynamic 
  surface tension causing the high overrun. 


The conclusion is that you will get the most volume of foam at moderately acidic pH around 4.8, but the best stability of the foam once created at the alkaline pH of egg whites.

The moral seems to be that for stable egg foam, not to add acid.

Later in the article, they also indicate that volume can be increased without ill effects by adding up to 40% more water to the egg whites.

Also, adding sugar to the egg whites inhibits foaming, requiring more agitation to form the foam.