Any idea what your pH was? I have heard it can affect yeast attenuation towards the end.
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"Nutrient uptake by yeast
Shortly after being pitched into fresh wort yeast will start lowering the pH of the surrounding medium (i.e. beer). This is the result of ammonium ion and amino acid uptake, secretion of organic acids [Briggs, 2004] and most importantly a proton pump which moves H+ ions from the yeast cell into the beer. By doing so the yeast also raises its internal pH. This proton pump is very important to the yeast and it is the most abundant protein in its cell membrane [Briggs, 2004]. The resulting pH gradient through the yeast's cell wall facilitates the uptake of nutrients like maltose.
Maltose uptake is a proton symport process through the cell membrane. The proton concentration outside the cell is greater (lower pH) than inside the cell (higher pH) and therefore a natural gradient exists which encourages protons to flow from the outside to the inside of the cell. Though the use of a symporter, a cell membrane protein, maltose can “piggy back” on the flow of protons into the cell. This is one of the reasons why yeast cells do better in an acidic environment and have means of lowering the pH.
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The ability of yeast to lower the beer’s pH is important for healthy and low yeast stress fermentation and is one of the reasons why sufficient pitching rates are important and why it is better to step up starters rather than starting a small amount of yeast in a large starter. The more yeast cells that are working on lowering the pH the faster the pH will be able to drop.
As yeasts age, starve or otherwise loose their vitality, it becomes increasingly difficult for them to pump H+ from their cells into the beer. After all, this goes against nature’s desire to equalize everything and therefore takes energy. The result is a slight rise of the beer pH after primary fermentation. The pH can rise more significantly if the beer is not taken off the yeast before a large number of yeast cells start to autolyze."
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