Odd attenuation numbers

So when I input a rest at 142F for 90min the estimated FG goes up instead of down as you would expect.  Any thoughts on why that is?

There are reasons.

First, there is a known bell curve of fermentability within the calculations used by BeerSmith. It's important to realize that the calculations used by BeerSmith are the same ones used throughout the brewing industry. So, this curve isn't unique.

This results in lower attenuation predictions both above and below the optimum mash temperature range. The upper end of the limit is accepted and understood by most brewers. The curve isn't as good at predicting fermentability in the lower range, but it's accurate to say that a reduction is likely without some increase in temperature.

I seem to remember Brad saying that a future update will increase fermentability at lower temperatures. I have no idea if this will be more accurate or just be a feel-good fix for intuitive brewers.

The lower end is dependent on a number of factors such as water ratio, pH, grain diastatic potential and time. There are more, but these four are enough for this discussion. Generally, low temperature mashes result in poor wort quality and reduced efficiency in terms of full conversion. The three main reasons is that Alpha amylase works slowly here and limit dextrinases are the main enzymes providing starch chains for Beta amylase, plus both work best in very different pH ranges. Limit dextrinase at pH 5.1 and Alpha Amylase at 5.6.

The third reason is that water gets bound into more amylopectin at this temperature because it is less effective as a solvent than it is at normal temperatures. This binds enzyme activity, limiting the speed and completeness of conversion.

In decoction, the 60C (140F) rest allows more time for limit dextrinase to act, thus reducing residual body and increasing fermentability. Beta amylase begins its work at this temperature, but the additional step of 70C (158F) speeds beta amylase up by 3X while at the same time, allows Alpha Amylase to create more open starch ends for the Beta to work on.

The trouble is that increased temperatures decrease pH, so it's not likely a brewer will get the 5.1 desirable at 60C and the 5.6 desirable at 70C without intervention or a sacrifice in wort quality.

In a mash, water is both a hydrolyzer and a solvent. There is a near logarithmic change to the solvent capacity of water with increasing temperature. Something on the order of 6X per 10C. The result is to break down beta glucan and amylopectin, thereby exposing more substrate to the enzymes. This has the net effect of making a lower temperature rest redundant to what happens at the "normal" infusion range of  66 - 67C (151 - 154F).

Something to keep in mind is that Beta Amylase has a pretty short half life. At "normal" mash temps, it is most effective for about 15-20 minutes. In real world terms, that means 45 minutes into your 90 minute, 142F mash, your enzyme activity has halved twice and you don't have much Alpha Amylase activity to increase starch surface area.

So, absolutely, very low temperature mashes would limit fermentability in a real-world beer.

Ok I understand what you are saying.  So right now I have a mash profile of 122F for 30min, 148 for 75min, 158 for 20min, 170 for 10min.  My grist is pilsner 98%, aromatic 1%. carapils 1%.  What i am trying to do is get the most fermentible wort as possible.  Would that work or can you think of a better way to go?

Crazyirishman34 said:

My grist is pilsner 98%, aromatic 1%. carapils 1%.  What i am trying to do is get the most fermentible wort as possible.

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Carapils runs counter to fermentability. It's main goal is to add body and head retention. Residual body is the exact opposite of fermentability.

OTOH, you're using malt. That means you want some flavor, right? Of course! Otherwise, you may as well substitute sugar for the malt and just make alcopop.

So, instead of "most fermentable," I'll go with "dry finish." This leaves plenty of room for body and aftertaste without too much malt sweetness. That definition also matches the grist you've described.

So right now I have a mash profile of 122F for 30min, 148 for 75min, 158 for 20min, 170 for 10min. 
Would that work or can you think of a better way to go?

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What follows are mash guidelines, not hard & fast rules. Try the mash schedule, but with the same grist, try others and see if there is any appreciable difference.

Assuming you're going to get the correct mash pH of about 5.3 without acidifying the mash, the protein rest of 122F is unnecessary. With the exception of Chit and Bohemian Pils malts, what's available is fully modified and doesn't get enough from a protein rest to carry over into the beer at homebrewer efficiency of 70-80%.

The 148F rest will give you the most fermentable wort and a crisp, dry finish. The time is longer than needed, if the pH is correct, perhaps 40 to 60 minutes is enough. If pH is high or low, a little extra time helps.

My flagship beer is an Export Pilsner at 5.5% that finishes out at 1.010. I happen to use 151F as my target mash temp and when the mash goes a little long (>50 min), we end up with low finishing gravity. During vorlauf, we add heat and target 168F for mashout. I get full conversion and net out at 94% mash efficiency. In my case, the single rest and mashout give me the profile I want.