Yield on Malted and Unmalted Grain

[Dr Griz]

A recent disappointing all-wheat wash has had me pondering -- I figure I'd share my thoughts for the good of the order.

With almost 200 ferments under my belt, I've dialed in my mash protocols so that I can regularly expect 1.070 or better OG -- with glucoamylase, that gives me a healthy 9-10% ABV wash for my boiler. I could get higher efficiencies by changing my kit, but I reckon reliability is more important than chasing numbers.

This time, however, it came in at 1.055. The mash bill was 78% unmalted wheat and 22% malted wheat. An iodine test showed a full conversion. Mind you, 1.055 isn't exactly a catastrophe -- the white whiskey tasted pretty good, and is settled down for nice age on oak -- but it left me with a process riddle. PPG charts usually show wheat and barley as pretty much the same, easily enough to hit my 1.070 target. So what the hell happened?

A little while ago, I went down a rabbit hole on bushel weights of malted and unmalted grains (viewtopic.php?p=7791655#p7791655), and learned that unmalted rye and barley are surprisingly heavier by volume than their malted counterparts. Thinking about it, it makes sense -- a bit of googling tells me that unmalted grains are ideally stored at 12-13% moisture, while malts are stored at 4-6%. That difference in water weight (let alone whatever the malting process itself does) is gonna give malted grain more available sugar by weight than unmalted grain.

And sure enough, when I did the math, it worked out reasonably closely: Assuming a typical PPG for malted wheat at 31, the unmalted wheat would have to be at about 27.6 PPG to yield the OG I experienced. That's about 73% of malted wheat. Not too far from the standardized bushel-weight ratio of malted (34 lb/bu) and unmalted (48 lb/bu) barley.

I know, I know -- this isn't exactly new information. I found two threads from a few years ago that touched on these differences, but they didn't really dig into the numbers.

viewtopic.php?t=93346
viewtopic.php?t=86955

So I guess I'm wondering if this more or less fits your experience? And maybe more importantly, does anyone have more systematic numbers we can all use to predict our gravities? I've recently bought a proper grain mill, so I'm starting to use a lot more unmalted grain. Unfortunately, the kind of bushel-weight and PPG charts I can find online (including online calculators like Brewer's Friend) inevitably leave out a lot of the grains we all use, so I'm running off a pretty dismal sample size for dialing in my mash bills going forward.