Batch sparging: let the grain bed run dry or not?

Question

I have a 3 vessel setup (hot liquor tank + mash tun + wort kettle) in a single tier setup so I need to pump liquid from one vessel to another, but I only have one pump, so I batch sparge: transfer wort from the mash tun to the wort kettle, then top up the mash tun with sparging water from the hot liquor tank -- rinse and repeat.

What is better, both in terms of wort quality and mash efficiency:

1. keeping the grain bed covered in water and topping up with more sparging water before it starts runninng dry,

or

2. letting the grain bed drain completely before adding more sparging water?

I've tried to avoid running the grain bed dry, but I'm often struggling to hit my target gravity, and at the end of the day there's still a ton of sugars left in the spend trains.

Your opinion on both options with pros and cons would be appreciated!

Answer 1

We sparge because we're interested in dissolving more of the sugars from the grain into solution. There are some physical principles when dissolving, like:

1. A more concentrated sugar solution dissolves sugar more slowly than a weak solution.
2. Hot water dissolves sugar more quickly than cooler water.
3. Sugar only dissolves if it's in contact with the water.

To maximise dissolved sugars, you want to maximise time in contact with the grain, minimize the concentration of sugars in the water, and maximise the water temperature (but not past ~80C to avoid extracting tannins).

Fly sparging maximises contact with a weaker solution as you're continually running off the stronger solution. But fly sparging runs foul of grains not being in contact with water if the grain bed is poor or if the water level drops below the level of the grain bed.

I don't think there's a right way to sparge, but batch sparging is IMO a whole lot simpler. You can simply give the grain bed a stir to maximise contact with lower concentration solutions - maybe lauter before running off the wort.

Answer 2

I have a similar system. I always take the complete lot of wort from the mash initially, and then during both batch sparges. The whole point of batch-sparging is not needing to maintain the liquor (water) level in the mash tun.

Then I add more water from the HLT (pumped over). I ensure the new batch-sparge water is mixed well with the mash. Then next step is to recirculate until it's clear, then pump it off completely again. Repeat once more, or more times for very high gravity beers (obviously not if the SG of the liquor is at or below ~ 1.010).

What you propose in the OP sounds like a mix of batch and fly sparging.