Ask Mr. Wizard

Can you mash overnight and sparge in the morning?


Trev Cox • Reading, Pennsylvania asks,

I have been brewing for almost 30 years. To increase mash efficiency, I mash overnight. I do this with beers of low to average original gravities, but especially with high-gravity beers. I will start the mash at 155–160 °F (68–71 °C) around 11 p.m. and sparge in the morning around 9 a.m. By then, the temperature of the mash is around 145 °F (63 °C). I have found this method to be successful at conversion and the beers have been good. Is there any reason why I should not be doing this? I am often asked for brewing recommendations and I want to recommend this, but I need to know if I am wrong.



I am a great fan of methods that make things easier and this method is certainly a time-saver when it comes to scheduling those precious weekend hours. To me, saving time is the best way to improve efficiency. Very long mashes also will improve the extraction of wort-soluble solids from malt and improve your mash efficiency, although mash efficiency is primarily a function of milling and wort separation techniques. I toured a small brewery in California that brewed a batch of stout, then mashed in the second batch and returned the next morning to finish the batch. There are no major problems with this method, but there are a few things to keep in mind to avoid potential problems.

Mashing is all about enzymes. The two key enzymes in mashing are alpha-amylase and beta-amylase. Beta-amylase produces maltose from starch and is most active between 140–149 °F (60–65 °C). The thing about beta-amylase is that it stops working when it runs into a branch in the starch molecule. That’s where alpha-amylase comes to the rescue. Alpha-amylase randomly reduces big starch molecules into smaller pieces. Its temperature optimum is right around 158 °F (70 °C).

When enzymes run out of their substrate — starch, in the case of amylases — the reactions just stop. No big deal. Brewers typically stop mashing when the mash is complete because they want to efficiently utilize their brewing equipment. In your case, the mash is over at midnight and you are more concerned about utilizing your bed than your mash-tun — so you let the mash wait for you to awaken.

The key to your method is keeping your temperature high. If you used this very long mashing method with a low mash temperature, say around 140–145 °F (63–65 °C), the result would be very fermentable wort because this temperature range is ideal for beta-amylase activity and also is high enough to get some alpha-amylase activity. When these two enzymes work together, the result is an increase in wort fermentability.

Dry beers and light beers typically begin with this sort of wort. If you want to brew something like Michelob Ultra, a very long mash in this temperature range is the ticket! Just make sure that you achieve complete conversion by using the iodine test before sparging. If you get a black iodine test result, you should heat the mash up to 158 °F (70 °C) for about 20 minutes for complete conversion. Most homebrewers don’t want to brew super-dry beers and should keep the temperature above 150 °F (66 °C) for the mash.

Keeping the temperature high is also critical for pest control! Wort bacteria don’t mess around and will quickly begin growing if the wort temperature falls into the 120 °F (49 °C) range. Malt is chock-full of bacteria that cause souring of wort, such as Lactobacillus and other bacteria that can lead to some really rank off-flavors. In fact, the most common application to overnight mashing is for sour mashes where the temperature is intentionally allowed to drop into the realm of these “bugs” that so effectively turn mash sour. Perhaps the best method to guarantee that the mash stays in the 150 °F (60 °C) range is to put the mash into an oven set at its lowest temperature. I strongly suggest you verify that the lowest temperature is not too hot for the mash before chucking your mash into your oven for a 10-hour bake! However, it sounds like you have a very well-insulated mash tun if the mash only drops 10 °F (5 °C) over 10 hours.

As long as the mash temperature is adjusted to address concerns about overly-fermentable wort and bacterial spoilage, your method is a real daylight time saver!

Response by Ashton Lewis.