Turbid Mashing is a labor-intensive process involves taking the mash through multiple temperature rests through infusions of hot water and the removal of 'turbid' starchy wort that is not fully converted. This process was used historically for a range of beers, both aged mixed-culture acidic beers and young beers. In modern brewing, this technique is known because of its use in the production of traditional lambic. The main goals of turbid mashing are to maximize extraction from a grist with a high percentage of unmalted grains and to produce a dextrinous wort, which can then feed wild yeasts and bacteria during the long fermentation that beers such as lambic undergo. Turbid mashing also pulls more tannin material from the grain due to especially hot sparge temperatures and prolonged sparging.
For the full process of spontaneous fermentation as is done in traditional lambic brewing and Méthode Traditionnelle, see the Spontaneous Fermentation page. This page refers only to the mashing process that is often incorporated in spontaneous fermentation (and also some historical saisons).
Typical Mash Procedure
Turbid mashing procedures are outlined well in Levi's writeup of Cantillon's procedure and Wild Brews by Jeff Sparrow (see the books page). Brewers conducting traditional turbid mashes usually have at least a 4 vessel system: a mash/lauter tun, a kettle to hold turbid wort, a hot liquor tank (or hot water on demand) and a boil kettle to receive the mash runnings. Many different specific turbid mashes may used by varying rest temperatures and the numbers and timings of turbid pulls. This was the case for historic turbid mashing (see, for example, Johnson, 1918). An example of a modern turbid mash based on records and observations of Cantillon's process is as follows:
- The grist, which traditionally in lambic is malted barley and ~30-40% unmalted wheat (see this MTF thread for tips for homebrewers and commercial brewers on milling raw wheat), is doughed to reach a temperature of 45°C (113°F). At this point the mash is very thick.
- By infusion of hot water the temperature is raised to 56°C (133°F). After a short rest the first turbid portion is pulled (liquid wort only is pulled; the grain is left in the mash tun). The volume pulled is about 4-5% of the total target brew volume. This turbid portion is then heated to ~ 90°C (~190°F). The main mash rests a bit longer at 56°C.
- By infusion the temperature is raised to 66°C (151°F). This is the main saccharification step and the rest here is longer. The second turbid portion (liquid wort only is pulled; the grain is left in the mash tun) is pulled from the mash during this step (~24% of the total brew volume).
- By infusion the temperature is raised to 72°C (162°F). After a short rest the mash is lautered into the boil kettle.
- After the first runnings are collected, the turbid portion is added back to the mash. This heats the mash to a mash-out temperature. After a short settling time this is also lautered to the boil kettle (see this MTF thread for thoughts from Dave Janssen on possible enzymatic activity on the turbid wort during re-adding it to the mash tun).
- The mash is sparged with especially hot water (~88°C, ~190°F). This pulls more tannins that a traditional sparging temperature would. The amount of sparge water used is typically significantly more volume than modern brewing methods, and results in a large volume of low gravity wort that is then boiled for 3-5 hours to reach the desired post boil gravity (see Lambic for the rest of the lambic brewing process).
Turbid mashing is possible with more simple systems with some modifications to the process. For example, if a second kettle is not available to hold turbid wort while keeping the main bottle kettle clear to collect runnings, the turbid wort can be collected into the main single boil kettle. It is then necessary to add this turbid wort back before collecting the first runnings. In this case, the turbid wort rather than an infusion of near-boiling water is used to raise the mash to the final temperature (step 4 above), therefore clearing the boil kettle to receive the wort. A system such as this is employed by 3 Fonteinen as they have only one boil kettle. Jester King follows a similar process, where the turbid wort is added back at the end of the mash before collecting wort for the boil, for their Spon series  (discussion starts ~9 minutes in). An extra fermentation tank can serve as a hot liquor tank as needed. Turbid mashing is also fully possible with brew in a bag (BIAB - see this MTF thread for tips on turbid mashing in BIAB).
Originally, this process was done to avoid high taxes because at the time breweries were taxed on their mash tun size. The resulting wort is high in unconverted starch. See "Lambic attenuation and carbohydrate composition " by Dave Janssen for more details. There is also evidence that this process was desired for other styles of beer such as low strength beers even after the tax laws were changed in Belgium (see "Thoughts on Johnson 1918 - a Belgian mashing system for low-strength beers" by Dave Janssen).
Alternative methods to yield starchy wort
One of the main goals of turbid mashing for mixed culture beer is to yield a starchy wort in order to carry carbohydrates which are unfermentable to Saccharomyces but fermentable to Brettanomyces and lactic acid bacteria into the fermentation. There are other approaches that brewers have taken to come to this same end result of starchy wort without the labor and equipment-intensive turbid mashing process. These alternatives will likely not yield the same exact results as turbid mashing, but depending on the brewer's goals they may be sufficient and/or preferred. Some of these methods, as outlined in the spontaneous fermentation page, include the addition of flour to the boil  passing hot mash runnings through flaked grains , adding flaked grains to the mash at the mashout step , soaking a huskless grain such as oats in the boil, pulling mash runnings before full conversion without the prolonged processing of a turbid mash , using maltodextrin in the boil, or even adding 100% wheat pasta. Although he mostly conducts traditional turbid mashes, James Howat of Black Project Spontaneous Ales has experimented with doing a 165-170°F (74-76.7°C) single infusion of a 60/40% pilsner/raw wheat grist for 15 minutes to simulate extracting the starch content of a turbid mash .
Carbohydrate Composition and Utilization Through Fermentation
Little work has been done to identify the types and amounts of carbohydrates in turbid mashes. One reference for an unknown lambic brewery showed that simple sugars (under 3 carbon chains) were 62%, 3-6 carbon chain sugars were 18%, 185+ carbon chain starches were 8%, 6-30 were 5%, 30-61 were 6%, and 61-185 were 1%. This study showed that the longest chain sugars were the third highest fraction in this instance of turbid wort. During the extended fermentation, the longest chain sugars were broken down into medium sized sugars during the by lactic acid bacteria and Brettanomyces. By the end of fermentation (nearing 1°P), these long chain sugars were mostly used up; however, there was surprisingly a higher amount of smaller and medium-sized sugars than the longer chain sugars. See this Hors Categorie blog article for more information.
Roos et al. (2018) reported that glucose (8.0 g/l), fructose (2.5 g/l), sucrose (4.0 g/l), maltose (60.0 g/l), maltotriose (12.0 g/l), and higher maltooligosaccharides (<4.0 g/l) were the most abundant sugars in turbid wort produced at an unnamed lambic brewery. The wort, which was pre-acidified to a pH of 4.5 with lactic acid, also had some organic acids: lactic acid (1.2 g/l), citric acid (250 mg/l), malic acid (200 mg/l), and gluconic acid (70 mg/l) .
MTF Wiki Pages
- Levi's write-up of Cantillon's procedure
- Traditional Lambic Using a Turbid Mash - BrewUnited Blog Post
- Lambic.info page on brewing lambic
- Wild Brews by Jeff Sparrow
- American Lambic blog post about traditional v. modern processes
- Early 20th Century Biere de Garde Mashing Technique; by Dave Janssen.
- Turbid mashing in historical Bière de Garde and the associated MTF thread.
- Michael Tonsmeire's turbid mashing steps.
- Blog post by Dave Janssen about a historic article (Evans 1905) dealing with turbid mashing for French bière de garde
- Blog post by Dave Janssen about Johnson's 1918 article dealing with turbid mashing for young beers
- Jester King on the Sour Hour, episode 55
- MTF discussion about using a fermenter as a HLT
- Burgundian Babble Belt discussion
- Flat Tail on the Brewing Network, ~1:04 in
- Mark's alternative process for a starchy wort
- Homebrewtalk Thread by Amos Browne. April 2012. Retrieved 02/08/2016.
- Howat, James. MTF discussion on doing very hot, short mashes to simulate turbid mashing. 03/31/2017.
- Wort Substrate Consumption and Metabolite Production During Lambic Beer Fermentation and Maturation Explain the Successive Growth of Specific Bacterial and Yeast Species. Jonas De Roos, Peter Vandamme, and Luc De Vuyst. 2018. DOI: 10.3389/fmicb.2018.02763.