Cold solids are turbid precipitates formed during the cooling process of wort. They are mainly composed of proteins and polyphenols. Their properties are basically the same as those of cold turbidity produced by beer and can be dissolved by heating. Cold solids have a great impact on beer fermentation. They should be separated and removed before entering the formal fermentation, otherwise they will adhere to yeast cells, affect normal fermentation, increase the burden of beer filtration, make the beer taste rough, and have poor foam performance and stability. The separation and removal of cold solids usually adopts the yeast breeding tank method, conical fermentation tank separation method and flotation method.
(1) Breeding tank method. The cooled wort enters the yeast breeding tank and yeast is added. After standing for 12 to 20 hours, the cold solids are allowed to precipitate naturally. As time goes by, the cold solids continue to precipitate and precipitate. When foam appears on the surface of the wort, it means that the yeast has multiplied to a certain extent. At this time, the wort containing a large amount of fresh yeast on the upper part is sent to the fermentation tank or conical tank for fermentation. The precipitated cold solids and a small amount of aging yeast cells are separated.
(2) Conical tank method. The cooled wort is fed into an open-air conical fermentation tank in batches, and sterile compressed air is introduced while the wort is fed. It can also be fed into the fermentation tank using a yeast addition tank. The temperature of the wort fed each time should be 0.5~1.0℃ higher than the previous time. After 18~24 hours of static sedimentation, the condensed solids and dead yeast cells are precipitated to the cone bottom of the fermentation tank and can be directly discharged from the bottom valve.
(3) Flotation method. Flotation is a very effective method for removing condensed solids from wort. During the wort cooling process, sterile air (30~70L/hL wort) is introduced through a venturi tube, and the air is broken into uniform and fine small bubbles with a mixing pump. The bubbles rise slowly, and the condensed solids precipitated from the wort are adsorbed on the fine small bubbles and rise to the liquid surface. Finally, a layer of bubble cover is formed on the liquid surface. After standing for a period of time, the clarified wort below is separated from the condensed solids.
The main equipment of the flotation method is the flotation tank, which is available in vertical and horizontal types. The wort inlet and outlet are at the bottom. When the wort is pumped in, the back pressure in the tank is 0.05~0.09Mpa, and the pressure is slowly reduced after the wort is pumped out. The wort is left to stand for 6~16 hours, and 50%~70% of the condensed matter can be removed.
Yeast can be added at the same time or not by flotation. If yeast is added, since the diameter of yeast is 6~8μm, it will not be adsorbed by bubbles and brought to the bubble cap, and sufficient ventilation and oxygen supply are conducive to the reproduction and metabolism of yeast.
