The bag is a critical part of the BIAB process. You need to have a bag that is of the right material, size, shape and strength.
Bags Designed by BIABrewer
BIABrewer has designed a bag that is tapered, elasticised, tabbed and stitched strongly. It is quite a complex design and therefore expensive and time-consuming to make as a one-off item. BIABrewer.info intends, as soon as possible, to wholesale/retail a range of high quality BIAB bags to retailers/brewers to suit a variety of kettle sizes.
There are currently no, "BIABrewer-approved," bags available on the market.
Material
The best material to make your bag from is sometimes known as “Voile.” This is a 100% polyester curtain sheer. It is a fine mesh, a lot finer than fly screen for your windows but not as fine as muslin cloth. If women’s stockings were the right shape and size, their porosity would probably be perfect! For instance, if you filled it up with flour and shook it, the flour would come through, but nothing else would. Water would fall through it without resistance. 35 threads per centimetre is BIABrewer.info's recommendation.
So, if buying your own material, print out the “Voile” page linked above and take this to your local fabric store . Doing this and bearing in mind the description above will ensure you end up with suitable material.
Size
Size does matter! Your bag must be BIG enough to line your pot completely and without restricting the grain in any way. In BIAB you are not mashing “in a bag” you are mashing in a pot that just happens to be lined with a bag.
A fail-safe rule of thumb is that you need to be able to fit your pot inside the bag.
Design/Shape
The ideal BIAB bag shape is tapered, difficult to sew and hard to describe but simple shapes and designs such as pictured at the bottom of this post work perfectly well. (These are not real BIAB bags, just examples of shapes so forget the carry straps etc
)The third shape pictured is very simple to make and works extremely well. It is based on a tapered pillow slip shape. All that needs to be done is to cut two pieces of material in the shape above and sew them together. Get your measurements from your kettle and cut your material to the following formula..
top = (kettle top circumference x 0.5) + 15%
height = kettle height + 15% + 2.5cm (1”) for drawstring or elastic.
base = kettle base diameter – 15%
For example, if you had an urn that was 40 cm wide and 45 cm tall then...
Top = (3.1416*40*0.5)*1.15 = 72.2 cm
Height = 45*1.15+2.5 = 54.25 cm
Base = 40*0.85 = 34 cm
A drawstring or elastic and tabs, hold the bag securely and allow lifting out of the kettle.
Stitching
Make sure all your stitching uses polyester thread. Use over-locking, or another form of strong stitch to sew your bag. Check the stitching regularly on your first few brews. The right material with strong stitching should give you a bag of indefinite life.
Credits: Dan Walker (ThirstyBoy) provided the pictures and some of the original writing above including the great description of the porosity of a BIAB bag and the, "rule of thumb."
If you have questions regarding this topic, feel free to ask them here.
