Brix is the amount of the total soluble solids present in a batch.
Example: Sugar is a soluble solid because you can hydrate it with water then you could boil the water off making the sugar re-crystallize. Most soluble solids are from sugar and less from acid. The reason is because all sugars including sugar in the fruit is soluble.
Brix can be read quite easily with a refractometer. If you plan on doing a lot of new product development, I would highly recommend the purchase of a refractometer and pH meter. In summary, A brix reading is the amount of all soluble solids (sugar, acid, ect.) in a given mixture.
Methal (alcohol) groups are added or taken away from the pectin molecule chain to give a specific environment to react in. As methal groups are added and the DM (Degree of Methelation) becomes greater than 50 the pectin become a high methoxly pectin requiring high sugar content to achieve gel. As the DM goes below 50 the pectin becomes reactive to calcium ions. These’s pectin’s are call Low methoxyl or LM pectin.