Brix should not be used to measure the quality of wheatgrass juice. You need to use a spectrophotometer.

Generally Brix is used with fruit juices. It's function is to analyze the sugar content of the liquid. In the case of fruit, the taste and concentration of nutrients are normally correlated with how high the sugar content is. The more sugar, the better but this is only for fruit, not wheatgrass. 1 Brix is associated with somewhere between 1-2% sugar content. In the case of orange juice, the various batches are mixed and blended to achieve appromiately 12 Brix.

I once heard someone rave about the excellence of their tray grown wheatgrass because they achieved nearly 20 Brix. All this actually means is that their wheatgrass is 20 - 40% sugar. Sugar in all forms is what most of us are trying to avoid and simply demonstrates that the plant is undeveloped and therefore has high levels of simple sugars.

Brix readings on outdoor grown wheatgrass juice are 0. There are no sugars whatsoever. When you grow wheatgrass outdoors, it matures quite differently. The roots reach 12" down into the soil and it has plenty of time to convert sun energy into chlorophyll as the plant grows to harvesting height (7"-11") in 90-120 days (instead of 10 days in a tray) and the simple sugars are converted into complex carbohydrates.

Longer growing cycles have been shown to increase anti-oxidant content:
http://www.medscape.com/medline/abstract/16521113

The way to measure the quality of wheatgrass is with a spectrophotometer. Using a spectrophotometer, you can measures chlorophyll levels and this is what we really want in our wheatgrass juice. If you would like to get superb outdoor grown wheatgrass juice that also has superb chlorophyll levels we invite you to check out our family company, DynamicGreens.com