Re: How do I measure glucose levels in food (banana) quantitatively?

Technically you are correct. Refractometers do not measure anything in absolute terms, and must always be used as an indirect measurement method.

What this means is that a calibration curve, which translates refractive index measured by the refractometer into concentration units of the substance being measured, must be used. However just about all commercial refractometers used for measuring sugar are pre-calibrated for sugar (and usually ethanol as well) so the scale is built into the device. However, this assumes a pure sugar-water solution, so for solutions containing other materials an external calibration curve must be established using some other primary sugar measurement method such as liquid chromatography.

In the case of fruit, what is usually done is the fruit is crushed or pulverized and a drop of liquid from the mash is placed into the refractometer, which will then give an approximate sugar concentration. In practice, if a more accurate sugar measurement is needed then a calibration curve would be used instead of the direct reading. The reading from the refractometer is compared to a chart of sugar readings (for banana in your case) for which the absolute sugar content was determined ahead of time for a wide range of banana sugar contents.

See http://www.education.com/science-fair/article/measure-sugar-ripe-fruit/ for a simple description of the refractometer method for fruit sugar measurement.

You had mentioned sugar test strips as well. This method works in a very similar way. A drop of liquid from the fruit is placed on the test strip (or the strip is dipped into the solution), a colorimetric chemical reaction takes place on the test strip producing visible color. The calibration of color intensity related to sugar concentration was done by the manufacturer of the test strip and comes with the test strips. The color of the test trip is compared to the color calibration chart in order to determine the sugar concentration. This is exactly the same idea used for the refractometer measurement and both are good examples of one of the most important concepts in analytical chemistry - indirect measurement through calibration.