I'm currently sipping on a cup of Sweet Bloom's roast of Thageini from Nyeri, Kenya, which according to my calculations with my Atago PAL-COFFEE refractometer, I extracted to 23%.
I used Matt's V60 recipe, and his coffee brewing water recipe, taken from here. In his brew instruction video, Matt says this recipe ended up giving him 20.8% extraction (1.40 TDS). The v60 recipe is as follows:
water at 97c (I used water just off of a rolling boil)
50g bloom, 50g water after 30s, 100g after 60s, finish time 2:20.
I also put the kettle back on the boiler in between pours.
I finally nailed the finish time, and my pour has been improving, but this level of extraction is still shocking. Like I said, my coffee is at 23% extraction (1.60 TDS with this recipe). However, I measured the cup I made yesterday and the TDS was even higher, around 1.64. I thought my refractometer wasn't calibrated correctly, because this came out to an extraction of around 24%, but lo and behold, distilled water tested at 0.00% and I let the coffee cool completely before testing. The flavors are incredible by the way, lots of sweetness, and I don't taste much, if any, overextraction.
What could be different about this coffee I'm drinking now that leads to such a high extraction? What factors could be affecting the solubility this much?