Previously we tried reusing old grounds in baking and in making sun coffee with generally good results.

However the sun coffee method might have needed some refinement. Two things immediately came to mind as possible improvements:

  1. there seemed to be a maximum level of extraction over time (given the ratio of coffee to water) leading us to think we could reduce the total brew time from four hours; and
  2. given the amount of extraction we saw when using only the sun as heat source, we thought we would be able to achieve similar results using fewer grounds.

We were correct about both.

Changing The Method and a Cautionary Note

This is the Rowan’s Royale Test Kitchen, it isn’t the Science Kitchen (coming soon?) so we have to be forgiven for changing three (major) variables – ratio of coffee::water, brew time, and grinding method.

Some variables were uncontrollably different. For example the second experiment was conducted as the sun waned from the Summer Equinox whereas the first was done as the sun waxed to it. This type of natural variation is understandable and even desirable since it is the way the recipe will be made for general consumption.

Other than the above caveats, we kept both attempts as congruous as possible: same water, same water amount, same vessel, same steeper, same location (down to a few millimeters), same grind, same roast, and most importantly, the same coffee.

This time we used about 70g of medium ground medium dark roast in about 1750 mL of water. Unlike last time, we did not dry the grounds first but used wet grounds from our coffee machine. Again we used an unbleached cheese cloth as a steeper and a clean glass canning jar. We set the brew time to three hours.

Besides the coffee::water ratio and brew time the biggest change was grinding method. For the first test we had used a standard blade grinder (I know, I know). This time we used our every day conical burr grinder – a Cuisinart DBM-8, which we selected due to its being used for science in the biomedical labs of a few friends.


Likely because they were already wet, the beans began steeping immediately and profusely. Certainly this helped with extraction, though we think not by a significant amount since this was likely existing brew left in the grounds due to surface tension and that the grounds were already saturated.

Unlike the first test, there was a noticeable lack of steeping and of convection above the lower edge of the steeper. This didn’t seem to have any lasting effect on the brew but it was an interesting difference, which we think shows that the wet grounds absorb more heat than the empty water below it (this may have an effect on the brew, but it wasn’t something we could measure by site or taste).

We also found that the brew held much more pigment/loose grounds than in the first test – an opacity which lasted past brewing, past settling, and into drinking. We think this is due to the consistency of grind provided by the burr grinder. More consistently sized grains means more even extraction, means more transfer into the water.

Compared to Test #1

Both our conjectures above were proved to be true. The brew had the same light, amaretto, nuttiness and toasted smoke flavor of the first brew but a little stronger.

Like the last, this brew also did not stand up terribly well to full-strength agave nectar, though it did fare better than the brew from the first test.

Finally, while we tested it with lime and lemon juice, we think lemon is a better choice to bring out the natural sweetness of the coffee as it enhances the contrast between the tart flavors and the amaretto but allowed the totality of the brew to shine forth.

A nice chilled glass of sun coffee with lemon from the garden.

Happy brewing!