When Haruki Nishimura first started testing granola recipes in his Setagaya apartment in 2017, he followed the temperature most recipes suggested: 180°C for 20 minutes. The results were edible but uneven. The edges of the tray burned before the centre had coloured. The oat oils smelled sharp rather than nutty. It took four months of Saturday-morning tests to land on 155°C for 28 minutes, and the difference was significant enough that we have not changed it since.
What happens to oats above 170°C ¶
Rolled oats contain a meaningful amount of natural fat, around 6-7% by weight in the Hokkaido variety we use. At temperatures above 170°C, those fats begin to oxidise quickly. The result is a slightly acrid note that most people do not consciously identify but that makes the granola feel heavy after a few spoonfuls. Commercial producers often roast at higher temperatures because it is faster and because the added sugars mask the oxidation. In a small batch with no refined sugar, there is nowhere to hide.
The role of kokuto in low-temperature roasting ¶
Okinawan kokuto (unrefined black sugar) caramelises at a lower temperature than white sugar, beginning around 140°C. This means that at 155°C, the kokuto is actively caramelising throughout the roast rather than sitting inert until the last few minutes. The result is a more even coating and a deeper, more complex sweetness that does not require a high-heat finish. If we switched to refined sugar, we would need to raise the temperature or extend the time, both of which would affect the oat oils.
Even colour as a quality indicator ¶
One practical advantage of the lower temperature is that the colour across the tray is far more consistent. At 180°C, the outer oats on a 12 kg batch will be noticeably darker than the centre oats, even with rotation. At 155°C, the colour difference across the tray is small enough that we do not need to rotate mid-roast. This matters for quality control: the colour you see in the bag is the colour the whole batch came out at, not an average.
Why 28 minutes and not 25 or 30 ¶
The 28-minute figure came from a specific test in November 2017. At 25 minutes, the oats were not quite dry enough in the centre, which affected the crunch after cooling. At 30 minutes, the almonds in the Original Sesame blend were beginning to over-roast. 28 minutes was the point where both conditions were satisfied. We have tested this again twice since 2018, with the same result each time.
What this means for home bakers ¶
If you are making granola at home, the 155°C principle is worth trying. Most home ovens run slightly hot, so you may need to calibrate with an oven thermometer. The key indicator is smell: when the kitchen smells like toasted nuts and caramel rather than burning sugar, you are in the right zone. Pull the tray at that point and let the granola finish on the rack. It will crisp as it cools.
Temperature is one of those variables that looks like a small detail until you change it. We landed on 155°C through repetition, not theory, and we stay there because the result is consistent. If you want to taste what it produces, the current range is a good place to start.