Grinding with mortar and pestle in the presence of liquid nitrogen: Freeze plant
material in liquid nitrogen and do not let the sample thaw at any time during
homogenization. Precool mortar and pestle using liquid nitrogen. Grind frozen
sample thoroughly to a fine powder and refill mortar occasionally with liquid
nitrogen to keep the sample frozen. Use a precooled spatula to transfer the
sample in precooled tubes. Make sure no liquid nitrogen is transferred or all
nitrogen has evaporated before closing the tube.