What will come out  is a dense foam  that can be applied directly to the yellow sweat stains. Rub it in for a few seconds and then leave it to act for 20 minutes or half an hour at most. Once the required time has passed, put it in the washing machine and then start washing at 30, 40 degrees at most. The end result  will be spectacular,  the clothes will be very white, they will be like new again, clean and with an enviable aroma.

Other alternative and hasty solutions