Cover a stained area on your white cotton fabric with table salt. Wet the area with lemon juice and lay the cotton fabric in the sun. Allow the salt and lemon mixture to dry on the fabric.
Add 1 gallon of water to a large cooking pot. Drop in one sliced lemon and bring the mixture to a boil. Remove the lemon mixture from the heat and allow it to cool until it is hot but no longer boiling.
Submerge the cotton fabric in the hot lemon mixture and allow it to soak for one hour. Remove the fabric from the lemon solution and wring out the excess moisture.
Wash the cotton fabric with laundry detergent. Treat any remaining discoloration or dinginess by adding 1 cup of lemon juice to the wash cycle.
Hang-dry your cotton garments and textiles in direct sunlight to finish the whitening process.