Why did my oatmeal and raisin cookies end up tasting soapy?

Here are some possible sources of soapy flavor in your oatmeal cookies.

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Issue: Dish detergent residue on your mixing bowl, utensils, or baking sheet.

• If the detergent residue was only on the baking sheet, the soapy flavor would be just on the bottom of the cookies. You can test for that by taste-testing only the top of a cookie.
• The simple way to test for soap residue on a dish is to lick it. If the dish has enough soap residue to impart flavor to cookie dough, you will definitely notice the taste by licking the dish.

Solution: If that turns out to be the problem, you have some trouble-shooting to do. The problem can be your dishwasher, or your brand of dish detergent, or that you use too much dish detergent, etc. Try adding a "rinse agent" or putting some vinegar in the "rinse agent" compartment of your dishwasher. If the dishes are hand-washed, perhaps you don't rinse enough. Try rinsing in hot water. Try adding a splash of vinegar to the rinse water (soap and detergent are slightly alkaline, and if your tap water is also alkaline the soap may not rinse off completely; adding vinegar will acidify the water and help rinse the remaining soap).

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Issue: The baking soda (bicarbonate of soda) has absorbed odors from the cabinet.

Baking soda is very good at absorbing odors, in fact leaving an open container of baking soda in a smelly cabinet or fridge is a good way of deodorizing it. It's a good idea to buy new baking soda every so often, and also to make sure not to store it in a smelly place.

Solution: Buy some new baking soda. If you haven't had that container of baking soda for very long, it may have sat for along time at the store or in a warehouse. Try a different brand, or purchasing it from a different store.

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Issue: One of the other ingredients is the source of the soap taste.

One by one, taste your other ingredients. If you don't find the culprit by a simple taste test, try baking or cooking an item with just that single ingredient, for example:

• To test the flour, make a simple thin (runny) pancake batter with just flour and water or milk. Cook it in a frying pan using butter to grease the pan.
• To test the oil, use it to grease the pan for the flour test. Or use it to sauté some mushrooms.
• To test the oats, cook them as oatmeal. (You say you did this "some time ago," but it's worth trying it again.)
• To test the cinnamon, sprinkle some on your oatmeal. (Be sure to taste-test the oatmeal alone before adding cinnamon.)
• To test the sugar, use it to sweeten some tea (not a strong-flavored tea). If you're not sure from this test, use hot water instead of tea.
• To test the raisins, just eat a few. Or soak a few raisins in hot water to plump them up, then eat them.

Solution: Once you find the "soapy" ingredient, buy a replacement item. If the soapy ingredient is old, it probably picked up the flavor by sitting around in your cupboard. Prevent the issue in the future by storing dry foods in air-tight containers. If the ingredient is not old, perhaps you got a bad batch, or perhaps it's a problem with that brand. Try a different brand.

If you find every brand of the problem ingredient has this flavor, the issue may be peculiar to you and your taste buds. Try substituting a similar ingredient. Instead of a generic "vegetable oil" blend try single-ingredient vegetable oils, eg canola oil, corn oil, olive oil (not extra-virgin) etc. Instead of golden caster sugar, try white sugar plus a splash of molasses. Instead of raisins, try golden raisins, dates, dried cranberries, etc.

Soap is effectively a base (eg baking soda) reacted with a fat (eg oil). Also, heating baking soda can cause it to convert into sodium carbonate, which is a much stronger base.

I would suspect you're on the mark with suspecting the baking soda. The recipe calls for only a quarter of a teaspoon, so it would be very easy too add too much.

I am betting on the oil

I bet the oil you used is not used often and the bottle is old. Rancid oxidized oil gets those bitter soapy flavors. I pick the oil over the baking soda because there is hardly any baking soda in these cookies! But a fair bit of oil.

I am curious what the vegetable oil you used is and when you opened the container.

Get some new oil. Sunflower is very nice for oatmeal cookies. Then only use a tablespoon or two and for the rest use butter. Butter is even nicer.