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

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 don't see anything acidic enough to react with the baking soda and neutralise it. That's often the culprit in a soapy flavour. Flour and egg are very slightly acidic but I don't think it's enough, especially if the eggs are old. Brown sugar is more acidic than white. I would assume golden caster is somewhere in between, but if you used ordinary caster (I would, I don't keep in golden caster nor is it in my closest supermarket) that would lose some acidity. Brown sugar is common in similar recipes.

Any residual alkalinity from the baking soda may be more noticeable if you omit the salt that isn't listed on the ingredients, only in the method.

Replacing the baking soda with baking powder would be an option here, but only because so little is used.

But looking at the comments, many people found them oily (though I have no way of checking whether they tasted them too soon). Perhaps look for a different recipe, though I tend to find the BBC recipes generally OK, and they've actually been tested in grams unlike some rather suspect conversions.

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

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).

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.

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.

The chemistry of the soapy taste for oats has been described in several studies.

The oat grain consists of the groat (caryopsis) and the surrounding hull (husk). Only the groat is required for milled products while the indigestible hull must be separated and removed in a de-hulling (shelling) stage. The groat has a lipid content which is 2-5 times that of wheat. It also has an active lipase that is separated from the lipid in the intact seed, which can lead to hydrolysis resulting in a ‘soapy’ taste in final products. This means that the lipase needs inactivation by a process called stabilisation to avoid these undesirable effects in processed products. This is usually achieved by kilning or cooking.

So the soapy taste suggests a default of processing. But could it also be caused or be aggravated after the processing: during the transport, storage, sell if the hygrometry is not tightly controlled? If it's the case, (if the soapy taste indicates that the oats have (also) taken moisture), it should alert on the possible development of aflatoxins produced by some common molds. "Aflatoxins are among the most carcinogenic substances known".

source:

The quote above comes from: Keith Scudamore, Harry Baillie, Sue Patel, Simon G Edwards. The occurrence and fate of fusarium mycotoxins during the commercial processing of oats in the UK. Food Additives and Contaminants, 2007, 24 (12), pp.1374-1385. ff10.1080/02652030701509972ff. ffhal-00577366

The process in detail: Welch, R. W. (2012). The Oat Crop: Production and Utilization. Springer Science & Business Media.

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.