Issue: marie biscuit having soapy taste
Root cause I have in mind: imbalance pH, saponification process which happened when ammonia bicarbonate reacted with shortening.
Thank you in advance
Ammonium Bicarbonate when used in biscuits totally breakdown in the baking process resulting in a neutral pH. The levels used by manufacturers can be quite high depending on the amount of volume they want the biscuit to achieve in the oven.
Sodium bicarbonate is not degrade by heat and unless neutralised by an acid will result in biscuits with an Alkaline pH. There are various acid ingredients that can be used to neutralise the Sodium Bicarbonate and the use of an acid also liberates more CO2 from the Soda. Each acid has a different neutralising value so the amount required to reach a neutral pH is different for each one. The most widely used is Sodium Acid Pyrophosphate (SAPP) which has a Neutralising value of 72 so to neutralise 1g of Sodium Bicarbonate you need to add 1.4g of SAPP
The soapy note in Marie biscuits is definitely due to excess sodium bi carbonate in the recipe. This we have faced same problem when I was in Britannia. When sodium bi carbonate in excess qty than required when due to heat it disintegrate into the sodium carbonate, carbon dioxide and water. When traces of sodium carbonate is left in the biscuit due to presence of oil and oxygen it forms soap in the biscuit. So normally in Marie biscuits we use to add 350gms to 400gms of soda for 100kgs of wheat flour. if any gauge problem comes(stack height) then try to adjust with ammonia. Ammonia will never shift the pH to abnormal limit. Then also you are not getting stack then try with SAPP or ACP. The desired dough pH is around 7.5.
There is not hard and fast rule for addition of Soda or ammonia. Needs to be fixed on experience base only.
Please also assure that you moisture is up to 2,0 % or lower, if you are using Ammonium Bicarbonate. The use of this leavening powder with high moisture index result in ammonia linking with water, and not being release in oven, as it should behave. This may cause unpleasant flavor. Try decreasing sodium bicarbonate as well, but also your acid leavening, accordingly, to keep pH balanced (max. 7,4).
All the previous answers are totally right and gives you very good points to work on in order to get good results.
The last one reason I would like to mention is the type of fat you are using. If it is coconut or palm fat, sometimes soap taste may appeasrs. But it will happen some days or months after production, along the shelf life of the product. On the contrary, if the reason is the excess of sodium bicarbonate, the soap taste will appear as soon as you bake the product, and sometimes even in the raw dough.
I hope to have helped. My kind regards.