i've been making SMBC for a few months, using egg whites but i have decided to go for a cheaper more reliable alternative by using MP instead of fresh egg whites. the first few times i made the SMBC, it tasted perfect, but for some reason, i've noticed that it taste slightly tart. what could be the problem and how can it be fixed? HELP!

this is the ingredients i use:

10 tsp MP

10 tbsp water

1 1/2 cups white sugar

2 C Shortening

1/2 C Powdered sugar

AI have no clue. SMBC is egg whites, sugar, and butter. I don't know what the list of ingredients produces, but it's not SMBC.

ANot saying it's not good, but as an SMBC user, that confuses me.

well i usually use 1 cup butter and 1 cup shortening in the fall season, but since i live in in a Sauna state (Florida. lol) and its always hot in my kitchen, i just use all shortening to ensure it doesnt give me problems when i mix it. 

AOh, ok then.

AI've heard people saying some things taste tart/sour with adding MP. Maybe try a new can

I make SMBC with powdered egg whites(DebEl), but I don't like the way it clumps so I mix it with Meringue Powder.  It mixes and flows well but does not have the off taste it does with the straight MP. I have always used all butter.   Also I have subbed either Pink Champagne or Cpt. Morgan spiced Rum for the water and they are delish.

thats what i was thinking..but i didnt wanna waste a whole new can of MP if MP is the problem..as a matter of fact, ill try the powdered egg whites. 

thanks guys!