I don't use high ratio but will try to answer. High ratio is used instead of shortening and reportedly makes a less greasy buttercream. It is more expensive than store shortening and available at bakery supply places and internet. A substitute would be shortening from a grocery store. Not sure about availability in Scotland.
Hi ratio shortening is a cake and icing shortening while the shortening found in grocery stores is an All Purpose shortening which can be used for everything from baking to icing to frying...
Hi ratio shortenings have more emulsifiers than AP shortenings so they can absorb more liquid without breaking down.
Here's a blurb on hi ratio:
http://www.thebakerskitchen.net/index.asp?PageAction=VIEWPROD&ProdID=266
I know of no equivalent substitute for hi-ratio shortening (Sweetex) unless you could acquire straight icing shortening (Alpine).... AP shortening could be used in icing recipes but it would be like substituting whole milk for heavy cream.
HTH
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