The piping gel or corn syrup used in icing are to make it hold its shape better specifically for petals and roses. The piping gel or corn syrup make leaves that tend to have better pointier tips and tends to give the icing a more elastic quality. It wasn't being suggested to make the icing less thin, it was being suggested to make the petals of roses not have cracked edges and such. It is fairly common for folks to have problems specifically with roses and leaves when using the half butter half shortening icing.
Regarding the refridgeration of icings using dairy products, when the icings contain butter or milk or cream, when the milk or cream is used in small amounts, as in buttercream icings with only a few tablespoons of milk or cream added, they can indeed be kept on the counter at a room temperature below 75F for 2-3 days. In fact Wilton recommends the same timeframe for both the all-shortening and water icings as they do for the half butter and cream or milk icing. Only in the commercial industry where kitchens can have much more intense heat, is it recommended and indeed, law, to keep these icings refridgerated. This is because many commercial kitchens do not have air-conditioning and with the amount of baking going on, the temperatures are extremely high.
Sugar is a preservative and indeed using a salted butter, also contributes to the preservation of the icing.
This is a common misconception regarding icings using milk or cream and butter..
Regarding the use of the Crisco butter flavoured shortening, actually this is not recommended because it has a higher moisture content that regular Crisco and makes for an icing that is sloppier to work with particularly when using it for roses and items that require stiffer consistency of icing.
Butter has a melting temperature of 83F, shortening has a melting temperature of between 89 and 99F.
Salted butter can be kept at a room temperature of below 75F for a week to two weeks. The salt preserves the butter, whereas unsalted butter has a much shorter shelf life both at room temperature and refridgerated temperature.
It is true that the heat of your hands can cause melting of butter. To counter that, adding a bit more icing sugar and using a ratio of 4-5 cups of icing sugar for every combination of 1/2 cup butter and 1/2 cup shortening helps.
Icings made with butter and cream and/or milk can be made up ahead, up to 2 weeks in advance and refridgerated. It is important to insure that the expiry dates of these products are within this 2 week period.
You cannot compare a glass of milk left on the counter to an icing with a dairy product in it, due to the other ingredients that act as preservatives. What you can compare it to is whipped cream icing. Actually that has the shortest life at room temperature because it is actually the nature of whipped, whipping cream to separate at room temperature. Where this will vary is in the case of a ganache which is equal amounts of scaleded cream and chocolate. Bear in mind that scalding the cream creates a condition where all bacteria is killed and this enables you to keep this icing at room temperature for a period of 2-3 days.
Generally the rule of thumb regarding the use of milk or cream in icing is, if it is a main ingredient, the icing or filling must be refridgerated, as in a pudding, a whipped cream topping, a mousse. If it is strictly a binding agent, one to add enough moisture to bind the mixture together, it does not require refridgeration as long as room temperature is below 75F and the period of counter time is below 3 days. Actually these icings hold up 4-5 days for most people but 2-3 days is used as extra precaution.
The way that shortening and butter react at room temperature has more to do with the moisture content of these two items than it does to the way the fats themselves react to the temperature. When the temperature rises above 83F, it comes into play. It also comes into play at oven temperatures as the effect on these two fats in baking is totally different.
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