Providing default elements in XMLSchema differentiated with attribute values

I want to create a schema which enforces that an element with different
values has to be present in the instance of an XML document. e.g.

I want the following elements to be part of the schema always:
<FoodItems>
  <!-- The following 2 elements have to be there in an XML document -->
  <Liquid name="water">
     <qty>100</qty>
  </Liquid>
  <Liquid name="milk">
     <qty>50</qty>
  </Liquid>
  <!-- The following could be optionally there too -->
  <Liquid name="coke">
     <qty>10</qty>
  </Liquid>
</FoodItems>
I can write an XMLSchema for Liquids, but how do I enforce that elements
with water & milk as attribute values are present every time an instance
document is created of the schema.
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