The more expensive rations is more durable than the low cost ones, so they are already more beneficial. That said I absolutely agree that the quality of the food should influence the company. As there is already a morale system in place, this would be the natural place to integrate it.
Maybe add a value to each type of food in the inventory, low for simple food like grain and fish, and high for luxury food. If the total value of your food is lower than X, then morale penalty occurs. If it is higher than Y, then a morale bonus takes effect.
Then you could get the similar effect of stocking up on lots of different low grade foods, or spend some money on a few valuable luxury foods. It would also give the option of letting grains, as an example, last long before getting spoiled, even if it’s cheap. It just don’t give much bonus to morale.
