What causes my mother's loss of appetite and how can I improve it so that she is getting adequate nutrition?

What causes my mother's loss of appetite and how can I improve it so that she is getting adequate nutrition? My mother, 88, has no appetite. She blames the condition on the poor cooks in her assisted living facility. However, she rarely touches the food I take to her, even foods that were past favorites. I'm concerned about her getting adequate nutrition. What are some causes of loss of appetite in the elderly? What are some suggestions for improving the situation?