I’m finally learning the importance of proper brew temperature in extracting a shot. Basically, too hot and the bitter elements of the coffee will be extracted. Too cool and the acidic elements of the coffee predominate. Also, the color of the extraction is affected by temperature. Excessive heat produces a very dark extraction. Not hot enough and the extraction and resulting crema will be light in color. Ideally, the crema on the shot should be show tiger striping, dark and lighter striations.
There are numerous opinions on how to achieve the proper temperature on an espresso machine. Silvia, in particular, has numerous theories, practices and articles on the subject. Stay tuned on this site for links to some of these articles.