The main pastry ingredient is flour. Yeast is sometimes added to cause ethanol fermentation, the production of ethanol and carbon dioxide from carbohydrates contained in flour. Carbon dioxide makes pastry spongy and therefore bigger.
Although flour and yeast are natural, industrially produced bread may contain chemical substances like antioxidants (L-Ascorbic acid), emulsifiers (lecithin, fatty acid esters), stabilizers, dyes, preservatives, and acidity regulators (sodium acetate, sodium lactate). They improve appearance, smell and shape, make pastry bigger and tastier, and increase shelf life. Bread may have been frozen at -18 degrees Celsius one year before being sold.
Pastry ingredients are to some extent regulated by The European Union.
Flour is made from cereal and contains gluten.