Any person, firm, or corporation owning or operating an assembling plant, workshop, or mechanical establishment employing one or more persons shall allow all of their employees not less than thirty consecutive minutes for lunch in each eight-hour shift, and during such time it shall be unlawful for any such employer to require such employee or employees to remain in buildings or on the premises where their labor is performed. This section does not apply to employment that is covered by a valid collective-bargaining agreement or other written agreement between an employer and employee.