Preemption is the rule that federal law reigns supreme above state law. This is known as the supremacy clause. Federal law can preempt state law in the following ways.

1. When the statute clearly and expressly says that the federal statute has full and complete authority over the state. This is called expressed preemption.
2. When the state and federal law is in conflict
3. When the state law stands as an obstacle to federal law

Expressed preemption is a clear declaration in law that a federal statute is the exclusive authority of the federal government. In this case, the government prevent states from creating laws that will conflict with the federal law or establish additional requirements above the federal law. An example of a case of expressed preemption can be see in the example of the Michigan Comminuted Law. Michigan had a standard for comminuted (minced) meat that was higher than the federal standard. This was a source of great pride for the state. However the Meat Inspection Act was amended in 1967 to preempt the standards for meat in Michigan. Consequently the legislature in Michigan amended the state law to require retailers selling meat who fail to meet the higher Michigan standard to include in the labeling a statement that says "Do not meet Michigan's high meat ingredients standards but do meet lower federal standards". It was determined that this law was a burden to interstate commerce and was therefore struck down. Today, state meat inspection programs exist, but only with permission and supervision of the USDA. State rules are upheld in cases where both state and federal statutes can be met at the same time. A case in point is Florida Lime and Avocado Growers, Inc vs Paul 373 U.S. 132 (1963). In this case, California law required that avocados have 8% oil to pass as mature. Avocados shipped from Florida to California struggled to meet this criteria. This was because Florida avocadoes were of a different variety, making them achieve maturity before the oil content reached 8%. Federal law for avocadoes did not stipulate oil content, but other criteria including size, which Florida Avocadoes met. Florida growers argued that California law was in conflict with the Federal Law. The Courts did not agree however. This was because it was possible for Florida’s avocado to meet both the State and Federal standards. Florida could allow their avocadoes to remain on the tree until the 8% oil level was reached. The down side would be the lost sales opportunity during that wait time. Regardless, since waiting allowed Florida farmers to meet both California’s Standard and Federal Standard, the Courts did not consider the California law to be in conflict with Federal law. When the state law is found to be an obstacle to federal law, the federal law invalidates the state law. This was the case of  Jones vs Rath Packing Company 430 US 519 (1977). Wheat flour made by Rath packing company was determined to be underweight by the Department of Weights and Measures in California. This weight difference was due to normal moisture loss during distribution. The loss was within the range that was acceptable by Federal statutes, but out of range with respect to the State law which had stricter requirements for weight differences allowed. Therefore, the packing company filed a suit. The Courts questioned whether or not it was possible to meet both the State and Federal standards with respect to weight. The answer was ‘yes’. However, this was possible only by over-packing the bags that were destined for intra-state distribution and regular packing, for interstate distribution. The problem the Courts found with this was that it would mean that different customers (in-state and out-of-state) would receive different amount of flour solids in their bags. Therefore this was determined to be an obstacle to the full purpose of the Federal Law; which was that every customers receive the same quantity of product. Reference: Fortin N. D. (2009). Food regulation - Law, science, policy, and practice. Hoboken, NJ: John Wiley and Sons Inc.