Which of the following is NOT an ingredient of the traditional pesto sauce?

Pesto is a sauce originating in Genoa, the capital city of Liguria, Italy. It traditionally consists of crushed garlic, European pine nuts, coarse salt, basil leaves, and hard cheese such as Parmigiano-Reggiano (also known as Parmesan cheese) or Pecorino Sardo (cheese made from sheep's milk), all blended with olive oil.

Pesto is thought to have two predecessors in ancient times, going back as far as the Roman age. The ancient Romans used to eat a similar paste called moretum, which was made by crushing garlic, salt, cheese, herbs, olive oil and vinegar together. The use of this paste in the Roman cuisine is mentioned in the Appendix Vergiliana, an ancient collection of poems in which the author details the preparation of moretum.

During the Middle Ages, a popular sauce in the Genoa's cuisine was agliata, which was basically a mash of garlic and walnuts, as garlic was a staple in the nutrition of Ligurians, especially for the seafarers.

The introduction of basil, the main ingredient of modern pesto, occurred in more recent times and is first documented only in the mid-19th century.