There are many people who struggle to duplicate recipes from their favorite Thai restaurants because it is difficult to reach a high wok temperature or find the perfect balance. But even more elusive than those challenges, the ingredients can make all the difference too.

"The Thai people use hundreds of ingredients that are not always so easy to identify once they are cooked," says Alan Wong, general manager of Kung Fu Plaza, which is the oldest and most authentic Thai restaurant in Las Vegas. "Many ingredients change the flavor, depending on when they are added to the dish."

As an example, Wong selected five less common ingredients that he considers essential to cooking some Thai dishes. One of them cannot be substituted. He said it is better to leave the ingredient out than add something else.

Five Thai Ingredients You Can't Go Without.

• Thai Eggplant. Thai eggplants come in a variety different sizes and colors, but the one used most often is greenish white and round like a ball. It's about the size of a small lime. They are essential in chili sauce and curry dishes (especially green curry). Thicker skinned eggplants have fewer seeds and a slightly bitter taste, which dissipates with cooking. The best substitute is the long Asian eggplant.

• Kaffir Lime Leaves. Kaffir limes are used for the zest and the leaves (but not the juice). The zest is used for a red curry paste, giving it an astringent taste. The leaves add a slightly bitter taste to dishes, especially if they are added at the last minute. Fresh leaves are much better than dry leaves. The only substitute is lime zest or juice, but it's hardly a replacement.

• Thai Thin Soy Sauce. Thai thin soy sauce is made from the first extraction of soy sauce. It's considerably stronger, saltier, and less sweet. It's used in many stir fries, sauces, and soups. Its consistency is more like fish sauce (but these are very different ingredients). The best substitute is a light Chinese soy sauce or Kecap asin, which isn't as sweet.

• Thai Hairy Basil. Thai basil has a milder flavor than sweet basil and a slightly lemony flavor. It's often sprinkled over salads, soups and some curry dishes as a garnish. This is not to be confused with the holy basil, which is peppery. As a substitute, Italian basil is fine.

• Galangal. It is a close cousin to ginger, but is much more pungent and peppery. It's often used for pastes and curries. It also minimizes fishiness in some dishes. There is no substitute, and please do not make the mistake of substituting with ginger. If you ask any Thai chef, they will say do not substitute with ginger. It's better to leave it out.

Founded in 1973, Kung Fu Plaza is the oldest and most authentic Chinese and Thai restaurant in Las Vegas. It is open daily from 11 a.m. to 11 p.m. for dine in, take out, and delivery. The average entree is under $10 and most patrons order family style.