Some time ago a friend of mine asked me if I’d consider doing a blog post on fajitas, but every time I made chicken fajitas I never felt as though they were interesting enough to feature on my website. However, I came to realise that this was down to two reasons. One, I’m utterly bored to death with eating chicken in fajitas when there are much more interesting options out there instead. And two, I needed to know more about how Mexicans created authentic fajitas (i.e. what meat did they tend to use? How did they marinate it? And what herbs and spices did they use?). This realisation led me into an investigative journey into the chemistry that creates a fantastic fajita.

WARNING! SCIENCE AHEAD! READ ON AT YOUR PERIL!

(But it’s quite interesting so I’d keep reading if I were you…)

The perfect fajita is made up of a number of components which come together to produce a wonderful medley of Mexican flavours: a warmed soft tortilla; juicy, slightly seared around the edges meat which is encrusted in paprika, cumin and chilli; and soft, buttery guacamole that’s sharp, but aromatic, with freshly squeezed lime juice. Bliss.

Although, there’s more to it than just serving the right combination of ingredients for people to cram into a tortilla, the meat’s got to be treated right in the first place in order for it to give its all to the diner’s palate. That’s where the chemistry comes in. Upon investigation, I’ve discovered that the best meat to serve when making fajitas is beef. To be precise, good quality lean skirt steak (also known as flank).

The unique structural fibres of steak enable it to absorb the oils, acids and salts of a marinade much better than chicken or pork ever could and allow it to retain the flavours of the herbs and spices we choose to add, but it’s the important chemical effect of the marinade that leads to the production of a beautifully soft and juicy piece of cooked beef.

The best steak fajita marinade will always contain three elements: oil; acid; and salt. The oil works on three levels: it emulsifies the marinade and allows it to coat the beef efficiently; it dissolves the oil-soluble flavour compounds within the spices, enabling them to be absorbed into the meat; and it also provides a protective layer around the meat when you cook it over a high heat, hopefully helping it to retain its natural moisture. The acid, in the form of fresh lime juice, tenderises the meat and breaks down the connective tissue, leading to a softer and easier to chew mouthful of beef. And lastly, the marinade’s salt content dissolves myosin (a muscle protein) which gives the beef a slacker texture and helps retain its moisture. Also, by using soy sauce instead of plain old salt it introduces glutamate and protease (found naturally in soy sauce) into the marinade which add umami flavours and tenderise the meat further.

I did warn you there’d be science.

In an ideal world I’d marinade the steak fajita strips overnight to really let the flavours be absorbed by the meat, but if you take the notion to make these I think you can get away with an hour’s marinating (that’s what I did, to be honest). And in terms of cooking the meat, cook it fast over a really high heat and try to cook the steak medium to enable the natural juices of the steak to remain.

Serve the steak fajitas with a plethora of delicious accompaniments so that the people at your dining table can build the perfect fajita to suit themselves. Sombreros and stick-on handlebar moustaches are entirely optional though.

Ingredients for the marinade :

500g of skirt steak (cut into strips)

1 heaped tsp paprika

1 heaped tsp smoked paprika

½ tsp celery salt

½ tsp ground cumin

¼ tsp ground chilli

¼ tsp ground black pepper

2 tbsps. tamari (gluten-free soy sauce)

1 tbsp. vegetable oil

The juice of 1 lime

Additional ingredients:

100g red bell pepper (cut into thin slices)

100g green bell pepper (cut into thin slices)

8 corn tortillas (or gluten-free tortillas)

To make a basic guacamole :

80g avocado

The juice of ½ a lime

8 cherry tomatoes (quartered)

¼ tsp fine salt

Method :

Put the steak strips in a large bowl and add all the ingredients into the bowl with it (apart from your guacamole ingredients, obviously). Stir it all thoroughly and leave to marinade.

When you’re happy that your meat’s marinated enough put a griddle pan or a large frying pan over a high heat.

Drain and discard the liquid from the steak marinade before putting the steak and the slices of pepper into the hot pan.

Cook the steak to your preferred liking. Once cooked, put the steak in a serving bowl and cover with foil and let it rest for 5 mins while you make the guacamole.

To make the guacamole:

Half your avocados and remove the stone. Use a spoon to scoop out the avocado flesh and mash it in a bowl before adding the rest of the guacamole ingredients. Mix them all together and place in a serving bowl.

Serve your steak fajitas with warm, soft tortilla wraps, the guacamole, chopped fresh coriander, salsa, crème fraiche or sour cream (or a non-dairy version), re-fried beans, grated cheese (or a non-dairy version), and slices of fresh chilli.