As anyone under 40 will be bored with hearing, at one time you could only get olive oil from the chemists. Today, more than 20 varietals are available, ranging from the delicate La Tanche from Provence, with its sweet hints of apples and pears, to the hefty Frantoio from Tuscany, which delivers a potent peppery hit of wood, rocket and watercress. Yet we retain certain misapprehensions about this invaluable culinary ingredient. Bottles of olive oil are among those residents of the kitchen cupboard whose use-by date should be observed. We are, after all, talking about a type of fruit juice.
Despite our growing familiarity with oils, we tend to ignore many of their best applications. In my view, a fine olive oil is never used to better effect than when added as a swirl to a bowl of thick Italian soup such as rebollita or pasta and bean. Orange-infused oil works wonderfully with green salads and, more surprisingly, shellfish. A drizzle of walnut oil with a few dots of soy sauce makes a transporting dressing for thinly sliced raw tuna.
More unusual oils have enlarged the culinary palette. Dark green pistachio oil works in Greek pastries or drizzled on steamed vegetable. Sumptuous avocado oil enriches dressings and stews. Roasted sesame oil can be used (with discretion – it's very strong) in stir-fries and salad dressings. Pine nut oil can provide an alternative to olive oil in pesto, mixed with chopped basil for a dressing or added to cooked meat.
Which oil to use?
Oils are merely fats that are liquid at room temperature, which explains why olive oil turns cloudy in the cold as a first step towards solidity. Just like animals, plants make oil as a form of concentrated chemical energy. Like animal fats, vegetable oils are highly flavoured, calorie-packed and have a high boiling point. Some oils are best suited for frying, such as refined sunflower, rapeseed and groundnut oil, while others are used at room temperature for their flavour in dressings and marinades. These include walnut, hazelnut and extra virgin olive oils. Infused oils take on the flavours of added ingredients such as basil, lemon zest and chilli pepper.
Sometimes you only need a little oil to add flavour. Giorgio Alessio of the Scarborough restaurant La Lanterna bakes fresh turbot and sea bass with butter in the style of his native Piedmont, but uses olive oil as a condiment on the cooked fish. "It adds a nice little bit of flavour." Similarly, mayonnaise made with olive oil is too strong. A combination of sunflower oil and olive oil in a ratio of 10:1 makes mayonnaise with a sufficiently rich flavour.
Nuts produce the tastiest of all edible oils, resulting from their remarkable nutritional richness. Walnuts and hazelnuts are more than 60 per cent oil. Mainly produced in France, both oils are excellent in salad dressings. A splash of walnut oil enhances stir-fries (heated hazelnut oil turns bitter). A newcomer in this field is Kentish cobnut oil from Hurstwood Farm near Sevenoaks in Kent (www.cobnutoil.co.uk). The first oil to be made from British cobnuts, it won the award of Supreme Champion in the 2010 Great Taste Awards. Along with an intense nuttiness, it has a distinctive freshness. The nuts are shelled and dried before being cold pressed to make the oil. More than a kilo of cobnuts is required for each 250ml bottle (£9.95 plus £4.50 p&p).
You can substitute cobnut oil for butter in baked potatoes or toss boiled new potatoes in the oil with a little black pepper. Cobnut oil works well in cooked dishes and adds an extra dimension if used to replace olive oil in dishes such as ratatouille, risotto, French onion soup and roast pumpkin soup. Like other nut oils, it is sensational in salad dressings. The traditional combination of nuts and blue cheese is utilised in a Roquefort salad dressing – stir cobnut oil into crumbled Roquefort cheese with a splash of sherry vinegar.
Liquid gold: rapeseed oil
Made from the golden crop that dazzles in early summer, cold-pressed rapeseed oil is the nearest homegrown equivalent to olive oil. When looking for it in the supermarket, make sure you get the more expensive cold-pressed rapeseed oil (around £4 for 500ml). I once tried hot-pressed oil at under £1 a litre and it tasted unpleasant and rancid – even when fresh. Unfortunately, soaring rape prices means more of our crop is going this route, since hot pressing has a far higher recovery level.
Cold-pressed rapeseed oil has notable healthy qualities. It is rich in the cholesterol-busting Omega 3 and has half the saturated fat of olive oil. A high smoking point means it performs better than olive oil for high temperature tasks such as roasting potatoes or wok cooking. In our house, it has become the default medium for shallow frying. Endowed with a gentle grassy flavour that carries hints of straw or even asparagus, rapeseed oil makes a good salad dressing and is excellent in mash. Farm-made brands such as Yellow Fields from Selby House in Northumberland and Gold from the Wold, made by the Jackson family at Carnaby near Bridlington, are available online.
Why do extra virgin olive oils differ so much in price?
Extra virgin is an indication of quality – it means that an olive oil is particularly sweet with less than 0.8 per cent acidity – but why do bottles bearing this phrase on the label vary so much in price? "The most expensive oils will come from handpicked olives from a single estate," explains Anne Dolamore, author of The Essential Olive Oil Companion.
"Brands such as Berio, Bertolli and supermarket oils are blended from oil brought by the tanker from local producers. As a consequence, they generally taste the same because they are blended to a particular 'house' style. Some are excellent value as everyday oils for salad dressings, marinades and shallow frying. Keep the expensive single-estate oils for pouring on soups or steamed vegetables and drizzling on salads without any lemon juice or vinegar.
"If you want a good olive oil 'kit', I suggest you should have three types: Greek (for its grassy zing), Spanish (rich and sweet) and Italian (full, fruity punch), so you can ring the changes. Colour is no indication of taste in olive oil. Some of my favourite oils are Ravida from Sicily and Morgenster from South Africa. All oils from the Fresh Olive Company are reliable. I especially like the late harvest Arbequina from Spain and Nunez de Prado estate oil from Spain."
The most expensive oil of all
More expensive than even the poshest single-estate olive oil is truffle oil – olive oil flavoured with the white winter truffle of northern Italy. A 55ml bottle of L'Aquila's concentrated Truffoil sells for £8.50. This is a bargain compared with the real thing. I once paid 100 euros (£85) for a truffle the size of a smallish mushroom. Though it does not match the elusive taste of the world's most expensive foodstuff – somehow both heavenly and earthy – truffle oil has a potency of its own. People tend to love it or (inexplicably) hate it. You use it in much the same way as you would a white truffle. A few drops added at the last minute will transform scrambled eggs.
Similarly, a tiny addition of truffle oil will magically infuse baked and mashed potato. Truffle oil with pasta and a sprinkling of Parmesan works well. Float a few drops on soups. It can also be used to amplify the flavour of mushrooms and dried porcini.
In Riverford Farm Cook Book, Guy Watson and Jane Baxter write that "a drizzle of truffle oil would make a fabulous addition" to their recipe for Jerusalem artichokes and mushrooms baked in a square of parchment and topped with crumbled goats' cheese.
A splat of infused oil
As a short cut in the furious rush of the kitchen, cooks use neutral oils such as corn or sunflower infused with hefty flavours. These infusions can be commercial or homemade. For chilli oil, simply puncture fresh red chillies with a pin and immerse in oil. Make sure the bottle is sterilised first and keep the resulting infusion in the fridge. Use in any dish from chilli con carne to puttanesca sauce that requires a bit of fire.
Horseradish oil (thin slices of peeled fresh horseradish marinaded in oil) can be drizzled on roast beef, added to a Welsh rabbit or used in a dip with soy sauce for sushi. Lemon oil (zest of four unwaxed lemons in 500ml oil) can be used in salad dressings or splatted on grilling fish. Oil can also be infused with herbs. Rosemary oil works well with lamb, while basil oil is a great partner for grilled chicken.
If you make your own infusions, do not presume that they will last for ever. (The exclusion of air encourages botulism bacteria.) Use sterilised bottles and store the infusion in the fridge. Preferably consume within one to two months of making.