Using residual heat for cooking

Energy Efficiency in your Kitchen

I love the change of seasons – the different colours, clothes, and perhaps most of all, the shift in seasonal foods and rhythms of cooking. The winter months usher in a heartier, comforting repertoire of soups, stews, braises, roasts, to complement both the weather and the deeper, robust flavours of winter greens, root vegetables and squashes.

And while I welcome these cold weather changes to my menu (the braai put away, and roasting dishes and casseroles come out of their hibernation), my heart sinks with every recipe that finishes: “cook slowly for five hours”.

Do these recipe writers live in a world without electricity or gas bills? Where they can instruct us to roast one head of garlic for fifty minutes in the oven – and then use that oven for no other part of the recipe?

So what is a modern home cook to do, in a world where rising energy costs and accompanying environmental concerns make it difficult to blithely roast a leg of lamb for eight hours?

One of the new terms to emerge out of the energy crisis last year was ‘hypermiling’ – trying to use as little fuel as possible in your car by using driving techniques that maximise your vehicle’s efficiency.

The answer to the conundrum above lies, I believe, in taking a similar approach to cooking. More than what you cook, it’s how you cook.

This article focuses on techniques and approaches to minimise energy use by using your stove, oven and other cooking appliances more effectively and creatively.

Here are six simple ways to use less energy in your kitchen:

1. Ensure your devices function properly

If your oven door doesn’t seal properly, your toaster insists on using two elements for one piece of toast, and your gas stovetop flame burns high and yellow, you’re fighting a losing battle. No one likes to spend money fixing kitchen devices, but in the long run, you will be saving money by making sure your equipment runs efficiently, without excessive use of electricity or gas.

This also extends to unpleasant but necessary chores, such as defrosting your freezer once a month (ice build up keeps the chill elements from working effectively, and sometimes affects door sealing). Stocking your fridge too full or leaving it very empty makes it harder for your fridge to keep food at the correct temperature, which leads to spoilage, and also wastes energy.

And finally, energy also should be a consideration in the equipment you buy. Look for energy star ratings when purchasing a new oven or fridge, and when buying equipment to be used with your energy-hungry devices, it’s worth going for quality.

Heavy-bottomed pots will not only last you longer, but will retain heat more effectively and evenly, leading to less food burning, and to less energy usage.

2. Understand how your energy-using devices work, and what they should (and should not) be used for

Sure, you can make one piece of toast in your oven’s grill – but using a toaster, which is meant for toasting one or two pieces of bread at a time, rather than browning an entire lasagne, is a better use of equipment. If using your stove, match the size of the pot to the size of the stoveplate. A small pot might heat faster on a large stoveplate, but all that heat emanating from the exposed stoveplate is lost energy.

To go back to your fridge, if you put food still piping hot from the oven into it, it heats up the inside, which shortens the shelf life of neighbouring foods, and forces your fridge to work harder to cool the inside temperature. Rather cool hot dishes in a sink of cold water, or by transferring to a cold dish, than increasing your fridge’s workload.

3. Cook more portions at one time

No one wants to waste food, but if you make more than you need and properly store the excess (for lunch the next day, or the freezer), you’ll be saving energy. Making four portions of soup in one pot doesn’t take double the energy as making two portions of soup in one pot. This is especially relevant for dishes that take a long time – and a lot of energy – to cook.

And if it’s a meal that won’t freeze well, get creative with the leftovers. For meat roasts and braises, think pasta sauces, sandwiches and stock from the bones. Leftover vegetables and legumes make amazing soup. There are very few dishes which can’t be super-sized and reinvented in leftover form.

4. Fill your oven

Unless we’re making a big meal, it’s unusual to use our entire ovens, which usually can fit three shelves-worth of dishes at a time. The worst offenders are small items, which often could be cooked using a different method and certainly don’t warrant the hour or so of the oven’s full blast, such as one head of garlic, four baked potatoes, one tray of muffins.

Whenever you turn on your oven, think, “What else could I be putting in the oven right now?” Why not save energy and time and cook tomorrow’s meal now, too? Rummage in the fridge, defrost things in the freezer that should be used sooner rather than later, and fill that oven up.

While the oven uses more energy when very full than it does when relatively empty, the long periods an oven is typically used for (mine rarely goes on for less than an hour at a time) and time needed to preheat means it’s an energy saving to cook several dishes at once rather than one after the other, or on different days.

If you’re roasting meat, roast your vegetables, too. Layered pasta, meat and vegetarian bakes reheat quickly and deliciously the next day, and are a good choice for tomorrow’s meal. Think of lunch, and pop in a couple of potatoes to bake (it could also shorten prep time for potato salad tomorrow).

I love to use extra oven space for root and hard veggies, such as beetroot (which take a long time, but rarely much space), foil-wrapped heads of garlic (the smoky resulting garlic mush can be frozen in ice cube trays or kept a while, sealed, in the fridge, and added to almost everything) and butternut and carrots (superlative hot out the oven, or as a soup tomorrow).

5. Use a microwave where possible

Microwaves have attracted an unusual level of controversy and furore in recent times. I’m not going to address that here — if you own a microwave, I’m going to assume you’re happy to be told to use it more.

It is extremely energy efficient, as anyone who has cooked a potato in one instead of in the oven or on the stove can attest.

Most of us rarely get beyond its abilities with reheating leftovers, defrosting things or heating up milk for tea (although in the eighties, many microwave cookbooks to my mind did go beyond the reasonable – I don’t think a microwave is the place to cook fillet steak or chiffon cakes).

Nonetheless, a microwave can save you time, energy and nutrients (although I expect Angry Letters for putting in the latter) with many simple vegetable dishes.

I find a simple, retro pyrex dish with matching lid is all the equipment I need to steam pretty much any vegetable. Pop in your trimmed veg, add a pinch of salt, a splash of water or stock and close her up, and in two to eight minutes (as opposed to ten to thirty on the stove), perfectly steamed vegetables, easily monitored through the door and transparent pyrex.

These can be shocked in cold water for salads, or drained and dressed while warm with vinaigrettes in the French style, or simply butter or olive oil and some chopped herbs. And it leaves the stovetop and your attention free to focus on the other elements of the meal.

6. Use other sources of heat

If you have a fireplace or even a charcoal or wood stove you use regularly for heating, you automatically have another source of heat to cook with. I’m not suggesting a pig rotating on a spit in a your living room, but rather a way to use your stove and oven a little less.

Of course, there’s nothing stopping you from bringing the braai indoors, with grills and potjies, but a fireplace can cook a meal as humble at a pot of stew. Anything that can be cooked in a cast iron pot, slowly and at a simmer, is eligible.

I love to do bean stews, which I start on the stove and then transfer to the side of my fire. Trial and error has shown me how far away to place it from the flames; I use tongs to rotate it once every forty minutes for even cooking.

And while I haven’t done it yet, I think bread toasted on the fire to serve alongside it would round out a fireside meal splendidly!

A simpler but eminently practical way to use an indoor fire is roasting potatoes, beetroot, heads of garlic, onions (split, smeared with mustard and butter and herbs, and then sandwiched back together), beetroot and brinjals wrapped in tinfoil in the coals. Rotate once in a while, and don’t put them on active flames, and that’s all there is to amazing flavour and effortless cooking. Children find it very exciting to extend this method to dessert – apples or my favourite, quinces, wrapped in foil and left in the coals until soft, are a wonderful winter dessert served with custard or cream. Or if, like me, you feel cold weather is no impediment, ice cream.

7. Use residual heat

By ‘residual heat’, I mean the heat that’s left in your oven or in the walls of your pot even once you’ve turned the electricity or gas off. As all cooks learn, food keeps on cooking once removed from the heat source. But aside from dried out fish or steak, this heat can have a positive impact. If wisely harnessed and factored into the overall cooking time, you can use this residual heat to use less energy.

One simple way is to put a natural terracotta tile (samples usually are free from tile shops), pizza stone or even several bricks on the bottom shelf of your oven. Put them in when the oven is cold (sudden heat causes tiles to crack) and preheat with your oven. Bricks and terracotta retain terrific amounts of heat (which is why pizza ovens are traditionally brick structures), which both stabilises the heat inside your oven every time you open and shut the door, and gives you a very hot surface to put dishes on directly (think lasagne and roasts, not delicate cookies) if you want to speed up the cooking process.

Other methods are even simpler; just turn off the oven and leave the door closed, and the heat remaining within the oven is sufficient to cook many dishes to completion. Similarly on the stovetop, a heavy pot (such as cast iron) can be taken off the heat and left, sealed, to let the pot finish the cooking. These methods take slightly longer, but often result in more gently cooked dishes. And they certainly save you energy. (See the chicken recipe to the right for one such example.)

Possibly the best way to use residual heat is to use a hotbox. Hotbox cooking, or retained-heat cooking, is an age-old slow cooking method used to conserve energy, both in fuel and labor. Food is brought to a boil, simmered for a few minutes then put into a well insulated box where it will continue to cook slowly for hours.

Since the insulated hotbox cooker prevents most of the heat in the food from escaping into the surrounding environment, no additional energy is needed to complete the cooking process. While cooking takes about twice as long as stovetop cooking, hotbox cooking can save between 20% and 80% of the energy normally needed. Your pot only needs to remain on the stove for a quarter of the time needed in conventional cooking. Hotbox cooking also prevents food from boiling over, overcooking, sticking to the bottom or burning. Food turns out perfectly cooked every time! `

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