With modern life presenting new challenges in the face of busy schedules, meal prepping serves as a time, and sanity, saver for many people. Setting aside time to prepare a few meals in large batches that can be reheated later on in the week is an effective time-management tool. However, what does this mean for the nutrient content of our food? What does cooking food do to the nutrients inside? There will always be a degree of nutrient loss when cooking or reheating food, but getting some nutrients is always better than getting none at all, and, cooking food assists with digestion and can increase absorption of some nutrients. In addition, some foods must be cooked to prevent bacteria growth, which can be harmful to health. As a general rule, the less cooking time and the lower the temperature used when cooking, the better.
Steaming is the preferred cooking method for retaining nutrient content, particularly those heat-sensitive, water-soluble vitamins like vitamin C and B vitamins. Research has revealed that steaming broccoli, spinach and lettuce decrease their vitamin C content by only 9–15%, which is less than most cooking methods. Although the thought of steamed veggies screams bland to most people, this can be remedied by adding a punch of seasoning and oil or butter after cooking. Sautéing and stir-frying is also optimal, as it uses shorter cooking times without water, preventing loss of B vitamins. The inclusion of fat from oils during cooking is also linked to improved absorption of antioxidants. One study has shown that absorption of beta-carotene was 6.5 times greater in stir-fried carrots than in raw, while another demonstrated that serum lycopene concentrations increased 80% more when participants consumed tomatoes sautéed in olive oil rather than without. However, stir-frying has been demonstrated to notably decrease total vitamin C in broccoli and red cabbage.
In comparison, boiling vegetables leads to increased losses of said water-soluble vitamins, and, has been shown to cause reductions of glucosinolate, the sulfur-containing compound found in broccoli that has been linked to the suspected anti-cancer properties of the vegetable. Whereas, most vitamin losses are minimal when roasting or baking food, including vitamin C, however, given the longer cooking times and use of higher temperatures B vitamin content can still decrease by up to 40%. This is also the case when grilling or broiling meat, as the B vitamins are lost in the juices. There are also concerns about polycyclic aromatic hydrocarbons (PAHs), which are potential cancer-causing substances that form when meat is grilled, as fat drips onto a hot surface. Thankfully, researchers have discovered that PAHs can be reduced by 41–89% if drippings are removed and smoke is minimized. Do you see the dilemma? Many of the enzymes in whole grains, beans and legumes are also destroyed during the cooking process, which are needed for digestion. However, it can be quite hard to digest raw whole grains, and, who is going around eating raw chickpeas, seriously. There are some things you just cannot get away with eating raw.
Now, most of us know that fried foods are bad for us, but why? When the oil is heated to extremely high temperatures for long periods of time, as is done when frying, the chemical nature of the oil is changed and aldehydes are formed, which are toxic substances that have been linked to increased risks of cancer and other illnesses. These are also amplified when oil is reheated, so my advice would be to steer clear of fried foods entirely, but we are human. To help you justify the occasional hot chip at the footy, frying has been shown to conserve vitamin C and B vitamins, and, potentially increase the fiber content of potatoes by converting their starch into resistant starch. But let’s not get carried away, lest we forget those aldehydes. Another reason to steer clear of frying is the effect it has on the delicate omega-3 fatty acids, which are rife in fatty fish like salmon, tuna, sardines, anchovies, mackerel, and herring. I know the latter isn’t quite as popular, but have you ever tried a deep fried sardine? Yum. But unfortunately, they offer little in the way of nutrition, as the brain-loving omega-3’s are damaged at high temperatures. Research has shown that frying tuna degrades its omega-3 composition by up to 70–85%, compared to minimal losses in baking. So bad news for the Friday fish n chip lovers. My advice; throw some fish and veggies in the oven instead, your brain will thank you for it if it remembers (insert awkward laugh here – you get it? because omega-3’s are good for memory…anyway).
So that’s the cooking bit covered, are you still with me? Time for the reheating discussion.
The microwave isn’t as great a villain as often thought. Despite its convenience, particularly when wanting to reheat your leftovers at work or college, microwaves are super-efficient at exciting water molecules, producing frictional heat. This means that the water content in food is ignited, causing them to heat up quite rapidly. Food is essentially steamed from the inside out in short periods of time, retaining more vitamins and minerals than most cooking methods. Further losses of vulnerable vitamins like, vitamin C and B vitamins, in particular, folate (B9) and thiamine (B1), are also lessened. Studies have found that microwaving is the best method for retaining the antioxidant activity in garlic and mushrooms, and, only about 20–30% of vitamin C in green vegetables is lost when microwaving, which is less than most cooking styles. One potential disadvantage of microwaving food is the uneven nature of heating, which could have food contamination risks. To avoid this my advice is to:
- Stir food frequently
- Use minimal water
- Cook vegetables until just firm
- Know your microwave to avoid over or under cooking
- Avoid reheating in plastic containers
- Use microwave-safe dishes - glass is best
Now I know it may sound like I’ve just given you a bunch of contradicting information but the take home message is that it is inevitable, there will always be some loss of nutrients when reheating and cooking food. But, let’s face it, most of us do not have time to prepare a meal from scratch every night of the week, let alone, for every meal of the day. So, being prepared is key to making healthier choices, which comes at the expense of some nutrient loss. In the wake of increasing home delivery services and take-away outlets providing convenient meal options for busy consumers, society’s obesity epidemic continues to rise. I would say that reheating food is definitely the lesser of two evils in this case, not to mention it can reduce the risk of foodborne illnesses. I mean, who wants to spend three days with their head in a toilet all because they were worried about a few extra milligrams of vitamin C in their reheated capsicum. No thanks.
To finish off, here are a few tips to maximise the nutrient content of your food:
- Store pre-prepared food in air-tight containers – if exposed to air, cooked food will continue to lose nutrients from oxidisation
- Keep pre-prepared food refrigerated to avoid spoilage
- Keep pre-prepared food for no longer than 3 days to avoid risks of foodborne illnesses
- Eat fruit and vegetables raw, where possible
- Wash vegetables thoroughly instead of peeling them – the peel contains many nutrients
- Steam vegetables instead of boiling or blanching
- If boiling or blanching vegetables, save the nutrient-laden water to use in soup or stock - 100% of minerals & 70–90% of B vitamins are retained
- Use minimal water when poaching or boiling
- Cook food whole and cut afterward to reduce exposure to heat & water
- Cook foods quickly or at low temperatures, where possible
- If frying, avoid overcooking & use coconut oil – it is stable at high temperatures & has numerous health benefits
- Immediately freeze freshly picked produce, if home grown
- See storage guide for vegetables, fruit & herbs to maximise shelf life & nutrient density