Currying flavour

I grew up in a house where curry sauce always came out of a jar. And despite becoming a fairly competent home cook, for some reason it never occurred to me that I could be making curries any other way. That all changed three years ago. My family and I spent a year in the US, where none of our local supermarkets stocked ready-made curry sauces. At first, I tried avoiding cooking curries. Second, I drove long distances in search of jars. And then, finally, I made one from scratch. With mouths watering, we eagerly anticipated our first curry in months, only to discover that what was on our plates was rather bland and underwhelming.

Fast forward three years and I’ve got better. In the UK, we live less than 100m from the village shop that stocks upwards of 10 different types of curry sauce at any one time. But I refuse to buy them. I’m the stubborn type. It has become a competition in my mind: Nina v the jar, who can cook the best curry? I suspect the jar still has the edge. But can learning about the chemicals and chemical transformations involved in making a curry sauce help me up my game? It’s time to find out.

A potpourri of flavours

One of the reasons why cooking Indian cuisine seemed daunting to me was the extensive ingredient lists, typically including vast numbers of spices. ‘Indian recipes, popularly known as curries, are a potpourri of flavours,’ explains Ganesh Bagler, a computational biologist at Indraprastha Institute of Information Technology in New Delhi, India.

And I was flummoxed by the fact that, unlike for most western recipes, I couldn’t see how the individual components fitted together to build the flavour of the final product. There is a reason for this: Asian and western cuisines don’t follow the same flavour rules.

This disparity was identified in 2011 by researchers at Northeastern University in Boston, US. The team was studying over 55,000 recipes from around the world to seek evidence for the so-called food-pairing hypothesis – the long-standing assumption that ingredients with shared flavour molecules are more likely to taste nice together than ingredients that do not. ‘The conclusion from our paper was that there is statistical evidence that this shared compounds effect plays a role in western cuisine,’ explains one of the lead researchers Sebastian Ahnert, now at the University of Cambridge, UK. In Asian cuisine, however, they found the opposite. The more flavour compounds two ingredients shared, the less likely they were to be used together.

An Indian-specific follow-up study by Bagler probed 2543 curry recipes and found the same thing. ‘We looked at the average number of shared flavour molecules across all the ingredients in the recipes,’ explains Bagler. ‘Our analysis pointed to the negative or contrasting food pairing in all these Indian recipes.’

Ultimately, big data insight such as this is expected to revolutionise the food industry – including curry sauce recipes. But for now, it isn’t clear how this knowledge is going to help me beat the jar. So, for this competition, I’m going to take recipe advice from a master: it’s Indian food expert Madhur Jaffrey’s recipe for rogan josh versus a jar of rogan josh sauce from a supermarket.

Spicing things up

The finickiness of spices is another off-putting component of curry sauce construction for a newbie. You can’t always just buy all the spices listed in a recipe dried and pre-ground, chuck them in with everything else at the same time in the slow cooker and return to a delicious meal eight hours later. I’ve tried. But why not? The simplest answer is: because chemistry says so. To form, and retain, the right flavour molecules in the end product, everything needs to be just so during the cooking process.

Let’s start with dried herbs and spices – can we use them as a substitute for fresh? ‘It depends on the spice: some of them definitely work better than others,’ answers Joanna Buckley, a chemist from the University of Sheffield, UK. During drying, flavour molecules can evaporate or undergo chemical transformations. ‘Oregano dries really well because its particular flavour molecules aren’t degraded,’ says Buckley. But she is no fan of dried chives.

It’s also important to note that dried and fresh spices ‘are not always one-on-one substitutes’, adds Nik Sharma, a food writer and lapsed molecular biologist in Oakland, US. ‘The chemical that gives ginger its heat undergoes a chemical transition during drying, and that new compound is two times hotter than the one that’s present in fresh ginger,’ he says. Gingerols in the root of the ginger plant are responsible for fresh ginger’s pungency or heat. These undergo dehydration reactions when the root is dried, to form the twice-as-spicy shogaol compounds.

And when using dried spices, is it worth going to the effort of grinding your own? ‘Yes,’ answers Nazira Karodia, organic chemist at the University of Wolverhampton, UK. The smaller, more volatile flavour molecules start evaporating as soon as you crush the seeds, fruit, bark or roots, she explains. ‘You smell the aroma coming out of the mortar and pestle as you’re grinding them.’ The longer a ground spice is stored, the more its chemical composition will change. ‘That’s why grinding a small amount and using it within a short space of time gives you the full punch flavour of the spice,’ Karodia adds.

Before grinding, recipes often request dry toasting whole spices briefly in a pan. ‘Toasting releases the moisture from the spices’ and can change their flavour molecule composition, explains Sharma. When ginger is heated, the gingerols undergo a retro-aldol reaction to produce zingerone (a sweeter and less pungent flavour molecule characteristic of gingerbread). So to toast or not to toast is a decision to be made depending on the desired flavour.

Flavour molecule evaporation is also why most recipes recommend adding spice mix garam masala towards the end of the cooking process rather than near the start with most of the other spices.

If you can’t stand the heat…

A curry sauce recipe often starts with frying whole or (freshly) ground spices briefly in oil. This solubilises and extracts flavour molecules from the spices into the solvent. ‘Flavour molecules are organic molecules, which are either insoluble or sparingly soluble in water, but they are soluble in oils,’ says Karodia. ‘The flavour is being extracted from the dry spices so it can be evenly released into the curry [sauce].’ Facilitating flavour molecule extraction is also why it is recommended to bash cardamom pods before use, if you are going to use them whole. It exposes the flavoursome seeds inside.

Capsaicin – the flavourless heat molecule in the chilli fruit – also extracts well into oil. ‘If you sink a chilli into warm oil, it will give you a much more intense result than it would with water,’ explains Sharma. Its long hydrocarbon chain renders capsaicin fairly insoluble in water.

Controlling the spice level of curry sauces, especially when using fresh chillies, is a skill that I’m still mastering. My curries swing from too hot to very mild, rarely hitting the hallowed middle ground. The Scoville scale gives us an indication of how hot a fresh chilli should be. Developed by US pharmacist Wilbur Scoville in 1912, the scale ranks chilli varieties by their Scoville Heat Unit (SHU). Regular bell peppers or capsicum, for example, have a 0 SHU, while the variety currently holding the ‘hottest chilli pepper’ Guinness World Record, the Carolina Reaper, has an average of 1,641,183 SHU.

SHU values are, traditionally, determined using a tasting panel. Members taste chilli pepper extract watered down with sugar water, to determine how many equal parts of water need to be added before the heat is no longer detectable. To no longer taste the heat of a jalapeño pepper with an 8000 SHU, for example, one millilitre of its extract would need to be diluted in around eight litres of sugar water. Today, high‐performance liquid chromatography (HPLC) is normally used to measure concentrations of capsaicin and dihydrocapsaicin in the fruits. This is then converted into SHU. Other common laboratory techniques also suitable include spectrophotometry and nuclear magnetic resonance (NMR).

For home cooks using fresh chillies, the biggest problem with the Scoville scale is that it provides a range. Habanero chillies, for example, vary from 100,000 to 350,000 SHU. And, contrary to the popular myth, a chilli’s size is not indicative of how much heat it packs. If you don’t want to run a tasting panel, or send samples off to a contract lab for analysis, prior to cooking with every fresh chilli, is it possible to figure out exactly how much heat it packs? Yes, is the – perhaps surprising – answer.

In 2007, Richard Compton’s group at the University of Oxford, UK, developed an electrochemical method for this purpose, which has since been commercialised as a handheld Scoville meter. It uses adsorptive stripping voltammetry with electrodes coated in multiwalled carbon nanotubes and converts the resulting current into an SHU. The ChilliPot is sold online by Zimmer & Peacock, an electrochemical sensor manufacturer. It gives a reading in less than a minute, although you do have to factor in the time taken to dissolve the chilli in a buffer solution first. A bigger issue for an amateur curry chef, however, is the price: €5395 (£4740). I’m going to have to learn to live with uncertainly in the context of my Indian dish spiciness.

Crying out for a good curry

Chillies – fresh, dried or both – are key ingredients in most curry sauces. So too are allium family members garlic and onions. The main flavour molecules in both start out as odourless S -alk(en)yl cysteine sulfoxides in the whole, raw vegetables. When chopped, the cell wall damage triggers their enzymatic transformation into the corresponding sulfenic acids. These then undergo rapid condensation and rearrangement reactions to form a variety of sulfur-containing organic molecules. Further reactions also occur during cooking.

I have a pair of old safety specs and chop onions under an extractor fan

In onions, one of the molecules formed during the chopping-initiated reaction cascade is the volatile lachrymator syn-propanethial-S -oxide. When this comes into contact with eyes it causes irritation and tear production. ‘The answer to that is to use a very sharp knife and to cut very, very quickly,’ recommends Karodia. Buckley, meanwhile, favours the use of laboratory safety equipment. ‘I have a pair of old safety specs and chop onions under an extractor fan,’ she says. ‘You can tell that it’s fairly potent stuff because over time a thin film-like coating will form on your safety specs that will eventually go crazed.’

Ghee – a type of clarified butter – is another tool Indian cuisine often utilises to pack in extra flavour. ‘Butter is heated until the water comes out leaving behind the fat, milk solids and lactose,’ explains Sharma. During this process, ‘the amino acids from the proteins react with the sugars in milk and undergo the Maillard reaction to produce different flavour molecules that add a nuttiness to the aroma of ghee’, he adds. ‘Everything is then filtered to remove the milk solids, leaving fat with the flavour molecules created by these chemical reactions.’ The decision of whether to use ghee, butter or oil in a recipe comes down to the desired flavour for the curry sauce.

I ask Sharma if he has any other tips to help me up my curry game. ‘I often only add salt towards the end [after tasting], once I’m confident that all the saltiness has been brought out of the other ingredients,’ he says. He also advises not to use too much oil, or so much chilli heat ‘that you can’t taste anything else’.

Sharma, unlike me, also uses his previous scientific training (he left the lab around five years ago to pursue his cookery career) in his note-taking during cooking. ‘I use flow charts for recipes because I find it easier to write it down that way and even use the symbols that I used to use in the lab like the triangle for heating and RT for room temperature,’ he explains. We do, however, both reject the cup system prevalent in US recipes for ‘measuring’ out solid food stuffs. ‘I still can’t understand why people use cups for solid chocolate or chopped nuts. There are interstitial spaces between all those things!’ Sharma protests.

Awesome accompaniments

While the game here is Nina v the jar, I decide to seek advice on using science to improve my curry accompaniments too. Indian cuisine in the UK is often served with multiple side dishes such as rice, naan bread, poppadoms, chutney and raita. I confess to still buying many of these pre-made, but how can I improve the fluffiness of my boiled basmati rice?

Soak rice before cooking in cold tap water for 30 minutes, answers Sharma. ‘Soaking is important because the water changes the cell structure,’ he adds. ‘If you don’t soak them, there is a significant chemical difference.’ Rice should also be rinsed after soaking. ‘The grains rub against each other [in the packet], and a lot of starches are released from friction. Especially with basmati rice, the goal is to have grains that don’t stick to each other,’ says Sharma.

Making curries is all about building layer upon layer of flavour

And, what should I be drinking with a curry? ‘Not water,’ answers Karodia. Buckley explains: ‘On your tongue and on the inside of your mouth you’ve got a set of vanilloid receptors. At high temperatures these allow the flow of calcium ions from one part of the cell into the other part and that’s what triggers a response. Your brain recognises that you’re eating something hot. Capsaicin is a vanilloid and it binds to those particular receptors fooling them into firing a signal to your brain to tell you that you’re eating something hot.’ Capsaicin binds strongly to the receptors and to stop feeling the burn, you need something to wash it off. Water isn’t up to the job, because – as mentioned earlier – capsaicin is sparingly water soluble. ‘You need something with fat to get rid of it from the receptor on your tongue,’ Buckley says. Milk and the yoghurt-based drink lassi are, therefore, the optimum choice to accompany an overly hot curry. It also explains why adding cream or yoghurt to a hot curry sauce helps cool the burn.

Capsicin does also sparingly dissolve in alcohol. But you need a stronger concentration that the beers the majority of us order in the local curry house. Spirits would work best, but ‘something that’s about 13%, like a glass of wine, is a sensible halfway house’, advises Buckley.

Armed with my chemistry knowledge – and safety specs – I head to the kitchen to unearth both my pestle and mortar and electric spice grinder. I start by making my own garam masala, using the recipe in Dan Toombs’ book The Curry Guy. I then follow Madhur Jaffrey’s rogan josh sauce recipe to the letter, apart from using ghee instead of vegetable oil . Flavourwise, I have an easy win – almost confusingly so. ‘The jar sauce is so bland,’ exclaims my husband. Both of us wondered if we had misremembered the taste of mass-produced curry sauces.

I am, however, reminded of a significant plus for the jar: time-saving. It took less than an hour from chopping board to table versus over three hours to cook the ‘same’ curry from scratch. And there was significantly less washing up! But as Toombs said in his book: ‘Making delicious [curries] is all about building layer upon layer of mouthwatering flavour.’ His premise is there is no short-cut available to awesome tasting Indian cuisine, and during this little experiment I have certainly proven him right.

Nina Notman is a science writer based in Salisbury, UK