The Chemistry Advent Calendar 2023

After a few fallow years, this year I’m resurrecting the Chemistry Advent calendar! The theme for this year is “A Festive Food Journey Around the World”, with each graphic in the calendar focusing on a seasonal speciality from a particular country and some interesting chemistry behind it. Over the next 24 days, we’ll journey from fermented fish, via crunchy caterpillars, to Christmas puddings.

Follow along below – a new graphic and a link to download a PDF will be uploaded every day.

| Day 1 | Day 2 | Day 3 | Day 4 | Day 5 | Day 6 | Day 7 | Day 8 | Day 9 | Day 10 | Day 11 | Day 12 | Day 13 | Day 14 | Day 15 | Day 16 | Day 17 | Day 18 | Day 19 | Day 20 | Day 21 | Day 22 | Day 23 | Day 24 |

Day 1: Finland: Lanttulaatikko

Lanttulaatikko is a swede casserole served as a traditional Christmas side dish in Finland. Swedes are in the same cruciferous family of vegetables as brussels sprouts, and like sprouts they contain bitter glucosinolate compounds. Breakdown of glucosinolates during cooking produces isothiocyanate and nitrile compounds which contribute to swede flavour and aroma.

Day 2: Russia: Herring under a fur coat

Herring under a fur coat is the name given to a layered salad which includes pickled herring, grated boiled eggs, vegetables, onions and mayonnaise. A layer of grated beetroot gives the dish a vivid purple colour. The colour of beetroot is due to the pigment betanin. Betanin can sometimes pass through our digestive system intact and give our urine an alarming red appearance!

Day 3: Poland: Carp

Carp for Christmas dinner is widespread in Central and Eastern Europe. Freshwater fish flavour differs from that of saltwater fish and can sometimes taste earthy. This earthy flavour is due to the compound geosmin, produced by bacteria. Geosmin breaks down in acidic conditions so acidic ingredients can reduce the muddy flavour.

Day 4: South Africa: Deep-fried caterpillars

In South Africa, deep-fried emperor moth caterpillars, also known as mopane worms, are a Christmas Day delicacy. After harvesting, the caterpillars are squeezed to remove their innards, then singed to remove irritating hairs. The caterpillars are a better source of protein than many common meats such as beef, and are also rich in iron, zinc, and riboflavin (vitamin B2).

Day 5: Iran: Pomegranate and watermelon

Day 6: South Korea: Kimchi

Kimchi is a common Korean side dish all year round and is also present at the Christmas table. It consists of salted, seasoned and fermented vegetables, most commonly napa cabbage. Lactic acid bacteria from the raw ingredients are the dominant bacteria that ferment sugars and starches in the vegetables, producing lactic acid and other compounds. Some key flavour compounds are shown below.

Day 7: Australia: Prawns

Australians often grill prawns for Christmas.In uncooked crustaceans, the compound astaxanthin is bound to the protein crustacyanin. The negatively charged enolate ion this creates is blue in colour. The crustacyanin protein denatures when cooked. This releases the astaxanthin, producing a red-orange colour.

Day 8: Brazil: Farofa

Farofa is a Brazilian side dish also used as a stuffing for turkeys or chesters (chickens with more breast and thigh meat than a standard chicken). Farofa is made from cassava flour toasted with bacon, onion, garlic, and butter. Cassava roots contain cyanogenic glycosides, which are broken down by enzymes to produce poisonous cyanide. Soaking (or crushing) and sun-drying the roots removes almost all of these compounds.

Day 9: Hungary: Bejgli

Bejgli is a sweet roll, filled with poppy seeds, walnut paste or chestnut paste, commonly eaten at Christmas in Hungary. While the poppy seeds do not contain significant quantities of the opioid compounds such as morphine and codeine found in the seed pods, they can still contain trace amounts. Research has shown that these small quantities can be detected in urine up to 48 hours after consuming foods containing poppy seeds, and can cause opiate-positive urine test results.

Day 10: Mexico: Tamales

Tamales are made from a corn-based dough, called masa, wrapped around meats, beans and cheese and steamed inside a corn husk or banana leaf. Masa is produced by grinding nixtamalised corn. Nixtamalisation is the name given to the process of cooking and steeping corn kernels in an alkaline solution, commonly of calcium hydroxide. This degrades the pericarp, the outermost layer of the corn kernel, and softens the corn, making it easier to grind and also increasing its flavour and nutritional value.

Day 11: Ethiopia: Rooster doro wat

Doro wat is a spicy chicken stew commonly eaten in Ethiopia after periods of fasting, including at Christmas (which Ethiopians celebrate on 7 January). A key ingredient in the stew is berbere, an orange blend of various spices including besobela (Ethiopian holy basil), cardamom, coriander, fenugreek and ginger. A selection of key flavour compounds found in these spices, which contain a range of other compounds which also contribute to their flavours, are shown below.

Day 12: Japan: Kentucky Fried Chicken

A marketing ploy from the first KFC in Japan in the 1970s has led to millions of Japanese citizens celebrating Christmas with fried chicken today. Christmas isn't a public holiday in Japan, so a bucket of fried chicken represents a practical way of celebrating the occasion.  Thermal breakdown of the sulfur-containing amino acid cysteine during cooking produces some of the characteristic flavours of fried chicken.

Day 13: Iceland: Hákarl

Hákarl is fermented Greenland shark, a national dish of Iceland. It's eaten throughout the year but is traditionally eaten as part of the mid-winter festival þorrablót. Greenland shark is poisonous when fresh due to high levels of urea and trimethylamine oxide. The traditional process of fermentation for up to 12 weeks and drying for several months makes it edible. However, hákarl still contains high levels of ammonia which dominates the smell and taste of the fish.

Day 14: Colombia: Dulce de noche buena

Dulce de noche buena (literally 'Christmas Eve dessert') is a combination of different fruits in syrup. Typically, the fruits in syrup are papaya, figs, and lime, and they are served with cheese, buñeulos (fried dough fritters), hojuelas (crispy fried pastry) and almojábana (cheese bread). Key flavour compounds in papaya include linalool and benzaldehyde. Unusually for a fruit, papaya contains butyric acid, which also contributes to the aroma of parmesan cheese – and may also be why the fruit smells like vomit to some.

Day 15: Nicaragua: Tres leches cake

Tres leches cake is a popular dessert throughout Latin America and is thought to have originated in Nicaragua. The cake is a standard sponge cake soaked in a mix of whole, condensed and evaporated milks, then topped with whipped cream. Lactose is the main sugar, and is a disaccharide of galactose and glucose. Evaporated milk is milk with approximately 60% of the water removed, while condensed milk has 60% of water removed and sugar (sucrose) added.

Day 16: China: Tangyuan

Tangyuan are commonly eaten in China on the Dongzhi Festival (Winter Solstice), as well as at Chinese New Year. They are dumplings made from glutinous rice flour and stuffed with a variety of fillings. Glutinous rice is stickier than other types of rice due to its low amylose content and high amounts of amylopectin. The branched chains of amylopectin can gelatinise and are what makes glutinous rice sticky.

Day 17: Romania: Piftie

Piftie (or răcitură) is a pork jelly encasing boiled meat and mashed garlic. The jelly is made by boiling collagen-rich meat parts, such as pig trotters, along with carrots and other vegetables. This produces a broth, with boiling hydrolysing the insoluble collagen to produce soluble gelatin. As it cools, the gelatin forms a network which traps water, forming the jelly.

Day 18: Indonesia: Nastar

Nastar is a small pastry-based dessert filled with spiced pineapple jam. They are commonly eaten on holidays, including Christmas (Hari Natal). Esters, such as ethyl 2-methylbutanoate and allyl hexanoate, are key contributors to pineapple aroma and flavour. Other contributors include pineapple ketone (furaneol). Pineapples are low in pectin relative to some other fruits, so extra sugar or additional pectin is added to get the jam to set.

Day 19: United States of America: Green bean casserole

Green bean casserole is a commonly eaten side dish in the US at both Thanksgiving and Christmas. It consists of green beans, cream of mushroom soup and french fried onions. In raw green beans, aldehydes such as (Z)-3-hexenal contribute green, leafy flavours, but these decrease in concentration as the beans are cooked. For cooked beans, methoxypyrazines such as 3-isobutyl-2-methoxypyrazine (also known as bell pepper pyrazine) are important odorants. Other cooked bean flavour compounds include 1-octen-3-ol and 1-octen-3-one, which are also mushroom flavour components.

Day 20: Sweden: Lussekatter

Lussekatter are sweet buns flavoured with saffron. They are eaten in Sweden, Norway and Finland, particularly during advent and on Saint Lucia's Day (13 December). The saffron added to the buns contributes their yellow colour, due to the carotenoid pigment crocin, though commercial buns often have additional colour added due to the high cost of saffron. Saffron's fragrance and flavour primarily come from picocrocin and safranal.

Day 21: Greenland: Kiviak

Kiviak is a traditional Inuit delicacy eaten on special occasions, including Christmas. It's made by packing hundreds of little auks into a seal skin. The air is removed from the seal skin and it is sewn shut, sealed with seal fat, and buried under a heap of stones. This creates the perfect conditions for the auks to ferment for up to 18 months before they are removed and eaten. During fermentation, lactic acid bacteria ferment carbohydrates such as glucose, producing lactic acid and other organic acids. These help with preservation and also react with other compounds to produce further flavour molecules.

Day 22: Philippines: Puto bumbong

Puto bumbong is a steamed purple rice cake, topped with muscovado sugar and grated coconut, eaten in the Philippines at Christmas. Anthocyanin pigments (including cyanidin 3-glucoside and peonidin 3-glucoside) give the rice its purple colour. The pigments are found in the bran of the rice grains. The rice appears dark purple when uncooked, but lightens when cooked as the bran mixes with the white endosperm.

Day 23: Spain: Turrón

Turrón is a Spanish confection typically eaten at Christmas in Spain, Italy, and Latin America. The key ingredients are honey, sugar, egg white, and toasted almonds or other nuts, though it comes in varieties with various other ingredients. Nutty and toasty flavours come from pyrazines (such as 2-methylpyrazine) found in toasted nuts, as well as from furans (such as furfural) and pyrroles. Phenylacetaldehyde is also thought to contribute to the aroma of toasted almonds, as well as being an aroma compound found in honey.

Day 24: United Kingdom: Christmas pudding

Christmas pudding is made with dried fruits, spices, and suet, and steamed for several hours before being left to mature. It used to be custom to include small silver coins in the mixture to bring good luck and fortune for the coming year. This is not possible with modern coins, as their copper and nickel content can lead to reactions with the acids in Christmas puddings, discolouring the coins and giving the pudding a metallic taste.