It's called the "Mad Men" effect: the rediscovery of classic cocktails like the Old Fashioned and the gin martini.

But those cocktails had one key ingredient that's not as well known today called bitters — alcoholic ingredients flavoured with herbal essences that are added to cocktails in very small quantities (think drops). Bitters like the yellow-capped Angostura are still recognizable today, and can often be found in supermarkets. But there are now a variety of longstanding classic brands, reinvented makers who'd gone quiet after Prohibition, and upstarts who are putting a new twist on an old ingredient.

Author Brad Thomas Parsons has long been a food and cocktail enthusiast, so it's natural that his curiosity was piqued when he started noticing homemade bitters and bottles he didn't recognize behind the bar beside the expected Angostura and Peychaud's. After some investigation, he became fascinated and published a book about the topic a few years ago. The popularity of bitters has only increased since.

"It seems trendy, it seems in the moment," Parsons says. "but it's an essential part of a well crafted cocktail, from the history of it to what's in the glass."

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  • Angostura Aromatic Bitters

    <a href="http://www.angosturabitters.com/" target="_blank">Angostura is probably the most recognizable bitters brand</a> thanks to its yellow cap and over-sized wrapper. (The story is that the label is the result of either an ordering mistake or the using the wrong label, but it's been that way for a century.) These aromatic bitters — with flavours like cardamom, nutmeg, and cinnamon — are a key ingredient in <a href="http://www.foodnetwork.com/recipes/ted-allen/classic-manhattan-cocktail-recipe/index.html" target="_blank">classic cocktails like the Manhattan</a>. "It's very versatile," Parsons says. "You can use it in so many drinks."

  • Peychaud's Bitters

    <a href="https://www.facebook.com/PeychaudsBitters" target="_blank">These bitters are named after Mr. Antoine Amedie Peychaud,</a> a pharmacist and Creole immigrant from what is now known as Haiti. Peychaud began dispensing curative bitters with anise notes out of his pharmacy in the French Quarter of New Orleans in 1838. Peychaud dispensed his bitters in Cognac and fans began to ask for them by name at bars throughout the city. The iconic New Orleans cocktail, the <a href="http://www.sazerac.com/cocktail.aspx" target="_blank">Sazerac,</a> uses these red-coloured bitters as a key ingredient.

  • Orange Bitters

    Round out your collection of essential bitters with a bottle of orange bitters. This is an area where you can try a few different types of bitters, based on the flavours you like best. <a href="http://www.angosturabitters.com/OrangeBitters" target="_blank">Angostura Orange Bitters,</a> the only new bitters ever introduced in the company's 180-year history, has a straightforward orange flavour. <a href="http://byobto.com/collections/bitters/products/regans-orange-bitters-no-6-296ml" target="_blank">Regans' Orange Bitters No. 6,</a> another classic choice, is spicy with notes of cardamom, great for stronger spirits like scotch. Many other bitters companies offer an orange bitters, so you can have fun with this one and try a few different kinds.

  • Grapefruit And Other Citrus Bitters

    Both Parsons and Voisey recommended choosing grapefruit bitters once you want to expand your collection. "It's a fun twist on the citrus," Parsons says. Vancouver company <a href="http://www.bitteredsling.com/bitters/grapefruit-hops/" target="_blank">Bittered Sling </a> makes a grapefruit and hops bitters, as does <a href="http://bittermens.com/products/hopped-grapefruit-bitters/" target="_blank">Bittermen's</a>, which recommends using it with tequila drinks. You can also try to <a href="http://www.chow.com/recipes/11285-grapefruit-bitters" target="_blank">make your own recipe.</a> Other options for citrusy bitters include <a href="http://scrappysbitters.com/lime/" target="_blank">Scrappy's Bitters</a> in lime and <a href="(http://www.feebrothers.com/products/bitters/lemon_bitters.php" target="_blank">Fee Brothers in lemon.</a>

  • Spicy Bitters

    Add a kick to your drinks with bitters that recall spicy cuisines like Mexican and Thai. Parsons recommends <a href="http://bittermens.com/products/xocolatl-mole-bitters/" target="_blank">Bittermens Xocolatl mole bitters,</a> which contains cacao, cinnamon, and spices and is recommended for use with aged liquors. <a href="http://byobto.com/collections/bitters/products/bad-dog-fire-and-damnation" target="_blank">Bad Dog's Fire and Damnation</a> bitters contains a hint of smoke and capsaicin spice. <a href="http://byobto.com/collections/bitters/products/bitter-end-jamaican-jerk-bitters" target="_blank">Jerk bitters from Bitter End</a> add considerable heat to a cocktail and black pepper bitters from <a href="http://www.victoriaspirits.com/bitters/" target="_blank">Twisted and Bitter</a> is great in a Caesar.

  • Fruit Bitters

    You don't have to stop with citrus flavours — there are fruity bitters options for every taste. Cherry bitters, like the one by <a href="http://www.feebrothers.com/products/bitters/cherry_bitters.php" target="_blank">Fee Brothers,</a> is nice in bourbon or adds a fun hint of cherry cola to a rum and coke, while <a href="http://byobto.com/collections/bitters/products/plum-and-rootbeer" target="_blank">Bittered Sling's</a> plum and root beer bitters create a different carbonated beverage. For an unexpected flavour, try <a href="http://byobto.com/collections/bitters/products/bar-keep-apple-bitters" target="_blank">Bar Keep's apple bitters.</a>

  • Celery Bitters

    This is one to add to your collection if you're a fan of <a href="http://www.canadianliving.com/food/motts_clamato_classic_caesar.php" target="_blank">Caesars</a> or <a href="http://allrecipes.com/recipe/classic-bloody-mary/" target="_blank">Bloody Marys</a> — it's a great compliment to the celery salt used in both drinks. They're also an excellent addition to a gin and tonic. <a href="http://cocktailquest.blogspot.ca/2011/09/underused-cocktail-ingredients-celery.html" target="_blank">Celery bitters</a> were reintroduced by <a href="http://the-bitter-truth.com/bitter/original-celery-bitters/" target="_blank">The Bitter Truth,</a> but you can also try other varieties like the ones from <a href="http://scrappysbitters.com/celery/" target="_blank">Scrappy's Bitters</a> or <a href="http://www.feebrothers.com/products/bitters/celery_bitters.php" target="_blank">Fee Brothers.</a>

  • Make Your Own

    If you're really ambitious, or looking for a flavour you can't find in stores, you can try your hand at making your own bitters. Parsons has several recipes for homemade bitters in his book, <em>Bitters</em>, and online search will bring up many more.

  • Cook With Bitters

    You don't have to stick to just cocktails — bitters can add some depth and flavour to your cooking as well. This is a great way to use bitters that recall a particular cuisine, like Chinese bitters by <a href="http://www.greenbar.biz/BAR-KEEP-Organic-Chinese-Bitters" target="_blank">Bar Keep</a> or Thai bitters by<a href="http://www.bitterendbitters.com/proddetail.php?prod=ThaiBitters" target="_blank"> Bitter End,</a> or to highlight a particular taste or ingredient with choices like Cardamom bitters by <a href="http://scrappysbitters.com/cardamon/" target="_blank">Scrappy's Bitters.</a>

  • Explore The History

    Bitters originated as a <a href="http://www.westonaprice.org/basics/bitters-the-revival-of-a-forgotten-flavor" target="_blank">medicinal ingredient and they do have health benefits</a> — a bottle is not the the cure-all it was sold as a century or so ago, but bitters are often used as digestive aids. You also don't have to drink them with alcohol — Voisey says pregnant women also enjoy a dash of bitters in tonic water or club soda.

Before Prohibition, bitters were an essential ingredient for any cocktail — key to the name itself, actually. But the U.S. government's crackdown on alcohol in the early part of the 20th century, and the passing of the Volstead Act in 1919, dealt bitters a huge blow: other than Angostura, Peychaud's, and a few orange bitters, they've all disappeared.

As people began rediscovering classic cocktail recipes, the reemergence of bitters followed. Kristen Voisey noticed the trend while she was living in Los Angeles, and brought it back to Canada when she opened her Toronto store BYOB, which specializes in vintage barwear and carries a wide selection of bitters. She now stocks more than 100 varieties and says she discovers a new brand nearly each week.

With all those options, bitters can be overwhelming, but they're definitely worth experimenting with. Both Parsons and Voisey suggest starting with the classics and branching out from there based on your own tastes and favourite drinks.

"It's sort of like building a liquor collection," Parsons says. "You don't run out and buy every bottle of gin."

Here are a 10 things to know about bitters—soon you'll be crafting a perfect Angostura Fizz with the best mixologists.

Angostura Aromatic Bitters
Angostura is probably the most recognizable bitters brand thanks to its yellow cap and over-sized wrapper. (The story is that the label is the result of either an ordering mistake or the using the wrong label, but it's been that way for a century.) These aromatic bitters — with flavours like cardamom, nutmeg, and cinnamon — are a key ingredient in classic cocktails like the Manhattan. "It's very versatile," Parsons says. "You can use it in so many drinks."

Peychaud's Bitters
These bitters are named after Mr. Antoine Amedie Peychaud, a pharmacist and Creole immigrant from what is now known as Haiti. Peychaud began dispensing curative bitters with anise notes out of his pharmacy in the French Quarter of New Orleans in 1838. Peychaud dispensed his bitters in Cognac and fans began to ask for them by name at bars throughout the city. The iconic New Orleans cocktail, the Sazerac, uses these red-coloured bitters as a key ingredient.

Orange Bitters
Round out your collection of essential bitters with a bottle of orange bitters. This is an area where you can try a few different types of bitters, based on the flavours you like best. Angostura Orange Bitters, the only new bitters ever introduced in the company's 180-year history, has a straightforward orange flavour. Regans' Orange Bitters No. 6, another classic choice, is spicy with notes of cardamom, great for stronger spirits like scotch. Many other bitters companies offer an orange bitters, so you can have fun with this one and try a few different kinds.

Grapefruit And Other Citrus Bitters
Both Parsons and Voisey recommended choosing grapefruit bitters once you want to expand your collection. "It's a fun twist on the citrus," Parsons says. Vancouver company Bittered Sling makes a grapefruit and hops bitters, as does Bittermen's, which recommends using it with tequila drinks. You can also try to make your own recipe.

Other options for citrusy bitters include Scrappy's Bitters in lime and Fee Brothers in lemon.

Spicy Bitters
Add a kick to your drinks with bitters that recall spicy cuisines like Mexican and Thai. Parsons recommends Bittermens Xocolatl mole bitters, which contains cacao, cinnamon, and spices and is recommended for use with aged liquors. Bad Dog's Fire and Damnation bitters contains a hint of smoke and capsaicin spice. Jerk bitters from Bitter End add considerable heat to a cocktail and black pepper bitters from Twisted and Bitter is great in a Caesar.

Fruit Bitters
You don't have to stop with citrus flavours — there are fruity bitters options for every taste. Cherry bitters, like the one by Fee Brothers, is nice in bourbon or adds a fun hint of cherry cola to a rum and coke, while Bittered Sling's plum and root beer bitters create a different carbonated beverage. For an unexpected flavour, try Bar Keep's apple bitters.

Celery Bitters
This is one to add to your collection if you're a fan of Caesars or Bloody Marys — it's a great compliment to the celery salt used in both drinks. They're also an excellent addition to a gin and tonic. Celery bitters were reintroduced by The Bitter Truth, but you can also try other varieties like the ones from Scrappy's Bitters or Fee Brothers.

Make Your Own
If you're really ambitious, or looking for a flavour you can't find in stores, you can try your hand at making your own bitters. Parsons has several recipes for homemade bitters in his book, Bitters, and online search will bring up many more.

Cook With Bitters
You don't have to stick to just cocktails — bitters can add some depth and flavour to your cooking as well. This is a great way to use bitters that recall a particular cuisine, like Chinese bitters by Bar Keep or Thai bitters by Bitter End, or to highlight a particular taste or ingredient with choices like Cardamom bitters by Scrappy's Bitters.

Explore The History
Bitters originated as a medicinal ingredient and they do have health benefits — a bottle is not the the cure-all it was sold as a century or so ago, but bitters are often used as digestive aids. You also don't have to drink them with alcohol — Voisey says pregnant women also enjoy a dash of bitters in tonic water or club soda.

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