TL;DR: Robusta has spent two decades being treated as the cheap bean you settle for. It carries twice the caffeine of arabica, more body and more bitterness, and the sweet-coffee traditions picked it on purpose. It is not a compromise. It is the correct tool for this particular drink.
The actual difference
Two species do nearly all the world's coffee. Arabica is the specialty industry's bean: grown high, fussy, disease-prone, lower in caffeine at 1.2 to 1.5 percent by dry weight, and prized for acidity and aromatic complexity. Robusta grows low, survives heat and disease, yields more, and carries 2.2 to 2.7 percent caffeine — roughly double — with heavier body and a firm bitterness.
Vietnam is the world's largest robusta producer, growing something like 70 percent of the global supply, and the second largest coffee producer overall. More than 90 percent of what it grows is robusta.
Why sweet coffee chose it
Put arabica behind sweetened condensed milk and the thing you paid for — the florals, the acidity, the delicate stone-fruit business — vanishes. You have made an expensive milkshake.
Robusta survives the milk. Its bitterness holds a line against the sugar, and its body keeps the drink feeling like coffee rather than like dessert. Vietnamese cà phê sữa đá, Thai oliang, southern Indian filter coffee — sweet, milky, tropical coffee traditions, and robusta or robusta-heavy blends across the board. That is not a coincidence and it is not poverty. It is a correct engineering decision, made repeatedly, by people who drink a lot of coffee.
The snobbery, briefly
Robusta's reputation was earned in the era when it mostly turned up as filler in cheap supermarket blends and bad instant coffee. Badly grown, badly processed, badly roasted robusta is genuinely unpleasant, and there was a lot of it.
The error is concluding that the species is the problem rather than the treatment. Third-wave coffee then built a whole value system around the bean it happened to prefer, and everything else became a punchline.
Where we stand
We are not going to pretend our coffee is a rare micro-lot. It is Vietnamese coffee powder, robusta-led, blended for a specific job: to taste like coffee through milk and sugar, cold, at 3pm.
If you want to taste the farm, buy specialty arabica from one of the good roasters here and enjoy it properly. If you want a cold sweet coffee that still tastes like coffee after the milk goes in, robusta was always the answer, and the ratio is the rest of it.
Questions people actually ask
Is robusta worse than arabica?
It is different, and it has been unfairly dismissed. Robusta carries roughly double the caffeine of arabica — 2.2 to 2.7 percent by dry weight against 1.2 to 1.5 — with more bitterness and more body. Those are faults in a delicate filter coffee and virtues in a sweet milk coffee, which is why the tropical sweet-coffee traditions chose it.
Why is robusta cheaper than arabica?
It grows at lower altitude, resists disease and heat far better, and yields more per tree. It is easier to farm, so it costs less. Cheaper to produce is not the same as worse to drink, though the two get conflated constantly.
Does Vietnamese coffee use robusta?
Overwhelmingly. More than 90 percent of Vietnam's coffee production is robusta, and Vietnam grows roughly 70 percent of the world's supply of it. Vietnamese arabica exists, mostly around Da Lat, but robusta is the national crop and the backbone of the traditional drink.
Why does specialty coffee avoid robusta?
Because specialty coffee is built around tasting a bean without anything hiding it, and by that standard robusta is blunt. It is a reasonable judgement inside that frame. It stops being reasonable when it gets applied to drinks that were designed around sweetened milk in the first place.
Ten bottles, $39 · Why ours tastes the way it does
Qwang Coffee, Singapore.