* Indonesian beans sold at $10 premiums
* Vietnam beans traded at $10-$15 discounts
By Lewa Pardomuan
SINGAPORE, June 26 (Reuters) - Roasters purchased several cargoes of robustas in a series of deals this week, with Indonesian beans traded at premiums to London futures due to uncertainties about the crop, dealers said on Thursday.
The current harvest in the world's third-largest coffee producer after Brazil and Vietnam was expected to reach its peak this month, but exporters were confronted with erratic daily arrivals from plantations to key export ports.
Indonesia competes with neighbouring Vietnam in the robusta market. The bitter-tasting robusta beans are either blended with higher-quality arabica for a lower-cost brewed coffee or processed into instant coffee.
Sumatran grade 4, 80 defect beans were traded at premiums of $10 a tonne to London futures, with offers unchanged from last week at $30 a tonne. Dealers dismissed talk that sellers in Indonesia had begun offering beans at prices below London.
"It's hard to get beans actually. I don't think farmers are holding back beans because they are happy with the current prices. They also need cash before the fasting month," said a dealer in Sumatra.
"Daily arrivals hit 5,000 to 6,000 tonnes on Monday. But from Tuesday to today, they reached between 2,500 and 3,000 tonnes each day. I think buyers are hoping that Indonesia will sell beans at discounts, but it's not happening."
Indonesia's coffee output is likely to plunge to a three-year low in 2014/15 due to unfriendly crop weather, while higher domestic consumption will soak up about half the produce, a Reuters survey showed, curbing sales by the key exporter.
In late June 2013, Indonesians beans fetched hefty premiums of $200, while Vietnamese robustas were offered at $90 above London after excessive rains disrupted deliveries from plantations and falling global prices prompted farmers to hold back their beans.
This year, Vietnamese beans were mostly offered at discounts to futures as the country is estimated to harvest another record crop of more than 29 million 60-kg bags in the next crop year starting in October 2014.
Grade 2, 5-percent broken beans were traded $10 to $15 below London futures, within sight of last week's offer prices of zero to discounts of $20.
"Trade houses are buying Vietnamese beans, and I think you can get the best offer at $20 discounts," said a dealer in Singapore, who trades Indonesian and Vietnamese beans.
"Indonesian differentials can't come down because local roasters are buying and trading houses are willing to pay up."
Dealers expect differentials in Vietnam and Indonesia to hold steady next week as demand offsets pressure from gains in London futures. London robustas have gained more than 19 percent this year, tracking rallies in New York arabicas after a drought in Brazil triggered supply concerns (Editing by Sunil Nair)