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SunOpta, a seller of organic and specialty foods, jumped the most in seven years after its largest shareholder pushed the company's board to explore a sale.

Tourbillon Capital Partners, which owns a 9.9 percent stake in Ontario-based SunOpta, sent a letter to the board of directors urging them to "immediately engage an independent investment bank to advise on a value maximization process — including the execution of a sales process," according to a document filed with the Securities and Exchange Commission.

Tourbillon highlighted the company's poor share price in the last two years and believes that SunOpta has a "uniquely attractive business" in a rapidly growing field of sourcing hard-to-find organic and non-genetically modified ingredients, according to the letter.

SunOpta, which sells a range of products from non-genetically modified soybeans to Nature's Finest organic juice, rose as much as 23 percent in New York, the biggest intraday gain since March 2009. The shares traded 21 percent higher to $4.61 at 1:43 p.m. in New York.

"Despite its strengths, the company has been unable to translate its quality products and services into a thriving business with an attractive public market valuation," Tourbillon Capital's Chief Executive Officer Jason Karp said in the May 27 letter.

The company is reviewing the letter and will have a response at the appropriate time, SunOpta spokesman Rob Litt said in a telephone interview.

Nathaniel Garnick, an outside spokesman for Tourbillon, said the firm declined to comment beyond the letter.