Double Processing for Better Cold-Pressed Grape Juice

Concord grapes are an important crop for New York state. Most of the crop is used for juice concentrate and jelly. There’s a state initiative to find more uses for Concord grapes, and Cornell University researchers thought that cold-pressed grape juice could be a good value-added product.

“Concord grapes come once a year, and normally the juice is concentrated thermally,” explains Olga Padilla-Zakour, professor of food processing at Cornell’s Department of Food Science. Before it’s sold, it has to be diluted and pasteurized again, requiring several heat treatments and the addition of an enzyme, she says, to break down the pectins and get the most color and nutrients out of the skins. “Even though they have been optimized, every time you heat treat your product, you lose a little bit of the nutrients and the color.”

The researchers wanted to maintain the tart taste and deep purple color for which Concord grapes are known, and cold-pressing is the process that comes closest to producing a product with the taste of freshly squeezed juices. But the process of cold-pressing has its drawbacks. It produces less juice, the juice’s color can degrade, and the product’s shelf life is short compared with other processes, like hot-pressing.

So why not combine two processes to get the benefits of both, Padilla-Zakour and doctoral student Yuanyuan Li thought. Their recently published study details the results of their work using both high pressure processing (HPP) and pulsed electric field (PEF) techniques on Concord grape juice. HPP makes the juice more shelf stable, and PEF can get more juice from the fruit, ensure a good color, and maintain nutrients.

Fresh Flavor and Color

The researchers made grape juice using five different approaches—PEF, HPP, HPP and PEF, heat treatment, and a control that wasn’t treated at all. First, they harvested the grapes locally and put them through a crusher. The resulting mash was pressed, apart from a portion that underwent PEF before pressing, followed by 24 hours of refrigeration. Then the juice was further processed using the various treatments. The juices were refrigerated for 5 months and assessed each month for quality, color, and chemical content. The combination of PEF and HPP was a clear winner.

“The combination of those two processes and the cold chain maintained the quality very well,” says Padilla-Zakour.

Even though the naturally occurring enzymes were still active—they’re only partially inactivated by the HPP and PEF combination—the color remained a vibrant purple. Skipping the enzyme treatment leaves the pectin and also the nutritional elements, like soluble fiber, making for a healthier product. “Without additives, we were able to get a good yield and a good extraction of the important components, the anthocyanins, the polyphenols, and maintain that Concord fresh flavor that is so typical,” she says.

They also ran a sensory evaluation with 101 untrained panelists. Their verdict? Thumbs up. Padilla-Zakour’s own taste assessment was also positive. “I thought it was a really good product,” she says. “I liked the mouthfeel, too, because leaving the pectin behind allows you to have a little bit more viscosity in the product.”

Padilla-Zakour is now researching other applications for this process, although she cautions that PEF isn’t ideal for all fruits. She tried apples, for example, and they browned too much. Better contenders would be fruits that have all the color in their skins, like cherries and blueberries, she says. The process may also be useful in winemaking.

PEF technology is already used for food processing, explains Padilla-Zakour, as with potatoes destined to be French fries. They’re treated to reduce fry breakage and so they’ll absorb less oil during frying. While this study was a pilot project, she sees the potential for further commercial applications.

“I believe that the PEF units are out there that can be used,” she says. “It was just a matter of providing the proof of concept that this can be done at scale to have this premium product.”ft