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Linda Harris Tim Hearden
Linda Harris, a food science and technology specialist at the University of California, Davis, discussed food safety at the Almond Conference in Sacramento in December.

Off-ground almond harvests won't stop microbes

UC expert says contamination of raw nuts could still occur in the trees

Using catch basins and dryers to harvest almonds may cut down on orchard dust, but it won’t stop salmonella or other contaminants from potentially showing up on raw nuts, an expert advises.

That’s because nuts still in the tree can be contaminated by birds and other wildlife, said Linda Harris, a food science and technology specialist at the University of California, Davis.

“If you think salmonella can’t be on nuts shaken onto a catch frame, that’s not the case,” Harris told session attendees at the Almond Conference in Sacramento in December.

During harvest, almonds are typically shaken from trees and allowed to dry on the ground before being collected and processed. To cut down on air pollution and contamination of nuts from the soil, some in the industry are considering shaking the nuts onto basins and sending them to dryers instead, as is done with walnuts and pistachios.

However, dehydrators alone don’t use high enough temperatures to kill harmful bacteria, said Harris, whose research focuses on microbial food safety. Dehydration doesn’t kill the bacteria in walnuts, she said.


Harris’ remarks about off-ground harvesting came as she discussed what the future may look like as growers and handlers settle into a new norm under the federal Food Safety Modernization Act.

She also highlighted the history of almond food safety programs and chronicled how recent technological advancements have aided the prevention and early detection of pathogens in nuts.

Historically, low-moisture foods such as almonds weren’t thought to be a source of salmonella, which can cause diarrhea, fever and abdominal cramps in adults, and result in serious harm to the immune-compromised, elderly and young children. However, outbreaks involving almonds in 2001 and 2004 showed this assumption to be false.

To respond to consumer concerns, the industry worked with the USDA and others to establish a groundbreaking mandatory pasteurization program in 2007 that has supported an enviable industrywide food safety record over the last decade. Pasteurization involves using high heat to kill microbes in food and beverages.

Under the program, it is illegal to manufacture and sell unpasteurized almonds in the U.S. unless they are sold directly to consumers or shipped outside North America. Since the program was initiated, there have been no foodborne illness outbreaks related to California almonds.


At the time of the outbreaks, consumer trends favored raw almonds rather than roasted; out-of-hand consumption; and volume, Harris said. Since then, consumers have trended toward more value-added products such as almond milk and nut butter.

For those who still enjoy almonds as a standalone snack, there’s no shortage of online advice for keeping it free of bacteria – much of it in error, Harris noted.

For instance, YouTube videos describe how to soak almonds in water, but tests have shown that after about 8 hours, when the nuts reach about 40 percent moisture, “salmonella is quite happy,” she said.

Other pathogens are just as happy, she said.

“One thing you’re getting if you’re soaking your almonds in water is awhole lot of bacteria,” she said.


While salmonella does not grow in low-moisture foods, the organism can survive for long periods of time. In raw almonds, the same salmonella did not decline over 1.5 years of refrigerated or frozen storage, and declined only slowly when nuts were held at room temperature, according to a 2011 UC study.

Overwhelmingly, consumers buy almonds (97 percent), pecans (78 percent), and walnuts (81 percent) that are already cracked out of the shell. The most common practice is to store nuts at room temperature, according to the study.

While two weeks to four weeks is the most common storage time for consumers, 11 percent or more of consumers indicated in the 2011 UC study that they store nuts at room temperature for two months to six months. Those who store nuts for longer typically keep them in the freezer, the study found.

Room temperature storage encourages insect growth and causes nut oils to become rancid quicker, the UC observes in an advisory for consumers. Nuts retain quality for a year or more at refrigerator temperature or up to 2 years in the freezer, according to the university.

While the UC cautions that refrigeration and freezing do not destroy harmful bacteria, temperatures as cool as 55 to 60 degrees can prevent growth of organisms over a short period, Harris said.

She encourages the industry to help consumers understand how what they’re doing could affect almonds.

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