Sounds Like a New Way to Make Spinach Safer
Joan B. Rose, PhD

spinach

Spinach leaves; photo from University of Massachusetts, Amherst website

“Eat your greens,” mom always said. And she was right. Fresh spinach, a powerhouse of nutrition, is a good example.  According to the How Stuff Works website, spinach provides twice as much fiber as other greens.  Raw spinach contains: beta-carotene, an anti-oxidant that can help reduce the risk of developing cataracts; lutein, a phytochemical that helps prevent age-related macular degeneration; and lipoic acid, which helps regenerate antioxidant vitamins C and E and may help regulate blood sugar levels.

Fresh spinach is a delicious and healthful addition to any salad, but if I haven’t washed the leaves myself I am sometimes wary of eating this vegetable.  As a microbiologist, I know that contaminated spinach has been the cause of serious foodborne outbreaks in recent years.  Some have involved the O157:H7 strain of E. coli, which produces a toxin that destroys red blood cells, causing the kidneys to fail (this disorder is called hemolytic uremic syndrome).  So, I read with interest reports on a new strategy to reduce the number of E. coli O157:H7 cells that may live undetected on spinach leaves. 

Combining Ultrasound + Chlorine: A 1-2 Punch

Log Reduction Explained

When scientists discuss how well disinfectants destroy germs, they use the term “log reduction.” This is a mathematical concept based on logarithms, which are short-cuts for denoting large numbers. For example, 100 = 10 X 10 or 102. The logarithm is “2”, the power to which 10 must be raised or the number of times it must be multiplied by itself to arrive at 100. If a particular disinfectant could achieve 100 times fewer germs on a contaminated surface, it would be capable of a “2 log reduction.” The researchers at the University of Illinois report between 99.99 and 99.9999% reduction in bacterial levels. This means their disinfection method reduced germs by between 10,000 and 1,000,000 times the original level, or a ”4 – 6 log reduction.”

Scientists at the University of Illinois recently reported dramatic reductions—over 99.99%–in the number of E. coli O157:H7 cells on spinach leaves when they subjected contaminated spinach to a combination of continuous ultrasound treatment and chlorine washing (abstract).  The researcher team, according to Dr. Hao Feng, is responding to the USDA’s quest for technologies to achieve reductions in bacterial levels ranging from 99.99% to 99.9999%. Current food processing methods involving chlorine wash alone reduce bacteria by only 90%, so the research news is very encouraging.

Ultrasound waves are sound waves of higher frequency than those that humans can hear. As ultrasound waves pulse through spinach-filled basins of chlorinated wash water, pressure differences are created in a process known as cavitation. It is this process that can help dislodge bacteria from spinach leaves making it vulnerable to destruction. The researchers note, however, that leaf blockage of other leaves can cause an uneven distribution of ultrasound waves and uneven disinfection results. Another challenge is adjusting ultrasound to levels that are not destructive to the spinach leaves. Crank up the ultrasound too high and leaves could develop watery spots and rot.

There may be more work to do before this novel produce disinfection technique is adopted, but kudos to Dr. Hao Feng and his research team for making huge strides toward the goal of making healthful food microbiologically safer.

Joan B. Rose, PhD, is the Homer Nowlin Chair in Water Research at Michigan State University and a member of the Water Quality and Health Council.

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