Ultra-thin coating of biocompatible silk extends
room-temperature shelf life of delicate berries,
bananas. Half of the world's fruit and vegetable
crops are lost during the food supply chain, due
mostly to premature deterioration of these
perishable foods, according to the Food and
Ag9.riculture Organization (FAO) of the United
Nations.

Tufts University
biomedical engineers have demonstrated that fruits
can stay fresh for more than a week without
refrigeration if they are coated in an odorless,
biocompatible silk solution so thin as to be
virtually invisible. The approach is a promising
alternative for preservation of delicate foods using
a naturally derived material and a water-based
manufacturing process. The work is reported in the
May 6 issue of Scientific Reports.
Silk's unique
crystalline structure makes it one of nature's
toughest materials. Fibroin, an insoluble protein
found in silk, has a remarkable ability to stabilize
and protect other materials while being fully
biocompatible and biodegradable.
For the study,
researchers dipped freshly picked strawberries in a
solution of 1 percent silk fibroin protein; the
coating process was repeated up to four times. The
silk fibroin-coated fruits were then treated for
varying amounts of time with water vapor under
vacuum (water annealed) to create varying
percentages of crystalline beta-sheets in the
coating. The longer the exposure, the higher the
percentage of beta-sheets and the more robust the
fibroin coating. The coating was 27 to 35 microns
thick.
The strawberries were
then stored at room temperature. Uncoated berries
were compared over time with berries dipped in
varying numbers of coats of silk that had been
annealed for different periods of time. At seven
days, the berries coated with the higher beta-sheet
silk were still juicy and firm while the uncoated
berries were dehydrated and discolored.
Tests showed that the
silk coating prolonged the freshness of the fruits
by slowing fruit respiration, extending fruit
firmness and preventing decay.
"The beta-sheet content
of the edible silk fibroin coatings made the
strawberries less permeable to carbon dioxide and
oxygen. We saw a statistically significant delay in
the decay of the fruit," said senior and
corresponding study author Fiorenzo G. Omenetto,
Ph.D. Omenetto is the Frank C. Doble Professor in
the Department of Biomedical Engineering and also
has appointments in the Department of Electrical
Engineering and in the Department of Physics in the
School of Arts and Sciences.
Similar experiments were
performed on bananas, which, unlike strawberries,
are able to ripen after they are harvested. The silk
coating decreased the bananas' ripening rate
compared with uncoated controls and added firmness
to the fruit by preventing softening of the peel.
The thin, odorless silk
coating did not affect fruit texture. Taste was not
studied.
"Various therapeutic
agents could be easily added to the water-based silk
solution used for the coatings, so we could
potentially both preserve and add therapeutic
function to consumable goods without the need for
complex chemistries," said the study's first author,
Benedetto Marelli, Ph.D., formerly a post-doctoral
associate in the Omenetto laboratory and now at MIT.
Other authors on the
work were Mark A. Brenckle, Ph.D., former research
assistant in the Omenetto Laboratory, now at
Columbia University, and David L. Kaplan, Ph.D.,
Stern Family Professor of Engineering, Tufts
University. Kaplan also holds Tufts faculty
appointments in the Department of Chemical and
Biomedical Engineering, School of Medicine, School
of Dental Medicine and Department of Chemistry in
the School of Arts and Sciences.
See also
TED
Silk, the ancient material of the future
Link...
For more information
Marelli, B. et al.
Silk Fibroin as Edible Coating for Perishable Food
Preservation.
Sci. Rep. 6, 25263; doi: 10.1038/srep25263 (2016)
Link...
Tufts University
Link...
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