On Food and Cooking

photosynthetic tissue. Some algae (e.g.,
nori, sea lettuce) are essentially all leaf,
just one or two cells thick, very tender
and delicate.
Their immersion in salt waters of varying
concentration has led algae to
accumulate various molecules to keep
their cells in osmotic balance. Some of
these molecules contribute to their
characteristic flavor. Mannitol, a sugar
alcohol, is sweet (and, since our bodies
can’t metabolize it, low-calorie; see p.
662); glutamic acid is savory; and certain
complex sulfur compounds give rise to
aromatic, oceany dimethyl sulfide.
Because water selectively absorbs red
wavelengths from sunlight, some algae
supplement their chlorophyll with special
pigments for capturing the remaining
wavelengths. Many seaweeds are either
brown or reddish-purple, and change
color when cooked.