On food and cooking the science and lore of the kitchen ( PDFDrive ) 578
Bạn đang xem bản rút gọn của tài liệu. Xem và tải ngay bản đầy đủ của tài liệu tại đây (126.52 KB, 2 trang )
concentratetheotheringredients.Theboiling
is continued until the temperature of the mix
reaches 217–221ºF/103–105ºC (at sea level;
2ºF/1ºC lower for every 500 ft/165 m
elevation), which indicates that the sugar
concentration has reached 65% (for the
relationship between sugar content and
boiling point, see p. 680). A fresher flavor
results when this cooking is done at a gentle
simmerinawidepotwithalargesurfacearea
for evaporation. (Industrial manufacturers
cook the water out under a vacuum at much
lower temperatures, 100–140ºF/38–60ºC, to
maintain as much fresh flavor and color as
possible.) Now supplemental acid is added
(late in the process, to avoid breaking down
the pectin chains), and the readiness of the
mix is tested by placing a drop on a cold
spoonorsaucertoseewhetheritgels.Finally,
themixispouredintosterilizedjars.Themix
sets as it cools below about 180ºF/80ºC, but
firmsmostrapidlyat86ºF/30ºCandcontinues
togetfirmerforsomedaysorweeks.
Two kinds of pectin gels. Left:In ordinary
fruit preserves, the cook causes pectin
molecules to bond directly to each other and
form a continuous meshwork by carefully
adjusting acidity and sugar content. Right:A
modifiedformofpectin(lowmethoxy)canbe
bondedintoacontinuousmeshworkbymeans
of added calcium ions (the black dots), no
matter what the sugar content. This is how
low-sugarpreservesaremade.
Theusualproblemwithpreservemakingis
failureofthemixtosetevenattheproper
boilingtemperatureandsugarconcentration.
Thiscanbecausedbythreedifferentfactors:
inadequateamountsofeitheracidorgoodqualitypectin,orprolongedcookingthat
