Chemistry Reference
In-Depth Information
The official AOAC 920.115 method requires drying a diluted sample of
SCM in the presence of sand. The CEM (microwave) method requires drying
a diluted sample on a glass fiber pad. The use of sand or a fiber pad provides a
large surface area for sucrose glass formation, thus reducing the ''blanketing''
effect of sucrose glass on the complete drying of the sample. The use of the
Mojonnier TS method, on the other hand, is very tricky (rapid drying in an
open pan with no sand or pad); depending on the dilution and the sample size,
the value obtained by this method can overestimate TS by as much as 1.7%.
Reproducibility of test results using the AOAC method depends on the
diligence of the analyst in making sure that the surface of the sample (prior to
complete drying) is mixed periodically with the sand. Since this method
requires 5 h for complete drying, it is not suitable for industrial application.
The Karl Fisher method, on the other hand, is very reproducible and accu-
rate, provided that the sample has been pre-conditioned with 2-3 parts dry
formamide at 608C for 2-3 min to facilitate the release of water held tightly by
the solutes, prior to the titration in dry methanol media (see Bruttel and
Schlink, 2003).
I.8.5.
Effect of Water Activity on the Microbial Stability of Sweetened
Condensed Milk
Many microorganisms have been found in SCM produced around the
world, but the a
w
or composition of these products are not known. Today,
more and more emphasis is given to water activity (a
w
) in foods; a
w
is now the
primary criterion for defining the so-called ''non-potentially hazardous food''
(2005 Food Code of US FDA). An a
w
above 0.88 is considered ''potentially
hazardous'' for non-acid foods.
Sweetened condensed milk is a low-acid, pasteurized product (as opposed
to high-acid or sterilized products). Its microbial stability at room temperature
is due to the preservative action of the sugar it contains. With the right level of
sugar (a
w
below 0.85), only molds and osmophilic yeasts and bacteria can spoil
the product. Since molds and osmophilic yeasts are typically very heat labile
(readily destroyed by proper heat treatment), their presence in the product is
due mainly to post-process contamination. With a sufficient level of sugar (a
w
below 0.85), organisms other than osmophilic yeasts and molds should never
grow in SCM to such numbers that cause spoilage.
The number of organisms required to cause spoilage varies with the
food item and the type(s) of microorganisms growing in it. As a rule of thumb,
10
7
bacteria/g, 10
5
yeast/g or visible mold signals the end of microbiological
shelf life. In SCM, however, various organisms die within weeks or months of
storage (Rao and Ranganathan, 1970; Koroleva et al., 1975; Farrag et al.,
