132 Chapter 5
when milk is heated by UHT treatment with
long heating times; late addition of sugar
during the evaporation step reduces age
thickening; and a high concentration factor
increases age thickening. Age thickening also
is infl uenced by the type, amount, and stage
at which stabilizing salts are added. For
example, adding 0.03% sodium tetra pyro-
phosphate delays age thickening, whereas
adding more promotes age thickening. Age
thickening increases with storage tempera-
ture (Q 10 = approximately 3.4) and in tropical
climates, SCM gelation occurs in about one
year. High temperature storage may also lead
to brown discoloration due to Maillard
reaction.Recombined Evaporated and
Sweetened Condensed Milk
In countries with a limited milk supply, evap-
orated milk and SCM are manufactured using
nonfat dry milk (NFDM) or skim milk
powder as the starting material. Other ingre-
dients include sweet cream buttermilk
powder, fat (anhydrous milk fat [AMF] or
vegetable fat), water, and stabilizing salts.
When evaporated milk or SCM are manufac-
tured using all dairy ingredients, the products
are called recombined evaporated or recom-
bined sweetened condensed milk, respec-
tively. When vegetable fat is used instead of
milk fat, the products are described as fi lled
milk (Fotheringham, 1979 ). The specifi c
requirement of the NFDM used for manufac-
ture of evaporated and sweetened condensed
milk is that it must be heat stable (i.e., must
not coagulate when subjected to intense
heating). There are three different classes of
NFDM based on their relative concentrations
of undenatured whey protein nitrogen, and
these are expressed as the whey protein nitro-
gen index (WPNI). The American Dairy
Products Institute (ADPI) (ADPI, 2002 ) clas-
sifi cation of NFDM is given in Table 5.4.
The most preferred NFDM used for steril-
ized milk products are medium - or high - heatDulce de Leche
Dulce de Leche (caramel jam) is an
Argentinean dairy product that is popular
throughout Latin America. It is a form of
sweetened condensed milk prepared by
boiling whole milk with added sucrose until
70% (wt/wt) total solids is reached (Monro
and Hough, 1985 ). The typical starting for-
mulation is 10 parts milk and 2 parts sucrose.
Sucrose usually is replaced partially by
glucose syrup to avoid crystallization. About
0.1% sodium bicarbonate is added to increase
browning and prevent protein coagulation.
The lactose concentration in dulce de leche
is 9.85% from milk with 12% total solids and
4.5% lactose (Hough et al., 1990 ).
Defects in Sweetened
Condensed Milk
Microbial Spoilage
Because SCM is not a sterile product, it is
prone to microbial spoilage, although its
low water activity (approximately 0.83)
and high sugar content prevent microbial
growth. However, osmophilic yeasts of genus
Torulopsis may grow and cause gas forma-
tion and bulging cans. They also cause coag-
ulation and produce fruity fl avors. Other
microorganisms that grow in SCM are micro-
cocci and molds such as Aspergillus repens
and Aspergillus glaucus, and Penicillium sp.,
which grow on the surface of SCM when
suffi cient air (and oxygen) is available.
Chemical Defects
Age thickening is the main chemical defect
in SCM, followed by gelation. Factors that
affect chemical defects include seasonal vari-
ation in milk composition, preheat treatment
of milk, the stage at which sugar is added, the
degree of concentration, and stabilizing salts
added. For example, early lactation milk is
more sensitive to age gelation than mid -
lactation milk; less age thickening occurs