|Abstract or Summary
- Recent studies have shown that when milk fat is heated, a
homologous series containing the n-alkyl members of methyl ketones
with odd numbers of carbon in their chains are produced (48; 44; 38;
and 3). The same series of compounds also is found in evaporated
and dried whole milk and in these products the concentration increases
during storage (70 and 46). It is believed by some investigators
that the methyl ketones play an important role in flavor deterioration
of milk fat and in the aforementioned concentrated products.
At the present time, however, there is disagreement on the factors
influencing methyl ketone production; some workers relate their
formation to autoxidation (25), while there are others who report
that heat and water are essential in the reaction (48; 38 and 3).
Finally, a recent report indicates that anhydrous milk fat will give
rise to methyl ketones when heated in the absence of oxygen (44).
The purpose of this investigation was to study the effect of
various factors on the qualitative and quantitative composition of
methyl ketones in heat treated milk fat. It is anticipated that the
resulting information will contribute to a more thorough understanding
of the reactions leading to ketone production in the fat; hence, to
development of suitable processing measures for prevention of this
type of deterioration in dairy products.
Milk fat was prepared from raw cream two days after milking.
It was washed free from phospholipids, centrifuged at 30,000
x G for 20 minutes and degassed at two to five microns pressure
for one hour. The fat was then heat treated in sealed vials at various
temperatures and time periods under controlled conditions.
The samples were quantitatively analyzed for methyl ketones by
direct conversion of the ketones to 2, 4-dinitrophenylhydrazine
(DNP) derivatives in the intact fat sample. The derivatives were
isolated from the fat, separated and identified by a combination of
column and paper chromatographic methods and by their absorption
Methyl ketone formation in heated milk fat was shown to be
non-oxidative. A plateau in ketone production was approached in
the 120°C to 140°C range when the time of heat treatment was 30
minutes. Added water enhanced total methyl ketone production at
140°C but not at 200°C. Air did not hinder ketone production. Maximum ketone production (1.733 mM/kg fat) was noted after three
hours of heat treatment at two to five microns pressure, and 140°C.
Milk fat centrifuged at 30,000 x G for 20 minutes and degassed at two
to five microns pressure for one hour was found to contain 0.27%
water. This quantity of water is sufficient for hydrolysis of β-ketoesters
assuming them as the precursors of the methyl ketones.
Conventional methods of preparing "anhydrous" milk fat were not
adequate for removal of trace amounts of water. When milk fat was
dried over calcium hydride (35) prior to heat treatment, total ketone
formation was significantly reduced indicating that water is a limiting
factor in ketone formation.
A homologus series of n-alkyl methyl ketones (C₃, C₅, C₇,
C₉, C₁₁, C₁₃, C₁₅) was isolated from heat treated samples. The
ketones produced in large amounts were the C₃, C₇, and C₁₅.
When the heat treatment was for three hours or longer the C₄ ketone
was detected and composed approximately 11% of the total. The possible
origin of the C₄, ketone is discussed. The reaction of intact
fat with DNP-hydrazine and the subsequent isolation and identification
of methyl ketones were quantitatively evaluated.