Thermal Food Processing

UHT Thermal Processing of Milk 319
Rattray and coworkers 104 studied UHT direct and indirect milk during storage,
and their results also showed that the amount of furosine is higher for indirect
heat treatment than for direct treatment (Table 10.3), probably due to the higher
amount of heat applied in the indirect process and to the temporary dilution effect
of steam in the direct process.
It is noted that standardization of milk protein will affect furosine as an
index of heat processing. While fat standardization of milk is already a common
practice in the dairy industry, steps are being undertaken to also standardize
the protein content. There are several methods to alter protein concentration of
milk: addition of ultrafiltrate milk retentate or permeate and addition of fractions
of whey protein. 105 Rattray and coworkers 104,106 have found that milk protein
standardization through addition of acid whey permeate results in lower levels
of furosine than protein standardization obtained with skim milk permeate. This
difference is probably due to a decrease in both lactose and protein concentrations
when using acid whey permeate, while skim milk permeate only causes
a decrease in protein.
10.3.2 FURFURAL COMPOUNDS: INTERMEDIATE MAILLARD
REACTION INDICES
Intermediate compounds of the Maillard reaction may be formed in heated milk,
but only in very small amounts compared to the Amadori product. Among these
compounds, the furfurals must be included. In particular, hydroxymethylfurfural
(HMF) and other furfural compounds (furfural, furylmethylcetonem, and methylfurfural)
can be generated during heat treatment of milk or storage at inadequate
temperature.
From a chemical point of view, HMF is the product of the dehydration of
hexoses (free or linked to protein) due to the presence of concentrated acids. HMF
is formed not only from Amadori compounds, but also from sugars (lactose
isomerization). It is considered a good index of the severity of milk heat
treatment 107 and is suitable as a marker of the most severe heat treatments (UHT
and sterilized milk).
Traditionally, HMF is determined by a colorimetric method, 108 but this
method has low specificity. At present, capillary electrophoresis 109 and RP-
HPLC 110 appear to be the most powerful techniques for HMF determination.
The analytical separation of HMF can be achieved by micellar electrokinetic
chromatography (MEEC) employing sodium dodecyl sulfate as the anionic
surfactant or by a simple isocratic HPLC method using reversed-phase
microbore columns. 111
Using HMF as a marker, Ferrer and coworkers 102 observed differences between
store-brand and name-brand UHT milk. Free HMF was found to be present only
in store-brand UHT milk, levels ranging from 8.24 to 50.9 µg/100 ml of milk,
probably due to lower milk quality.
Several factors possibly affecting the use of HMF as a marker have been
investigated. In particular, the total HMF level in milk appears to be affected by