Thermal Food Processing

246 Thermal Food Processing: New Technologies and Quality Issues
phospholipids affected the most during cooking, in particular lysoforms. Lipolitic
enzymes in fish are mostly active at ordinary cold storage temperatures. Heat
treatment resulting in inactivation of enzymes is of considerable practical importance.
During fish thermal processes, hydrolysis of triacylglycerols gives as a
result a significant increase of free fatty acids. 52 The application of 13 C-nuclear
magnetic resonance ( 13 C-NMR) spectroscopy has shown a preferential stereospecificity
of lipid hydrolysis in the internal position of the glycerol moiety
after thermal stress, resulting in a larger increase of free DHA. 52 The ratio of
1,3-diacylglyceride and 1,2-diacylglyceride quantities confirms this finding.
These results contrast with enzymatic hydrolysis during fish frozen storage,
occurring preferentially in the sn-1 and sn-3 positions of triacylglycerols, with a
consequent cleavage of saturated and monounsaturated fatty acids. After thermal
processing, an increase in free PUFAs can be of special relevance for suffering
oxidation. The most susceptible compounds at thermal stress are plasmalogens,
1-O-alk-1-enyl-glycerophospholipids. 53 At sterilization temperatures (115°C), the
weak ether bonds of these compounds are broken, and as a result, the amount
of plasmalogens in canned brine and canned oily tuna decrease by 50%. Plasmalogens
have been more susceptible to damage during heat processing than
their corresponding acyl derivatives. This behavior may be explained by the
aldehydogenic chemical structure easily open to acidic attacks.
Overall quality and flavor of the thermally processed product can be highly
influenced by lipid oxidation rending to rancidity. Initial raw composition, process
conditions, and packing substrate have a special relevance. The canning process
provokes increases of primary, secondary, and tertiary lipid oxidation products
in canned tuna. 54 The highest thiobarbituric acid index (TBA-i) values were found
in tuna canned muscle using brine as a dipping medium, thus indicating lower
protection in the muscle kept in a highly aqueous environment than that kept
using oily packing media.
13 C-NMR spectroscopy has been applied to elucidate the mechanism of lipid
oxidation occurring during thermal treatment of fish. 55 Effects of temperature and
time of processing have been studied by means of a model system of lipids extracted
from salmon (Salmo salar) muscle to simulate industrial conditions of canning.
Unsaturated fatty acids located at the sn-2 position of the glycerol moiety are most
prone to suffering from oxidative damages. Regarding the mechanism of the reaction,
results inferred from olefinic and methylenic resonances indicated a higher susceptibility
of the closest allylic sites to the carbonyl group, followed by those placed near
to the methyl terminal group. Unsaturations located in the middle of the carbon chain
did not show notorious damages. The glyceryl region provided an unusual resonance
at 53.4 ppm that could be assigned to a hydroxilic compound formed during oxidation.
Oxidation in fish during thermal processing has also been studied by determining
volatile production with a static headspace gas chromatographic system. 56
The major volatiles formed included acetaldehyde, propanal, heptane, 2-ethylfuran,
pentanal, and hexanal. The formation of 2-ethylfuran had the highest independent
contribution for the prediction of oxidative stability of fish muscle at 4 days at
40ºC and 150 min at 100ºC. The formation of 2-ethylfuran in oxidized n-3 PUFAs