1. INTRODUCTION
The problem of food adulteration is by no means a contemporary
phenomenon and is likely to be as old as the food processing and
production systems themselves (Ellis, et al., 2012). Authentication of
food products is of primary importance for consumers and industries at
all levels of the production chain. The adulteration of food products
normally becomes a means for fraudulent economic gain, as fraudulent
actions are commonly applied to high-commercial-value products or to
those which are produced in high tonnage. Food products, such as jam,
fruit juice, meat, honey, milk, wine or vegetable oils are often
adulterated with the addition of cheaper products, dilution processes or
mislabeling (Cordella et al., 2002). Nevertheless, the detection of
adulterations or the discrimination between different product categories
become a very complicated issue when chemical compositions are similar.
Several methodologies have been used for foods authentication, such as
spectroscopic techniques, methods based on mass spectrometry,
chromatography or calorimetry. Characteristics of spectroscopic
techniques are their rapidity and portability, and they can provide the
fingerprint of a food product. On the other hand, calorimetric
techniques, such as differential scanning calorimetry (DSC), are also
rapid and highly sensible, and they can show the authenticity of a food
product or the effect of an adulteration on the physicochemical
properties of the sample (Chiavaro et al., 2008a and 2008b).
Among high-commercial-value products, traditional Spanish Iberian
dry-cured ham is greatly appreciated for its particular and intense
sensorial characteristics and associated with top quality gastronomy
worldwide. Mainly produced in the southwest regions of Spain, Iberian
pigs (pure breed or crossed with Duroc) are raised according to
different systems, so that their dry-cured products are classified into
different commercial categories. Four are the categories of Iberian
pigs, namely bellota , recebo , cebo de campo andcebo. Although according to the current Spanish legislation
(Royal Decree 4/2014), recebo category has been eliminated and,
therefore, nowadays three are the existing categories (bellota,
cebo de campo and cebo ), it is of primary importance to
characterize all of them in order to find a robust method to
authenticate the product and prevent fraudulent practices. Regarding the
rearing systems which define each Iberian pig category, Table 1
summarizes their characteristics and requirements.
Bellota products, which are the most appreciated and those of
highest commercial value, come from pigs with a final fattening period
in oak forests (period called montanera ) during which they
exclusively eat acorns, grass and other natural resources. Then, the
minimum duration of this period is 60 days and the minimum weight gain
of the pig must be of 46 kg. Recebo products are obtained from
pigs that combine the consumption of acorn and grass with a
supplementation of concentrated feeds (minimum duration of montanera: 60
days; minimum weight gain: 29 kg). By contrasts, cebo de campoand cebo products come from pigs fed with commercial feeds
throughout their life. However, in the case of cebo de campo ,
pigs roam in pasture for at least 60 days prior to slaughter, although
they keep eating mainly commercial feeds. The differences in animal
nutrition in the final stage of the fattening period may determine the
sensory quality of final products, due to different lipid deposition in
adipose tissue and intramuscular fat (Tejeda et al., 2002).
Several methods have been used to discriminate among different Iberian
dry-cured ham of different categories. Almost all the procedures are
based on fat properties, such as melting and slip point, or fatty acid
profiles. These methods rely on the fact that bellota usually
contains higher amounts of oleic acid than other categories (Ruiz et
al., 1998). By contrast, other methods include the determination of
hydrocarbons (Narváez-Rivas et al., 2008) and volatile compounds
profiles (Timón et al., 2001; Narváez-Rivas et al., 2011). Moreover,
isotope analyses on the adipose tissue of the animals have also been
applied to characterize and differentiate Iberian pork meat as a
function of the diet of the animal (González-Martín et al., 1999). As to
spectrometric techniques, near-infrared spectrometry (NIR) has also been
applied with the same purpose (Arce et al., 2009), although this method
could not perfectly discriminate among the different pig categories.
Recently, more attention has been paid to the determination of the TAGs
composition, as analyses become easier and faster, avoiding the use of
saponification and the formation of methyl esters (Díaz et al., 1996;
Gallardo et al., 2012). The TAGs composition has been also applied to
elucidate the effect of genotype (Tejeda et al., 2002; Petrón et al.,
2004; Viera-Alcaide et al., 2008). However, there are still some
ambiguities in the results obtained by the methods used until now when
high amounts of samples have been analyzed. Although the purpose of the
new Spanish regulation consists of protecting the purity of Iberian pig,
controlling the breeding and feeding and properly labelling the final
product, there is still a lack of a normalized and robust method to
discriminate among the different categories.
Some studies have focused on the characterization of physical properties
of pork fat. Svenstrup et al. (2005) analyzed the influence of changing
the cooling and reheating rates applied to pork fat. By using DSC and
XRD techniques, they characterized samples of lard (pork dorsal fat) and
leaf pork fat (surrounding the kidneys) in their raw state, their
extracted fat fraction, and in liver pâté. The crystallization behavior
observed permitted defining the textural properties obtained when pork
fats were used in liver pâté. Campos et al. (2002) also applied
different cooling rates to lard samples, but in order to observe the
effects of such variations on the hardness of fat crystal networks. By
contrast, Davenel et al. (1999) studied solid fat content variations of
pig adipose tissues in relation to their lipid composition. As to
spectroscopic techniques, Raman spectrometry has been applied toin situ analyze the crystalline states of fat in porcine adipose
tissue (Motoyama et al., 2013). The purpose was to establish a tool for
routine monitoring the physical conditions of meat carcasses in
refrigerators. The same technique was also proposed to discriminate
between pork and beef fat, by analyzing the polymorphic features of the
characterized samples (Motoyama et al., 2010).
Our group has recently analyzed the polymorphic behavior of the two most
differentiated categories of Iberian dry-cured ham (bellota andcebo ), and obtained very promising results (Bayés-García et al.,
2016), as significant differences, related to the polymorphic behavior
of the samples, were detected and permitted to clearly discriminate
among the two categories. Crystallographic techniques, such as
differential scanning calorimetry (DSC) and X-ray diffraction (XRD, with
both laboratory-scale or synchrotron radiation source) have been widely
used to study the polymorphism of lipid systems, in order to
characterize their physical properties, such as melting, morphology,
rheology and texture (Larsson et al., 2006; Bayés-García et al., 2020).
Nevertheless, these techniques have been rarely used in food
authentication.
In the present work we used DSC and XRD techniques to study the
polymorphic behavior of the four Iberian pork categories, including
intermediate recebo and cebo de campo to make a step
further. Moreover, we used raw lipid samples, obtained before the curing
of the ham, in order to define a more robust method, as curing becomes a
very complex procedure in which it is not possible to control the
variables which may modify the behavior of the samples. This study
demonstrates that crystallographic tools may be promoted in the food
authentication field to combat food fraud.