Fat loss during cooking of duck "foie gras": the biochemistry of lipids

7ABSTRACT
The technological yield, which is evaluated by the fat loss during pasteurization or 
sterilization, is the principal quality trait related to the French Foie Gras production: it participates 
in the uniformity of the end product and the profitability of a sales unit.
Aim of this work is to identify biological characteristics, more than liver weight and lipid rate, 
which contribute to the variability in fat loss during cooking because, despite the progressive 
industrialization of the market and homogenization of the product, a great individual variability 
still exists.
Particularly, the attention has been focused on lipid’s classes in order to determine if they are
involved in this kind of problem, and the principle was to compare the amount of each class 
between the high and low fat loss during cooking.
In order to (i) improve the repeatability of the overall results, (ii) confirm and validate the 
former results and (iii) include the fasting before slaughtering during the second step as a variation 
factor, two field trials of 97 and 238 animals have been planned.
Analyzing the global results it has been observed that there are certain lipid’s pool components 
involved in technological yield of recurrent importance: cholesterol is an interesting parameter 
which has shown statistical relevance when it has been compared for different levels of 
technological yield. It shows a good correlation value with technological yield, with the dry matter 
content and finally with the fatty liver weight. Correlations, as a possible linkage to fat loss, have 
been found also between free fatty acids, some triglycerides, diglycerides and light fatty livers 
characterized by a low technological yield. 
Overall, even though it has not been possible to link the lipid profile with fat loss during 
cooking, confirming the complexity of the problem, the correlations among certain parameters 
which have been above mentioned could be an interesting starting point for future research 
projects to highlight the mechanisms involved in this phenomenon.

9RIASSUNTO
Il rendimento tecnologico, che viene valutato attraverso la determinazione  della perdita 
di lipidi durante i trattamenti termici (pastorizzazione o sterilizzazione), è il principale parametro di 
qualità del prodotto francese Foie Gras: esso determina l’uniformità del prodotto finale ed il
profitto dell’unità di vendita.
Scopo di questo lavoro è quello di identificare le caratteristiche biologiche, al di la del peso del 
fegato e del tenore lipidico, che contribuiscono alla variabilità della perdita lipidica durante la 
cottura perché, nonostante la progressiva industrializzazione del mercato e conseguente 
omogeneizzazione del prodotto, esiste ancora una considerevole variabilità individuale.
L’attenzione è stata rivolta in particolare alle diverse classi lipidiche al fine di determinare se esse 
siano o meno coinvolte nella variabilità del prodotto e il principio è stato quello di comparare 
ciascuna di esse con le due categorie: elevata o ridotta perdita lipidica durante cottura.
Al fine di (i) migliorare la ripetibilità dei risultati complessivi, (ii) confermare e validare i
primi risultati e (iii) includere il digiuno pre-macellazione come fattore di variazione, sono state 
pianificate due prove sperimentali di 97 e 238 animali, rispettivamente.
Dall’analisi dei risultati è stato osservato che certi componenti del pool lipidico, coinvolti nella resa
tecnologica, sono di ricorrente importanza: il colesterolo è un parametro interessante che ha 
evidenziato significatività statistica quando è stato comparato con differenti livelli di resa
tecnologica. Esso ha evidenziato una stretta correlazione positiva con la resa tecnologica, con il 
contenuto di sostanza secca e con il peso del fegato. Altre correlazioni, come possibile elemento di 
collegamento con la perdita lipidica, sono state osservate tra acidi grassi liberi, alcuni trigliceridi,
digliceridi e i fegati più leggeri, caratterizzati da un basso rendimento tecnologico.
Complessivamente, anche se non è stato possibile trovare un preciso collegamento tra 
profilo lipidico e resa tecnologica confermando la complessità del problema, le correlazioni tra i 
parametri sopramenzionati possono essere un interessante punto di partenza per ricerche future 
con l’obiettivo di evidenziare i meccanismi coinvolti in questo fenomeno.

11
1. INTRODUCTION
1.1. History of Foie Gras
The first record about fatty liver in history came from Egypt and is dated back 2500 before 
J.C. For the Egyptians the goose was sacred: it represented the soul of the pharaoh, which means 
that it was like an intermediary between the real world and the world beyond. Egyptians noticed 
that geese, cranes and ducks used to overfeed themselves to be able to survive their migrations: 
this natural behavior takes them to an overproduction and storage of lipids into the liver as an 
energy reserve and this finally changes significantly their body composition (Guy et al., 2008). 
They also discovered that this overconsumption of food had a positive effect on their meat’s 
quality and flavor and so that they started to overfeed geese and cranes with a mixture of grains 
and ground grains and there are some archeological findings that testify this. Later Jewish people, 
who were slaves in Egypt, imitated and then spread the Egyptian’s overfeeding technique to 
Palestine as an alternative to the pork’s meat which they couldn’t eat. Geese were also well 
known and present in Greece from 1200 to 300 before J.C., where they were appreciated not only 
for their meat but also for their fatty liver and this is testified by some famous authors of that age 
such as Homer and Pliny. 
Afterwards in Egypt and Greece, the Romans discovered this product that was soon appreciated 
especially by the aristocracy; geese were also employed for gardening and their fat was used in 
pharmacology. But above all they were appreciated as a gastronomic product, particularly their 
liver which was prepared by Jewish slaves; some authors spoke about a particular feeding 
technique with figs boiled and soaked in the water and the final product was called “Jecur 
ficatum” which means the liver fattened with figs. Afterwards the word became “ficatum”, 
“figido” in the VIII century, “feie” in the XII century and finally “foie” in France (Guémené and Guy, 
2004). 
Unfortunately we don’t have many information about the Middle-Age even if we know that it was 
in that period that Jewish people spread this tradition in Hungary, Poland, Alsace and finally South-
West of France (XVI century) where rearing and feeding goose was cheaper than feeding a pork; 
but it is only in the XVIII century that the fatty liver became a French delicacy.
12
Afterwards the goose and the fatty liver were officially recognized as ‘a jewel’ of the 
French gastronomy becoming a typical and representative French product which is now known 
and appreciated also beyond national borders. 
But we can’t understand the meaning of this kind of product in the South-West of France (which is 
the actual region of Foie gras du Sud-Ouest IGP) unless we know the facts that have mainly 
contributed to the development of this industry.
In the 16
th
century there were two important discoveries:
 The Muscovy duck from South-America;
 The maize cultivation (rich in starch) which substituted the usual cereals such as millet, 
barley, wheat and figs.
In addition, the invention of piston funnel used to force feed geese (which were the only 
animals used for the fatty liver production until the 20
th
century), the improvement of the 
transportation system and subsequently the creation of sterilization and conservation systems for 
this product, all contributed to the diffusion of the fatty liver in France. In the Regions where cattle 
are raised people commonly produce butter, in the South-East the olive oil while in the South-
West the fat of goose is produced as a lipid source.
But why the South-West of France? The production of geese and ducks, associated with 
poultry, has been developing in this Region because of some important favorable aspects such as a 
rural economic system of the peasants with small holdings characteristics of 5-10 hectares (Foie 
gras du Sud-Ouest, 1998), a climate which is perfect for the maize cultivation and finally because 
almost all goose and the duck products have nutritional and also commercial interest.
In addition to this the waterfowls used for fatty liver production are strictly linked to conservation 
techniques:
 Salting combined with drying;
 The use of fat as a conservative;
 Tinned or canned conservation thanks to the Appert method.
Moreover, since the introduction of maize in the 16
th
century, the economic importance 
of farmyard animals have became more and more significant; during the Second World War the 
production of fattened waterfowls was not very important and it was limited to the area of South-
13
West of France (Guy et al., 2008). After the Second World War the “foie gras” became a luxurious 
product which could be found only in the greatest restaurants or during very special occasions.
Since the beginning of the Fifties, fatty liver production has been increasing over and over 
because of the development of new equipments, new management techniques and finally thanks 
to the new genotypes involved, so it can now be also found in supermarkets as a common 
luxurious product even if it has conserved its image of excellent and noble food for celebrations.
1.2. French Foie Gras
This French particular product is obtained from the overfeeding of ducks or geese with 
maize for a period which lasts at minimum ten or twelve days, depending on the breed and the 
genotype (mule duck, muscovy duck, Landes goose). The main aim of this kind of feeding is to 
induce the steatosis of the bird’s liver due to the progressive accumulation of triglycerides caused 
by the massive amount of maize (which is rich in carbohydrates). At the end of the force-feeding
period the liver’s weight is increased almost ten folds (Cazeils, 2000).
In France there are three species of waterfowls which can be used for the production of 
Foie gras: the grey goose of the South-West (Anser anser), the Mule duck (Anas platyrhynchos) 
and the Muscovy duck (Cairina moschata). It’s the graylag goose that is the origin of the most 
selected genotypes of domestic geese which are generally used for foie gras production.
About 96% of the French Foie gras is produced from ducks (Baéza et al., 2005). There are 
two genotypes which can be used for the Foie gras production: the Muscovy duck (Cairina 
moschata), a breed that originally came from Central-South America and the Mule duck which is 
the result of the interspecific crossing of a Muscovy male duck with a Pékin female (Anas 
platyrhynchos). There are two different phenotypes: the colored and white type (it depends on the 
parental color). The mule duck is sterile and does not show an evident sexual dimorphism. 
Nevertheless the female duck presents a veined liver which considerably decreases its quality. 
Thus, since 1996, the denomination “Foie gras de canard” is only allowed for the male’s duck fatty 
livers and because of this, female’s livers cannot take part to the French market.