NOTE
Stabilization of duck fatty liver by high pressure treatment. Inactivation of Enterococcus faecalis
Abdelhamid EL MOUEFFAK
1 *, Christian CRUZ
1, Monique ANTOINE
1, Michel MONTURY
2, Gérard DEMAZEAU
3, Alain LARGETEAU
3, Bernadette ROY
4, François ZUBER
4RÉSUMÉ Stabilisation de foie gras de canard par l’application de hautes pressions.
Inactivation d’Enterococcus faecalis.
Des foies gras de canard sont traités par l’application de hautes pressions (350, 550 MPa), associées à des températures réduites (55, 65 °C), durant 10, 20, et 30 min. Les effets observés sur la flore mésophile aérobie totale (FMAT) sont comparés avec ceux observés sur une flore inoculée d’Enterococcus faecalis. Après un traitement de 550 MPa/55 °C de plus de 20 min, on obtient une réduc- tion de sept logarithmes décimaux de la FMAT et Enterococcus faecalis. Néan- moins, la FMAT endogène apparaît plus résistante à la pression que Enterococcus faecalis, en particulier à 350 MPa.
Mots clés : haute pression, foie gras de canard, Enterococcus faecalis.
SUMMARY
The effects of combined high pressure (350, 550 MPa) and reduced temperatu- re (55, 65°C) treatments for 10, 20, 30 min, on the total aerobic mesophilic flora (TAMF) of duck fatty liver, were compared with those on inoculated Enterococ- cusfaecalis flora. Results were compared with those obtained from a traditional pasteurization. Seven decimal reductions of the TAMF and of Enterococcus faecalis were obtained at 55°C, 550 MPa for at least 20 min. Nevertheless, the natural TAMF appears more resistant to the pressure than the Enterococcus faecalis especially at 350 MPa.
Key-words:high pressure, duck fatty liver, Enterococcus faecalis.
1. Équipe de recherche agroalimentaire Périgourdine (ERAP-EURIA), Université Bordeaux IV, 39 rue P. Mazy, 24019 Périgueux cedex, France.
2. Équipe Périgourdine de chimie appliquée (EPCA-EURIA), Université Bordeaux I, 39 rue P. Mazy, BP 1043, 24001 Périgueux cedex, France.
3. Interface hautes pressions, Université Bordeaux I, 131 cours de la Libération, 33405 Talence, France.
4. Centre technique de la conservation des produits agricoles, 44 rue d’Alésia, 75014 Paris, France.
* Correspondence.
moueffak@montesquieu.u-bordeaux.fr
72 Sci. Aliments 21(1), 2001 A. El Moueffak et al.
1 - INTRODUCTION
Duck fatty liver is a renowned high value added product with high organoleptic qualities. Various traditional processing treatments exist such as half-cooking, pasteurization and sterilization, yielding products with a conservation period of just a few days at 4°C, 6 months at 4°C and 1 to 3 years at room temperature, respec- tively. Nevertheless, the partial melting of lipids, occuring during each of these treatments, along with a loss of organoleptic properties, is a critical factor.
High pressure processing (100-700 MPa) can be a “soft” method preserving food organoleptic qualities (LAMBERT, 1999). First Industrial applications appea- red in Japan (SHIGEHISAet al., 1991) and extended to USA and Europe (CHEFTEL and DUMAY, 1997). Today, the applications include meat and seafood products or cooked ham slices (CARLEZ, 1994; ELMOUEFFAKet al., 1994-1995).
Test studies on the bacterial stabilization of the whole duck fatty liver, using pressure greater than 700 MPa and reduced temperature (50°C) combinations, have provided microbiological results similar to those from a traditional pasteu- rization.
In order to better appreciate the efficiency of the high pressure processes, it is necessary to inoculate samples with micro-organisms that are considered as resistant to heat and to high pressure treatment, such as Enterococcus faecalis.
This micro-organism will be treated in a range of pressures (350, 550 MPa), temperatures (55, 65°C) and process times (10-20-30 min) and results will be compared to a traditional pasteurization process in order to identify the best combination reproducing results similar to pasteurization.
2 - MATERIAL AND METHODS
Forty eight samples (30 g), were taken from the main lobe of the liver of six Mulard ducks. Samples were inoculated with Enterococcus faecalis to obtain approximately 106 to 107 CFU/g and wrapped in a multilayered film PE/PA/PE for storage at 4°C before treatment (5 days after evisceration).
Two processing temperatures were selected: 55 and 65°C, chosen because they are too low to ensure pasteurization while ensuring minimum cooking with good flavor and texture preservation. The samples were preheated to 45°C/
10 min, then two high pressures (350 and 550 MPa) and three process times (10, 20, 30 min) were used for each temperature.
The reference samples were submitted to industrial pasteurization (90°C). So, an internal temperature of 85°C was reached after 33 min in the samples which were then immediately chilled to 4°C prior to storage.
The high pressure treatments were performed in a hollow cylinder (0.85 L) filled with heated and pressurised water. After treatments and 2 days of storage at 4°C, the same analysis were carried out on the same day by two laboratories
(EURIA and CTCPA) with the same operating conditions as follows: for each sample, plate count agar and KF-Triphenyl tetrazolium chlorure-2,3,5 mediums were used for the study of TAMF at 30°C and of Enterococcus faecalis at 37°C, respectively; the colonies were counted and the results from the two laborato- ries, expressed as CFU/g of duck fatty liver, were compared. The mean conta- mination was calculated and expressed as log N0/N where N0 is the initial con- tamination and N is the final one.
3 - RESULTS AND DISCUSSION
TAMF: 350 MPa
During the first 10 min of the treatment at 55 and 65 °C, 2 and 6 decimal reductions were obtained respectively. On the following 20 min, there is practi- cally no further flora reduction whatever the temperature (figure 1).
Thus, treatment at 55°C was never sufficient to reduce TAMF by a significant amount whatever the duration of pressurization while at 65°C the reduction was much more significant.
Enterococcus faecalis: 350 MPa
During the first 10 min of treatment at 55 and 65°C, between 4 to 5 and 5 to 6 decimal reductions in contamination were observed, respectively.
During the following 20 min, conditions remain stable at 65°C, but we can observe a continuing reduction in contamination at 55°C (1 decimal reduction for every 10 additional min). After a 30 min treatment, the results are practically simi- lar for both temperatures.
Figure 1
Effect of temperature on the reduction of the Total Aerobic Mesophilic Flora (TAMF) and Enterococcus faecalis (E.F) at 35O MPa for 3 high pressure durations
0 10 20 30
0 1 2 3 4 5 6 7
Time (min)
Log (NO/N)
55°C TAMF 65°C TAMF 55°C E.F 65°C E.F
74 Sci. Aliments 21(1), 2001 A. El Moueffak et al.
In contrast to the results for TAMF, there is no significant difference between the effects of the two temperatures for Enterococcus faecalis. Nevertheless, the lower temperature appears to be more effective for Enterococcus than for TAMF which seems to be significantly more pressure resistant.
These results are in agreement with the litterature (CARLEZ, 1994) and can be explained by a different pressure sensibility and growth stage between TAMF and Enterococcus.
The results point out that at 350 MPa temperature is a significant variable in the rate of microbiological reduction.
TAMF: 550 MPa
At a temperature of 65°C, the curves for 350 and 550 MPa are similar, except that the higher pressure leads to an increase in the reduction by 1decimal (figure 2). Whereas a reduction of contamination had practically stopped after 10 min at 55°C and 350 MPa (figure 1). At 550 MPa the reduction factor continued to increase until the TAMF had been reduced by between 6 and 7 decimals after 30 min of pressurisation. It seems that an increase in temperature has relatively little effect on the rate of reduction of TAMF after 10 min at 550 MPa (figure 2).
Enterococcus faecalis: 550 MPa
The reduction up to 7 decimals is similar to that for TAMF whatever the tem- perature and the duration (figure 2). The curves for the elimination of TAMF and Enterococcus faecalis present similar characteristics. This indicates that the pressure is the most significant variable and that a 20 min treatment at 550 MPa is equally effective, whatever the temperature chosen and the flora. Indeed, the
Figure 2
The effect of temperature on the reduction of TAMF and Enterococcus (E.F) at 550 MPa for 3 high pressure durations
0 10 20 30
0 1 2 3 4 5 6 7 8
Time (min)
Log (NO/N)
55°C TAMF 65°C TAMF 55°C E.F 65°C E.F
pressures used are close to that recorded by SHIGEHISAet al. (1991), for the same micro-organism inactivation (600 MPa/25°C/10 min) in ground pork.
In order to specify the conditions for a heat/pressure process that would give results similar to those achieved by a traditional pasteurization one, a compari- son was made with samples that had undergone the classical heat processing.
Our results show an apparently complete elimination of the TAMF and Ente- rococcus faecalis withthe traditional pasteurization process. Comparable results can only be achieved at 550 MPa applied for 20 to 30 min whatever the tempe- rature used (55 or 65°C) in the case of the pressure process. Since the endoge- nous contamination of fatty liver can rarely exceed 106 CFU/g, this latter pressure treatment seems sufficient to obtain a product that is “pasteurized” with low level endogenous floras during the storage at 4°C.
In order to retain as many organoleptic qualities as possible and to avoid the melting of fat, a temperature of 55°C and a pressure maintained during 20 min are sufficient. These conditions ensure the necessary cooking process. Indeed, as ELMOUEFFAKet al. (1995) have shown, the texture of the pressurised samples is similar to that observed by the traditional half-cooking method.
4 - CONCLUSIONS
Results show that TAMF and Enterococcus faecalis can be significantly redu- ced by 7 decimals with the low heat treatment of 55 or 65°C applied during 10 to 20 min at a pressure of 550 MPa. This pressure is significantly more effective than 350 MPa, while the effect of temperature is more pronounced at 350 MPa than at 550 MPa, especially for the 20 min treatment.
A marked elimination of Enterococcusfaecalis can be mainly observed in the first 10 min. The leveling off of the rate of reduction has been noted for duck fat liver thermal treatment. The TAMF appears much more pressure resistant than Enterococcus at 350 MPa.
From a practical point of view, it is possible to consider that a process of 550 MPa/55°C/20 min gives a product similar microbiologically to a pasteurized one, with a quality comparable to half cooked products. The required tempera- ture of 55°C is much lower than the classical temperature of 75°C/80°C presently used for the pasteurization process.
ACKNOWLEDGMENTS
With the support of Electricité de France and the Comité interprofessionnel des palmipèdes à foie gras.
Receveid 28 September 2000, accepted 4 December 2000.
76 Sci. Aliments 21(1), 2001 A. El Moueffak et al.
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