Elsevier

Food Control

Volume 23Issue 1January 2012Pages 128-136
Food Control

Characterization and control of microbial black spot spoilage in dry-cured Iberian ham

https://doi.org/10.1016/j.foodcont.2011.06.023Get rights and content

Abstract

The presence of black spots on dry-cured Iberian ham surface is an alteration related to microbial population. Although it provokes important economic losses for the meat industrythe microorganisms responsible for this spoilage still remain unclear. The aim of this study was to identify the microorganisms involved in dry-cured Iberian ham black spot spoilage and to study the conditions affecting their growth. Several microbial strains were isolated from Iberian hams spoiled with black spots at the beginning of post-salting stage. However just one strain produced black coloration in both culture media and fat from Iberian ham. This strain was tentatively identified as Pseudomonas fluorescens by using the API 20 NE system and phylogenetic analyses based on the 16S rRNA and carA genes. It was able to grow and produce blackening at 525and 30 °C and with salt concentrations of 2 and 5% at 25 °C and with 2% NaCl at 5 °C in culture media. When it was inoculated in sterile pork fat tissuesgrow and blackening were only detected in samples without added salt at water activity of 0.94 and 0.97. In order to control the incidence of the alteration temperature throughout salting and post-salting stages should be under 5 °C until salt reaches at least a concentration of 2% or water activity is reduced on the surface of dry-cured Iberian ham below 0.92.

Highlights

► P. fluorescens S12N was responsible for blackening in dry-cured Iberian ham. ► To identify S12N a combination of morphologicalbiochemicaland molecular methods is necessary. ► To control black spots in hams temperature below 5 °C should be ensured at salting and post-salting. ► A homogenous distribution of salt in the whole surface of hams during salting is essential. ► Salt concentrations of 2% or water activity 0.92 inhibit black spots at the surface of hams.

Introduction

Dry-cured Iberian ham is a traditional meat product obtained by a process which involves several months (18–24) of ripening. At the beginning of the processthe growth of undesirable microorganisms has to be prevented by adding salt and chilling until the stabilisation is reached by the gradual decrease of water activity throughout the ripening process. Thus at the end of the maturation periodhams usually present pH values ranging from 5.6 to 6.5 and water activity values down to 0.80 or even lower (Rodríguez et al.1994). Although these ecological conditions do not favour microbial growthseveral defects caused by spoilage microorganisms may happen throughout their long ripening period. One of these alterations only reported in Serrano dry-cured ham is the presence of black spots (GarrigaEhrmannArnauHugas& Vogel1998) which was first described by Hugas and Arnau (1987) on the surface of raw cured meat products. Black spots are localized very superficially on the hams mainly in the fat tissues and when the spot is removed by trimmingthe contiguous muscular and connective tissues are unspoiled. Besidesthe browned area is not characterized by an anomalous odour or texture. Howeverthe presence of black spots on the surface of dry-cured Iberian ham is an important factor for consumer acceptance because this kind of meat products is mostly commercialized as whole pieces. As a resultblack spot spoilage provokes important economic losses for dry-cured Iberian ham manufacturing industries.
Although some microorganismssuch as the Gram-negative bacterium Carnimonas nigrificanshave been related to black spot spoilage in some types of cured meat products (Garriga et al.1998)the microorganisms responsible for this one in dry-cured Iberian ham still remain unclear. Despite the fact that water activity values on the ham surface decrease throughout the ripening process below the growth limit of most spoilage microbial populationthe appearance of the first black spots has been observed at the beginning of the post-salting stage. It would be possibly due to the still high water activity values at this time that allow the growth of potential spoilage microorganisms. For that reasonto reduce the risk associated with these microorganisms in dry-cured Iberian hamthe environmental conditions should be carefully controlled during its maturation process. Firstlyit should be necessary to know what conditions throughout the dry-cured Iberian ham ripening may be used to avoid the growth of microorganisms responsible for black spots. These conditions should be based on temperature and salt concentration because they are the main preservative factors used during the maturation of dry-cured Iberian ham.
This study was primarily aimed to identify the microorganisms responsible for black spots on dry-cured Iberian ham by morphologicalbiochemicaland molecular methods. An additional objective was to study the environmental conditions affecting the growth of the microorganisms involved in this type of dry-cured ham spoilage.

Section snippets

Sampling and microbiological analyses

Samples were taken from 5 dry-cured Iberian hams showing black spots at the beginning of post-salting stage. In order to perform microbial counts and isolates 10 g of the black spots were taken aseptically and homogenized in a Stomacher lab-blender with 90 mL of sterile peptone water (0.1% w/v) at room temperature. Appropriate serial decimal dilutions were made with the same diluent and 0.1 mL was spread onto the surface of Plate Count Agar (PCA) and Malt Extract Agar (MEA) (2% malt extract2%

Microbial counts and identification using phenotypic methods

In samples from black spots spoiling dry-cured Iberian hamsthe microbial counts in MEA were higher than those obtained in PCAbeing at levels of about 107 cfu/g and 105 cfu/g respectively (Fig. 1).
Characterization analyses by morphological and biochemical tests of the microbial strains obtained in the former culture media showed Staphylococcus xylosus as the most frequently isolated species in black spot samples (Table 2). Other strains were found at lower proportion such as Moraxella spp.

Discussion

Microbial counts found in black spots of dry-cured ham at the beginning of processing (post-salting stage) at levels of 105-107 cfu/g in the two assayed culture media were in those levels reported in unspoiled dry-cured Iberian hams at the same ripening stage (García et al.2000Núñez et al.1996). In additionmost of the isolated strains were characterized as microorganisms usually found on the surface of unspoiled dry-cured hams such as S. xylosus or Rhodotorula spp. (Núñez et al.1996

Conclusion

In the present work it was concluded that P. fluorescens was the microorganism responsible for black spot spoilage in dry-cured Iberian hams. To control the incidence of this alteration an adequate combination of temperaturesaltand water activity should be ensured. This should consist of a temperature below 5 °C until salt reaches at least a concentration of 2% or water activity is reduced on the surface of the hams below 0.92.

Acknowledgements

This work has been funded by the Spanish Comisión Interministerial de Ciencia y TecnologíaCarnisenusa CSD2007-00016Consolider Ingenio 2010 and GRU08100 and GRU09158 of the Junta de Extremadura and FEDER. Authors are grateful to M. M. García for her technical collaboration during the development of this research.

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