Christina E. Adamsen

Zn-porphyrin formation in cured meat products: Effect of added salt and nitrite.

Zn-porphyrin (Zn-pp) was quantified by fluorescence spectroscopy in the cured and dry cured meat products: Parma ham, Iberian ham, dry-cured ham with added nitrite, cooked ham with added nitrite, raw ham meat, raw bacon and Karree-Speck. The highest amount of Zn-pp was found in dry-cured Parma ham and Iberian ham, while the use of nitrite as curing agent was found to inhibit completely the formation of Zn-pp in meat products. A positive correlation between both Zn content and Fe content and the logarithmic transformed Zn-pp content (measured as fluorescence intensity I(fl)) was found for the different cured and dry cured meat products, with correlation coefficients of 0.79 (p<0.001) and 0.71 (p<0.01), respectively. Log I(fl) correlates best with the Zn content, indicating that the formation of Zn-pp is proportional to the Zn content. A model system with vacuum packed pork in brine with different added levels of sodium chloride with or without nitrite and Zn acetate was investigated in order to further elucidate the mechanism of Zn-pp formation. Zn-pp increased with time (up to 42 days investigated) in non-cured meat and for meat cured solely with NaCl lower than 9%. Addition of nitrite or Zn(II) in the curing brine was found to inhibit formation of Zn-pp confirming the observations from the various cured meat products. It is suggested that a chloride anion assisted dissociation of iron from myoglobin could be rate-determining for Zn-pp formation in meat products.

Mass spectrometric evidence for a zinc–porphyrin complex as the red pigment in dry-cured Iberian and Parma ham

Extracts containing red pigment complexes from the two types of dry-cured hams, Italian Parma and Spanish Iberian ham, were obtained using water and acetone as extraction solvents followed by a crude purification with C18 column filtration. The purified extracts were then analyzed spectroscopically by recording absorption and fluorescence spectra (λ(ex)=420nm), which both indicate the presence of chemically identical red chromophores with properties similar to a complex of transition metals and protoporphyrin IX. Electrospray ionization mass spectrometry (ESI-MS) in the positive ion mode confirms the presence of identical chemical compounds. ESI-MS in the negative ion mode detects a cluster of seven isotopologue ions (that of m/z 623.2 as the most intense) with a pattern matching that of a Zn protoporphyrin IX complex. Based on mass spectral data it is concluded that a Zn-porphyrin complex constitutes a major chromophore in dry-cured Iberian ham as well as in Parma ham.

Changes in Zn-porphyrin and proteinous pigments in italian dry-cured ham during processing and maturation

Substitution of iron with zinc in myoglobin during maturation of Parma ham to yield zinc porphyrin extractable by 75% vol/vol acetone/water solution and detectable by fluorescence spectroscopy, was found to occur concomitant with protein modification in myoglobin. The content of zinc porphyrin increases throughout the whole processing and maturation of Parma ham, from I(fl) 0.1±0.06 for green ham to I(fl) 84.4±48.8 for fully matured Parma ham. In an aqueous extract of Parma ham with pH 6.0 protein alteration in myoglobin, as detected by size-exclusion chromatography, is initiated during the resting period following salting and seems to precede formation of zinc porphyrin. During maturation the results indicate that the modified myoglobin could undergo polymerization, and it is suggested that initial protein denaturation or degradation facilitates substitution of iron with zinc. The pigment polymerization may be a result of non-covalent protein association to zinc porphyrin in denatured or partly degraded myoglobin.

Quantification of zinc-porphyrin in dry-cured ham products by spectroscopic methods Comparison of absorption, fluorescence and X-ray fluorescence spectroscopy.

Zinc-protoporphyrin (Zn-pp), which has been identified as the major pigment in certain dry-cured meat products, was extracted with acetone/water (75%) and isolated from the following meat products: Parma ham, Iberian ham and dry-cured ham with added nitrite. The quantification of Zn-pp by electron absorption, fluorescence and X-ray fluorescence (XRF) spectroscopy was compared (concentration range used [Zn-pp]=0.8-9.7μM). All three hams were found to contain Zn-pp, and the results show no significant difference among the content of Zn-pp quantified by fluorescence, absorbance and X-ray fluorescence spectroscopy for Parma ham and Iberian ham. All three methods can be used for quantification of Zn-pp in acetone/water extracts of different ham types if the content is higher than 1.0ppm. For dry-cured ham with added nitrite, XRF was not applicable due to the low content of Zn-pp (<0.1ppm). In addition, XRF spectroscopy provides further information regarding other trace elements and can therefore be advantageous in this aspect. This study also focused on XRF determination of Fe in the extracts and as no detectable Fe was found in the three types of ham extracts investigated (limit of detection; Fe⩽1.8ppm), it allows the conclusion that iron containing pigments, e.g., heme, do not contribute to the noticeable red colour observed in some of the extracts.