Estrella Fernández-García

Volatile compounds in fresh meats subjected to high pressure processing: Effect of the packaging material.

The effect of high pressure treatment (400MPa, 10min at 12°C) on the volatile profile of minced beef and chicken breast, packaged with or without aluminum foil in a multilayer polymeric bag, was investigated. The analysis of the volatile fraction was carried out by dynamic headspace extraction coupled to gas chromatography-mass spectrometry. Pressurization produced significant changes in the levels of some volatile compounds presumably coming from microbial activity. Some alcohols and aldehydes decreased, while other compounds, such as 2,3-butanedione and 2-butanone, were more abundant in high pressure processed meats. A significant migration of compounds from the plastic material was observed, mainly branched-chain alkanes and benzene compounds. Two functions built by the principal component analysis explained a high percentage of the variance and could be used to separate the samples into four distinct groups, according to high pressure treatment and packaging material.

Preservation of the microbiological and biochemical quality of raw milk by carbon dioxide addition: a pilot-scale study

Carbon dioxide treatment of refrigerated raw milk was evaluated as a method for extending storage life by inhibiting growth of psychrotrophic bacteria and other bacterial groups in raw milk. The effect of CO2 acidification followed by degasification and pasteurization on biochemical and microbiological properties of cold stored milk was studied on a pilot scale, Two CO2 treatments (acidification to pH 6.2 and to pH 6.0) were compared with a control (untreated) milk during 4 days of storage at 4°C. Total bacterial counts in the categories of milk established in this study were mainly determined by the proteolytic psychrotroph levels. The inhibitory capability of CO2 was greater in the low-quality than in the high-quality milk category. Acidification at pH 6.0 was more inhibitory than that at pH 6.2, especially against proteolytic psychrotrophs. Neither caseins nor whey proteins were affected by CO2 treatment and pasteurization. Organic acid (orotic, citric, uric, formic, acetic, propionic, and hippuric) concentrations did not change after CO2 treatment, cold storage, or the pasteurization process; the lactic acid content of CO2-treated milks remained constant during the refrigeration time but increased slightly in the control. In general, lower amounts of volatile compounds were produced in CO2-treated milks during refrigeration than in control milk. Ethanol and 2-propanol levels were most affected by degasification and pasteurization. Sensory evaluation revealed no significant differences between CO2-treated milk after degasification and pasteurization and the untreated milk used as control. It was concluded that degasification and pasteurization on a pilot scale eliminated CO2 from milk with minimum detrimental effects on its biochemical and sensory properties, making this process acceptable for milk preservation.

Formation of biogenic amines in raw milk Hispánico cheese manufactured with proteinases and different levels of starter culture.

Two proteinases, a neutral proteinase from Bacillus subtilis and a cysteine proteinase from Micrococcus sp., were used to accelerate the ripening process of raw cows milk Hispánico cheese, a semihard variety. Two levels (0.1% and 1%) of a commercial starter culture containing Lactococcus lactis subsp. lactis and L. lactis subsp. cremoris were added for cheese manufacture. The influence of both factors, proteinase addition and level of starter culture, on the growth of amino acid-decarboxylating microorganisms and on the formation of biogenic amines during cheese ripening was investigated in duplicate experiments. The population of tyrosine decarboxylase-positive bacteria, which represented less than 1% of the total bacterial population in most cheese samples, and tyrosine decarboxylase-positive lactobacilli was not influenced by proteinase addition or level of starter culture. Tyramine was detected in all batches of cheese from day 30. Its concentration was significantly (P < 0.05) influenced by proteinase addition but not by the level of starter culture and increased with cheese age. After 90 days of ripening, 103 to 191 mg/kg of tyramine was found in the different cheese batches. Histamine was not detected until day 60 in cheese with neutral proteinase and 1% starter culture and until day 90 in the rest of the cheeses. The concentration of this amine did not exceed 20 mg/kg in any of the batches investigated. Phenylethylamine and tryptamine were not found in any of the samples.

Caseinolysis in cheese by Enterobacteriaceae strains of dairy origin

Aims: To study the effect of Enterobacteriaceae strains of dairy origin on caseins under cheese manufacture and ripening conditions.

Formation of volatile compounds by wild Lactococcus lactis strains isolated from raw ewes’ milk cheese

The formation of volatile compounds in fresh cheese by 32 wild strains of Lactococcus lactis was investigated, and compared to volatile compounds formed by six collection strains of lactococci. Hierarchical cluster analysis of the relative abundances of volatile compounds determined by GC-MS was used for the classification of strains. Seven compounds (1-propanol, 2-propanol, 2-methyl propanol, 3-methyl butanol, 3-methyl butanal, 2,3-heptanedione and isoamyl acetate) separated the strains into two main branches, one of which contained all collection strains. The standard addition method was used to quantify these seven compounds in cheeses. All but 1-propanol and 2-propanol were at levels exceeding their respective odour thresholds. Sensory evaluation by a trained panel was carried out on cheeses. Abnormal odours were detected in cheeses made with 27 out of the 32 wild L. lactis strains. Odour quality of cheeses was positively correlated to 2-propanol, α-pinene and descriptors butter, yoghurt and fresh cheese, whereas odour intensity was positively correlated to 13 volatile compounds (six alcohols, two aldehydes, four ketones and one ester) and descriptors branched-chain volatiles, fruity and roasted hazel nuts.

Volatile compounds produced in cheese by Enterobacteriaceae strains of dairy origin

The formation of volatile compounds in fresh cheese by 10 Enterobacteriaceae strains of dairy origin (4 Hafnia alvei, 2 Serratia liquefaciens, 1 Enterobacter cloacae, 1 Enterobacter sakazakii, and 2 Escherichia coli strains) was investigated. Small cheeses were made from pasteurized cows milk separately inoculated with 1-3 x 10(3) CFU/ml of each of the Enterobacteriaceae strains, with glucono-8-lactone added to achieve a pH value of 5.2 in the curds. All strains reached counts close to 10(8) CFU/g in 1-day-old cheeses and survived well from day 1 to day 8. Cheeses were analyzed for volatile compounds by gas chromatography-mass spectroscopy, after extraction by dynamic headspace using a purge and trap apparatus. Sixty-one volatile compounds were determined in cheeses, 31 of which were further investigated. Significant increases of aldehydes, sulfur compounds, and aromatic compounds were recorded from 2-h curd to 1-day-old cheese, and of ketones, alcohols, and acids from 2-h curd to 8-day-old cheese. Acetaldehyde, 2-methyl propanal, and 3-methyl butanal predominated among aldehydes; 2,3-butanedione, 2,3-pentanedione, and 3-hydroxy 2-butanone among ketones; ethanol, 2-methyl propanol, and 3-methyl butanol among alcohols; and ethyl acetate among esters. Hierarchical cluster analysis of strains using the data of 31 volatile compounds separated clearly the strain of E. sakazakii, which produced high amounts of volatile compounds, from the other Enterobacteriaceae strains.

Enzyme accelerated ripening of Spanish hard cheese

Abstract The acceleration effect of lipolytic, proteolytic, and glycolytic enzymes on the ripening of a Spanish hard cheese made according to Manchego cheese manufacturing methods was studied. The results indicated that proteolytic enzymes caused substantial alterations in the nitrogen fractions. The addition of lactase resulted in early hydrolysis of lactose as well as a noticeable increase in the amino acid nitrogen fraction. The free fatty acid content increased in cheeses treated with lipases. The cheeses treated with lactase received the best taste panel rating, and a slightly bitter taste was detected in the cheeses treated with protease.

Volatile fraction and sensory characteristics of Manchego cheese. 2. Seasonal variation.

An automatic purge and trap apparatus, coupled to a GC-MS was used to study the seasonal variability of the volatile fraction of raw milk Manchego cheese. Both season and dairy significantly affected abundance of most volatile compounds. Most aldehydes, methyl ketones, n-alcohols, and secondary alcohols reached significantly higher concentrations in spring cheeses. Branched chain alcohols showed significantly higher concentrations in autumn and winter cheeses, while significantly higher amounts of diketones were found in summer cheeses. Most ethyl esters reached higher concentrations in spring and winter cheeses and lower in autumn cheeses. Lower concentrations of alpha-pinene were found in spring cheeses, and higher amounts of limonene were observed in winter cheeses. Heptane and octane were significantly more abundant in summer cheeses. No significant seasonal differences were found either for quality or intensity scores.

Volatile compounds in dry-cured Serrano ham subjected to high pressure processing. Effect of the packaging material

The effect of high pressure treatment (400MPa, 10min at 12°C) on the volatile profile of Spanish dry-cured Serrano ham, packaged with or without aluminum foil in a multilayer polymeric bag, was investigated. The analysis of the volatile fraction was carried out by dynamic headspace extraction coupled to gas chromatography-mass spectrometry. Pressure treatment only had a slight effect on the volatile fraction of Serrano ham. Most compounds affected by pressurization, such as alkanes (C(9)-C(12)), 2-undecene, 2-nonanone, 1-octen-3-one, 1-heptanol, 2-hexanol, 2-heptanol, ethyl pentanoate, benzaldehyde and styrene, presumably originated from the metabolism of moulds. A significant effect of pressurization on the migration of compounds from the plastic material was found. Linear and branched chain alkanes, alkenes as well as benzene compounds, were generally less abundant in pressurized samples than in untreated samples. A scalping effect was also observed for compounds such as butanal, pentanal, ethyl esters and pyrazines.

Proteolysis during ripening of Manchego cheese made from raw or pasteurized ewes' milk. Seasonal variation.

Changes in nitrogen compounds during ripening of 40 batches of Manchego cheese made from raw milk (24 batches) or pasteurized milk (16 batches) at five different dairies throughout the year were investigated. After ripening for six months, degradation of p-kappa- and beta-caseins was more intense in raw milk cheese and degradation of alpha(s2)-casein in pasteurized milk cheese. Milk pasteurization had no significant effect on breakdown of alpha(s1)-casein. Hydrophobic peptide content did not differ between raw and pasteurized milk cheese, whereas hydrophilic peptide content was higher in raw milk cheese. There were no significant differences between seasons for residual caseins, but hydrophobic peptides were at a higher level in cheese made in autumn and winter and hydrophilic peptides in cheese made in winter and spring. Raw milk cheese had a higher content of total free amino acids and of most individual free amino acids than pasteurized milk cheese. The relative percentages of the individual free amino acids were significantly different for raw milk and pasteurized milk cheeses. The relative percentages of Lys and lie increased, while those of Val, Leu and Phe decreased during ripening. There were also seasonal variations within the relative percentages of free amino acids. In raw milk cheeses, Asp and Cys were relatively more abundant in those made in autumn, Glu and Arg in cheeses made in winter, and Lys and Ile in cheeses made in spring and summer. Biogenic amines were detected only in raw milk cheese, with the highest levels of histamine, tryptamine and tyramine in cheeses made in spring, winter and spring, respectively.