Pablo Nieto - Academia.edu (original) (raw)
Papers by Pablo Nieto
Science, 2006
The Born-Oppenheimer approximation of uncoupled electronic and nuclear motion is a standard tool ... more The Born-Oppenheimer approximation of uncoupled electronic and nuclear motion is a standard tool of the computational chemist. However, its validity for molecule-metal surface reactions, which are important to heterogeneous catalysis, has been questioned because of the possibility of electron-hole pair excitations. We have performed experiments and calculations on the scattering of molecular hydrogen from a catalytically relevant metal surface, obtaining absolute probabilities for changes in the molecule's velocity parallel to the representative Pt(111) surface. The comparison for in-plane and out-of-plane scattering and results for dissociative chemisorption in the same system show that for hydrogen-metal systems, reaction and diffractive scattering can be accurately described using the Born-Oppenheimer approximation.
Physical Review Letters, 2004
We have measured in-plane and out-of-plane diffraction of H2 and D2 molecular beams scattered by ... more We have measured in-plane and out-of-plane diffraction of H2 and D2 molecular beams scattered by reactive Pd(111) and nonreactive NiAl(110) surfaces at 140-150 meV. A comparison with six-dimensional quantum dynamics and classical trajectory calculations shows for the first time that accurate diffraction patterns can be obtained from state-of-the-art potential energy surfaces based on density functional theory. Our measurements show that, at general incidence conditions, out-of-plane diffraction is much more important than was assumed in previous experiments.
Chemical Physics Letters, 2004
We have measured the angular distribution of H 2 molecules with incident energies 100-150 meV ref... more We have measured the angular distribution of H 2 molecules with incident energies 100-150 meV reflected by the Pd(1 1 1) surface at 430 K. In spite of the low diffraction reflectivity ($0.5%), diffraction peaks have been resolved at nearly grazing incidence. Apart from specular reflection, out-of-plane diffraction has been found to be dominant, in agreement with classical simulations. We show that this behavior is a generic one for grazing incidence and discuss the relevance of this finding for the study of molecule/surface dynamics in systems for which dissociation is a non activated process.
Physical Review Letters, 2004
We have measured in-plane and out-of-plane diffraction of H2 and D2 molecular beams scattered by ... more We have measured in-plane and out-of-plane diffraction of H2 and D2 molecular beams scattered by reactive Pd(111) and nonreactive NiAl(110) surfaces at 140-150 meV. A comparison with six-dimensional quantum dynamics and classical trajectory calculations shows for the first time that accurate diffraction patterns can be obtained from state-of-the-art potential energy surfaces based on density functional theory. Our measurements show that, at general incidence conditions, out-of-plane diffraction is much more important than was assumed in previous experiments.
Chemical Physics Letters, 2004
We have measured the angular distribution of H 2 molecules with incident energies 100-150 meV ref... more We have measured the angular distribution of H 2 molecules with incident energies 100-150 meV reflected by the Pd(1 1 1) surface at 430 K. In spite of the low diffraction reflectivity ($0.5%), diffraction peaks have been resolved at nearly grazing incidence. Apart from specular reflection, out-of-plane diffraction has been found to be dominant, in agreement with classical simulations. We show that this behavior is a generic one for grazing incidence and discuss the relevance of this finding for the study of molecule/surface dynamics in systems for which dissociation is a non activated process.
European Food Research and Technology, 2005
The effect of calcium chloride addition used in combination with starter cultures (lactic acid ba... more The effect of calcium chloride addition used in combination with starter cultures (lactic acid bacteria and staphylococci) was studied in the development of dry-fermented sausage aroma and final sensory acceptance. Volatile compounds were extracted by solid-phase micro extraction and analysed by gas chromatography-mass spectrometry. A paired comparison test was done to determine which sausage was preferred. The addition of calcium chloride 0.05% showed an important effect on volatile generation during ripening by inhibiting the generation of lipid oxidation products and promoting the generation of methyl branched alcohols and ethyl esters that contribute to proper sausage aroma. However, other compounds such as 3-methyl-butanoic acid and acetic acid, detected in high quantities in the 0.05% calcium added batch, may impart rancid and cheese-like notes that can negatively affect the final aroma. Moreover, the amount of calcium chloride used is important because larger amounts (0.5%) favoured the lipid oxidation producing a rejection from consumers.
Food Chemistry, 1993
Muscle lipases and esterases were assayed for activity at different stages ((~ 15 months) in the ... more Muscle lipases and esterases were assayed for activity at different stages ((~ 15 months) in the processing of dry-cured hams. The evolution of volatile and nonvolatile free fatty acids, during processing, was also determined. All the assayed enzymes were found to be quite stable and active even after 15 months of processing. Most lipolysis seemed to occur during the first 5 months when maximal generation of free fatty acids was detected. Neutral and basic lipases are more active at the beginning of the process, while lysosomal acid lipases are active through the entire process. These enzymes seem to be the main ones responsible for the observed lipolysis. Muscle esterases showed excellent stability, but the amounts of volatile free fatty acids were very low, suggesting only a minor role for these enzymes. The study indicates that lipolysis plays a role in the ripening of Spanish dry-cured ham.
European Food Research and Technology, 1998
The effect of nitrite and nitrate curing salts on lipolysis in a typical non-smoked Spanish saus... more The effect of nitrite and nitrate curing salts on lipolysis in a typical non-smoked Spanish sausage (salchichón), manufactured using a rapid dry fermentation process, was studied. From the results obtained, one could deduce that nitrite and nitrate per se do not affect the endogenous hydrolysis of the lipids during the rapid fermentation process used to produce dry sausage. Polyunsaturated fatty acids (PUFA) are liberated in greater quantities than monounsaturated (MUFA) and saturated (SFA) ones, which suggests that they are derived mainly from phospholipids. The samples that contained nitrate curing salts contained more free fatty acids (FFA) (548 mg/100 g) than those that contained nitrite curing salts (478 mg/100 g) at the end of the fermentation step, as a consequence of having undergone a preripening stage, during which significant amounts of FFA are formed. However, at the end of the ripening process, although there is a significant increase of FFA levels in all the sausages, no significant differences between samples with nitrite and those with nitrate could be found. This result could have occurred if the oxidative effects of the samples with nitrate balanced the antioxidative properties of nitrite, such that, through different processes, similar concentrations of FFA are obtained at the end of ripening. Phospholipids (PL) and free fatty acids (FFA) are the lipid fractions that undergo the most important changes during ripening. PL are moderately degraded during the drying process, probably due to the combined effects of the drying temperature (10°C) and the presence of starters. However, this degradation does not differ significantly between the samples that contain nitrite and and those that contain nitrate in each control.
Food Science and Technology International, 2001
Two batches of dry-cured fermented sausage were made by industrial methods at pilot scale. The di... more Two batches of dry-cured fermented sausage were made by industrial methods at pilot scale. The difference between them was the drying temperature applied (5 or 12°C) for a period of time extending from the moment when pH = 5 was attained until a weight loss of 20% was achieved. The other conditions were the same for the two processes. Changes in pH, total acidity, D-and L-lactic acids, acetic acid, total volatile basic nitrogen (TVBN), free amino acids (FAA) and free fatty acids (FFA) were studied, and a discriminatory sensory analysis of the ready-to-eat product was carried out. Only L-lactic acid and total acidity within the acidity parameters showed substantial differences halfway through the drying process. No significant differences were found in any of the acidity variables at the end of the process. The drying temperature encouraged the production of TVBN, but no direct relationship was established between it and the quantity of FAA. The three fractions of FFA [SFA (saturated fatty acids), MUFA (monounsaturated fatty acids) and PUFA (polyunsaturated fatty acids)] increased during the curing period, always higher at 12°C than at 5°C throughout the process. No differences between the two processes were detected by the discriminatory sensory analysis.
European Food Research and Technology, 1997
This paper deals with the influence of industrial practices such as use of starters (Lactobacill... more This paper deals with the influence of industrial practices such as use of starters (Lactobacillus sake and Staphylococcus carnosus), preripening (3 days at 5°C) and drying temperatures (8 and 16°C) on proteolysis, ammonia production and the taste of “salchichón”, a traditional Spanish dry-cured sausage lacking surface mould. pH dropped more sharply in the presence of a starter and at a high drying temperature (16°C); preripening did not affect the pH. The highest value for non-protein nitrogen (NPN), 16% of total nitrogen, was observed to occur in samples with the lowest pH (4.7–4.8). Total volatile basic nitrogen (TVBN) increased during drying due to the deaminase activity of internal microbial flora, which was not great (0.5–1.0% of total nitrogen) enough to affect the pH, and significantly in samples showing a larger decrease of pH. The greater proteolysis in samples with starter did not affect the assessors’ taste preference. Assessors clearly preferred those samples without starter that were dried at low temperature (8°C); however, samples with starter showed excellent commercial appearance since they dried more homogeneously. An equilibrium point must be found between acid production and taste.
International Journal of Food Microbiology, 1997
The effect of the use of either nitrite or nitrate curing salts on microbial changes and sensory ... more The effect of the use of either nitrite or nitrate curing salts on microbial changes and sensory quality in rapid ripened sausages inoculated with a mixed starter culture (Lactobacillus sake and Staphylococcus carnosus) was investigated. Lactic acid bacteria and Micrococcaceae were not greatly affected by the added curing salt. Conversely, the inhibition exerted by nitrite on the undesirable flora (Enterobacteriaceae and psychrotrophs) was evident from the early stages of the processing keeping highly significant differences (P , 0.01) with respect to nitrate made sausages till the end of the ripening stage. The use of nitrite in sausage processing was found to reduce hygienic risks. © 1997 Elsevier Science B. V.
Meat Science, 1988
Melting point, slip point, iodine value (fast analyses) and percentage contents of fatty acids we... more Melting point, slip point, iodine value (fast analyses) and percentage contents of fatty acids were determined in subcutaneous fat of hams from Iberian pigs in order to study the possibilities of characterizing the type of pig feeding and the relationships between the results of fast analysis and fatty acid contents. Both types of analyses showed important differences between samples from pigs fed on acorn and samples from pigs fed on mixed feeds. When a combination of feeding materials (acorn and mixed feeds) was used, the analyses gave intermediate results, closer to those obtained with mixed feeds only. On the other hand, generally high simple and multiple correlation coefficients (greater than 0·85) were obtained between fast analyses and fatty acid contents, which allows accurate predictions using regression and canonical correlation analyses.
European Food Research and Technology, 2001
The purpose of this work was to study the effect of nitrate and nitrite curing salts on FFA gene... more The purpose of this work was to study the effect of nitrate and nitrite curing salts on FFA generation and oxidation during an industrial process of country-style dry sausages (35/40 mm casing diameter) to determine their role in flavour development. The pH values of the sausages are around 6.0 during the dry processing. FFA liberation is two or three times greater in the samples with nitrite than in those with nitrate. However, the intensity of the oxidative phenomena during the drying process does not depend on the quantity of FFA. In the samples with nitrate more intense oxidative phenomena take place, which lead to a stronger typical dry-cured flavour than in samples with nitrite.
Science, 2006
The Born-Oppenheimer approximation of uncoupled electronic and nuclear motion is a standard tool ... more The Born-Oppenheimer approximation of uncoupled electronic and nuclear motion is a standard tool of the computational chemist. However, its validity for molecule-metal surface reactions, which are important to heterogeneous catalysis, has been questioned because of the possibility of electron-hole pair excitations. We have performed experiments and calculations on the scattering of molecular hydrogen from a catalytically relevant metal surface, obtaining absolute probabilities for changes in the molecule's velocity parallel to the representative Pt(111) surface. The comparison for in-plane and out-of-plane scattering and results for dissociative chemisorption in the same system show that for hydrogen-metal systems, reaction and diffractive scattering can be accurately described using the Born-Oppenheimer approximation.
Physical Review Letters, 2004
We have measured in-plane and out-of-plane diffraction of H2 and D2 molecular beams scattered by ... more We have measured in-plane and out-of-plane diffraction of H2 and D2 molecular beams scattered by reactive Pd(111) and nonreactive NiAl(110) surfaces at 140-150 meV. A comparison with six-dimensional quantum dynamics and classical trajectory calculations shows for the first time that accurate diffraction patterns can be obtained from state-of-the-art potential energy surfaces based on density functional theory. Our measurements show that, at general incidence conditions, out-of-plane diffraction is much more important than was assumed in previous experiments.
Chemical Physics Letters, 2004
We have measured the angular distribution of H 2 molecules with incident energies 100-150 meV ref... more We have measured the angular distribution of H 2 molecules with incident energies 100-150 meV reflected by the Pd(1 1 1) surface at 430 K. In spite of the low diffraction reflectivity ($0.5%), diffraction peaks have been resolved at nearly grazing incidence. Apart from specular reflection, out-of-plane diffraction has been found to be dominant, in agreement with classical simulations. We show that this behavior is a generic one for grazing incidence and discuss the relevance of this finding for the study of molecule/surface dynamics in systems for which dissociation is a non activated process.
Physical Review Letters, 2004
We have measured in-plane and out-of-plane diffraction of H2 and D2 molecular beams scattered by ... more We have measured in-plane and out-of-plane diffraction of H2 and D2 molecular beams scattered by reactive Pd(111) and nonreactive NiAl(110) surfaces at 140-150 meV. A comparison with six-dimensional quantum dynamics and classical trajectory calculations shows for the first time that accurate diffraction patterns can be obtained from state-of-the-art potential energy surfaces based on density functional theory. Our measurements show that, at general incidence conditions, out-of-plane diffraction is much more important than was assumed in previous experiments.
Chemical Physics Letters, 2004
We have measured the angular distribution of H 2 molecules with incident energies 100-150 meV ref... more We have measured the angular distribution of H 2 molecules with incident energies 100-150 meV reflected by the Pd(1 1 1) surface at 430 K. In spite of the low diffraction reflectivity ($0.5%), diffraction peaks have been resolved at nearly grazing incidence. Apart from specular reflection, out-of-plane diffraction has been found to be dominant, in agreement with classical simulations. We show that this behavior is a generic one for grazing incidence and discuss the relevance of this finding for the study of molecule/surface dynamics in systems for which dissociation is a non activated process.
European Food Research and Technology, 2005
The effect of calcium chloride addition used in combination with starter cultures (lactic acid ba... more The effect of calcium chloride addition used in combination with starter cultures (lactic acid bacteria and staphylococci) was studied in the development of dry-fermented sausage aroma and final sensory acceptance. Volatile compounds were extracted by solid-phase micro extraction and analysed by gas chromatography-mass spectrometry. A paired comparison test was done to determine which sausage was preferred. The addition of calcium chloride 0.05% showed an important effect on volatile generation during ripening by inhibiting the generation of lipid oxidation products and promoting the generation of methyl branched alcohols and ethyl esters that contribute to proper sausage aroma. However, other compounds such as 3-methyl-butanoic acid and acetic acid, detected in high quantities in the 0.05% calcium added batch, may impart rancid and cheese-like notes that can negatively affect the final aroma. Moreover, the amount of calcium chloride used is important because larger amounts (0.5%) favoured the lipid oxidation producing a rejection from consumers.
Food Chemistry, 1993
Muscle lipases and esterases were assayed for activity at different stages ((~ 15 months) in the ... more Muscle lipases and esterases were assayed for activity at different stages ((~ 15 months) in the processing of dry-cured hams. The evolution of volatile and nonvolatile free fatty acids, during processing, was also determined. All the assayed enzymes were found to be quite stable and active even after 15 months of processing. Most lipolysis seemed to occur during the first 5 months when maximal generation of free fatty acids was detected. Neutral and basic lipases are more active at the beginning of the process, while lysosomal acid lipases are active through the entire process. These enzymes seem to be the main ones responsible for the observed lipolysis. Muscle esterases showed excellent stability, but the amounts of volatile free fatty acids were very low, suggesting only a minor role for these enzymes. The study indicates that lipolysis plays a role in the ripening of Spanish dry-cured ham.
European Food Research and Technology, 1998
The effect of nitrite and nitrate curing salts on lipolysis in a typical non-smoked Spanish saus... more The effect of nitrite and nitrate curing salts on lipolysis in a typical non-smoked Spanish sausage (salchichón), manufactured using a rapid dry fermentation process, was studied. From the results obtained, one could deduce that nitrite and nitrate per se do not affect the endogenous hydrolysis of the lipids during the rapid fermentation process used to produce dry sausage. Polyunsaturated fatty acids (PUFA) are liberated in greater quantities than monounsaturated (MUFA) and saturated (SFA) ones, which suggests that they are derived mainly from phospholipids. The samples that contained nitrate curing salts contained more free fatty acids (FFA) (548 mg/100 g) than those that contained nitrite curing salts (478 mg/100 g) at the end of the fermentation step, as a consequence of having undergone a preripening stage, during which significant amounts of FFA are formed. However, at the end of the ripening process, although there is a significant increase of FFA levels in all the sausages, no significant differences between samples with nitrite and those with nitrate could be found. This result could have occurred if the oxidative effects of the samples with nitrate balanced the antioxidative properties of nitrite, such that, through different processes, similar concentrations of FFA are obtained at the end of ripening. Phospholipids (PL) and free fatty acids (FFA) are the lipid fractions that undergo the most important changes during ripening. PL are moderately degraded during the drying process, probably due to the combined effects of the drying temperature (10°C) and the presence of starters. However, this degradation does not differ significantly between the samples that contain nitrite and and those that contain nitrate in each control.
Food Science and Technology International, 2001
Two batches of dry-cured fermented sausage were made by industrial methods at pilot scale. The di... more Two batches of dry-cured fermented sausage were made by industrial methods at pilot scale. The difference between them was the drying temperature applied (5 or 12°C) for a period of time extending from the moment when pH = 5 was attained until a weight loss of 20% was achieved. The other conditions were the same for the two processes. Changes in pH, total acidity, D-and L-lactic acids, acetic acid, total volatile basic nitrogen (TVBN), free amino acids (FAA) and free fatty acids (FFA) were studied, and a discriminatory sensory analysis of the ready-to-eat product was carried out. Only L-lactic acid and total acidity within the acidity parameters showed substantial differences halfway through the drying process. No significant differences were found in any of the acidity variables at the end of the process. The drying temperature encouraged the production of TVBN, but no direct relationship was established between it and the quantity of FAA. The three fractions of FFA [SFA (saturated fatty acids), MUFA (monounsaturated fatty acids) and PUFA (polyunsaturated fatty acids)] increased during the curing period, always higher at 12°C than at 5°C throughout the process. No differences between the two processes were detected by the discriminatory sensory analysis.
European Food Research and Technology, 1997
This paper deals with the influence of industrial practices such as use of starters (Lactobacill... more This paper deals with the influence of industrial practices such as use of starters (Lactobacillus sake and Staphylococcus carnosus), preripening (3 days at 5°C) and drying temperatures (8 and 16°C) on proteolysis, ammonia production and the taste of “salchichón”, a traditional Spanish dry-cured sausage lacking surface mould. pH dropped more sharply in the presence of a starter and at a high drying temperature (16°C); preripening did not affect the pH. The highest value for non-protein nitrogen (NPN), 16% of total nitrogen, was observed to occur in samples with the lowest pH (4.7–4.8). Total volatile basic nitrogen (TVBN) increased during drying due to the deaminase activity of internal microbial flora, which was not great (0.5–1.0% of total nitrogen) enough to affect the pH, and significantly in samples showing a larger decrease of pH. The greater proteolysis in samples with starter did not affect the assessors’ taste preference. Assessors clearly preferred those samples without starter that were dried at low temperature (8°C); however, samples with starter showed excellent commercial appearance since they dried more homogeneously. An equilibrium point must be found between acid production and taste.
International Journal of Food Microbiology, 1997
The effect of the use of either nitrite or nitrate curing salts on microbial changes and sensory ... more The effect of the use of either nitrite or nitrate curing salts on microbial changes and sensory quality in rapid ripened sausages inoculated with a mixed starter culture (Lactobacillus sake and Staphylococcus carnosus) was investigated. Lactic acid bacteria and Micrococcaceae were not greatly affected by the added curing salt. Conversely, the inhibition exerted by nitrite on the undesirable flora (Enterobacteriaceae and psychrotrophs) was evident from the early stages of the processing keeping highly significant differences (P , 0.01) with respect to nitrate made sausages till the end of the ripening stage. The use of nitrite in sausage processing was found to reduce hygienic risks. © 1997 Elsevier Science B. V.
Meat Science, 1988
Melting point, slip point, iodine value (fast analyses) and percentage contents of fatty acids we... more Melting point, slip point, iodine value (fast analyses) and percentage contents of fatty acids were determined in subcutaneous fat of hams from Iberian pigs in order to study the possibilities of characterizing the type of pig feeding and the relationships between the results of fast analysis and fatty acid contents. Both types of analyses showed important differences between samples from pigs fed on acorn and samples from pigs fed on mixed feeds. When a combination of feeding materials (acorn and mixed feeds) was used, the analyses gave intermediate results, closer to those obtained with mixed feeds only. On the other hand, generally high simple and multiple correlation coefficients (greater than 0·85) were obtained between fast analyses and fatty acid contents, which allows accurate predictions using regression and canonical correlation analyses.
European Food Research and Technology, 2001
The purpose of this work was to study the effect of nitrate and nitrite curing salts on FFA gene... more The purpose of this work was to study the effect of nitrate and nitrite curing salts on FFA generation and oxidation during an industrial process of country-style dry sausages (35/40 mm casing diameter) to determine their role in flavour development. The pH values of the sausages are around 6.0 during the dry processing. FFA liberation is two or three times greater in the samples with nitrite than in those with nitrate. However, the intensity of the oxidative phenomena during the drying process does not depend on the quantity of FFA. In the samples with nitrate more intense oxidative phenomena take place, which lead to a stronger typical dry-cured flavour than in samples with nitrite.