Classification of Brazilian honeys by physical and chemical analytical methods and low field nuclear magnetic resonance (LF H NMR)

This study evaluated the possibility of differentiating the botanical origin of honeys using Low Field Nuclear Magnetic Resonance (LF ¹H NMR). Eighty samples of honey from 8 different botanical sources (eucalyptus, “assa-lipto”, oranges, Barbados cherry, cashew tree, “assa-peixe”, “cipó-uva” and polyfloral) were analyzed. A close correlation (p < 0.01) was established between the LF ¹H NMR analysis and physical and chemical measurements, including water content, water activity, pH and color. Bi-exponential fitting of the transverse relaxation (T₂) data revealed two water populations in all samples, T₂₁ and T₂₂, corresponding to relaxation times of 0.6–1.8 ms and 2.3–5.4 ms respectively. The observed differences in the relaxation times suggest that these were influenced by the differences in botanical origins. Good linear correlations were observed between the T₂ and T₂₁ parameters and the physical and chemical data. This study demonstrated that LF ¹H NMR can be a viable technique for use in classifying honeys by their botanical origin.     

The effects of pre-salting methods on water distribution and protein denaturation of dry salted and rehydrated cod - A low-field NMR study

Low field Nuclear Magnetic Resonance (LF-NMR) relaxation time measurements were used to evaluate the effect of different pre-salting methods (brine injection of salt and/or phosphates followed by brining, solely brining, pickling and kench salting) on the protein denaturation and change in muscle properties during the production steps of dry salted cod fillets followed by rehydration. The NMR relaxation curves were affected by the salting method and represented well the structural differences between the salting methods at each processing step. Significant correlations were observed between the NMR relaxation parameters and all physicochemical quality properties measured, except the cooking yield, when samples from all processing stages were analyzed together. The longitudinal relaxation time T₁, and the faster relaxing transverse relaxation time T₂₁ were shown to be especially sensitive to protein denaturation in the fillets. The water distribution indicated that the salting and rehydration processes changed the cells irreversibly. The study indicated that pre-brining by brine injection followed by brining, with low salt concentrations, led to the least protein denaturation during the dry salting and rehydration process.     

Monitoring changes in feta cheese during brining by magnetic resonance imaging and NMR relaxometry

Magnetic resonance imaging (MRI) and NMR relaxometry were used to monitor changes in feta cheese during 169 h of brining at 4.8%, 13.0% and 23.0% salt solutions. Image and relaxation data were acquired to study salt uptake and water loss due to dehydration of cheese during brining. Saturation recovery and Carr-Purcell-Meiboom-Gill (CPMG) sequences were used to determine the longitudinal relaxation (T₁) and the transverse relaxation (T₂) times, respectively. Signal intensities of T₂ weighted images decreased during 169 h of brining. An excellent linear correlation between the average signal intensity and the water content was obtained (R² = 0.984). The T₁ values of cheese brined at 4.8% were almost constant but T₁ values decreased for both 13.0% and 23.0% salt brined cheeses. Analysis of the CPMG decays gave relaxation spectra containing two components which decreased during brining. The short component T_2a was highly correlated with salt content (R² = 0.974). Results showed that NMR and MRI can be used to follow salt uptake and changes in water content in cheese during brining.     

Ageing-induced solubility loss in milk protein concentrate powder: effect of protein conformational modifications and interactions with water

BACKGROUND: Protein conformational modifications and water‐protein interactions are two major factors believed to induce instability of protein and eventually affect the solubility of milk protein concentrate (MPC) powder. To test these hypotheses, MPC was stored at different water activities (a_w 0.0–0.85) and temperatures (25 and 45 °C) for up to 12 weeks. Samples were examined periodically to determine solubility, change in protein conformation by Fourier transform infrared (FTIR) spectroscopy and water status (interaction of water with the protein molecule/surface) by measuring the transverse relaxation time (T₂) with proton nuclear magnetic resonance (¹H NMR). RESULTS: The solubility of MPC decreased significantly with ageing and this process was enhanced by increasing water activity (a_w) and temperature. Minor changes in protein secondary structure were observed with FTIR which indicated some degree of unfolding of protein molecules. The NMR T₂ results indicated the presence of three distinct populations of water molecules and the proton signal intensity and T₂ values of proton fractions varied with storage condition (humidity) and ageing. CONCLUSION: Results suggest that protein/protein interactions may be initiated by unfolding of protein molecules that eventually affects solubility. Copyright © 2011 Society of Chemical Industry     

Low-field NMR relaxation and NMR-imaging as tools in differentiation between potato sample and determination of dry matter content in potatoes

Correlations between dry matter content and parameters determined by low-field nuclear magnetic resonance (NMR) relaxation and T₁ weighted magnetic resonance imaging (MR-imaging) were investigated in four raw potato varieties using partial least-squares regression. The low-field NMR relaxation parameters T₁(r=−0.85) and full Carr–Purcell–Meiboom–Gill (CPMG) relaxation curves (r=0.95) were correlated with the dry matter content of the potatoes. Furthermore, the amplitudes M₂₁ and M₂₂ of the two characteristic transverse relaxation (T₂) time constants T₂₁ and T₂₂ obtained upon bi-exponential analysis of CPMG relaxation curves, were also highly correlated with the dry matter content (r=−0.66 and 0.78, respectively). MR-images showed very large visual differences between tissues, varieties and maturity. However, the applied image analysis features based on the histogram analysis were not correlated with dry matter content in whole potatoes in the present study. Thus, MR-imaging did not complement the correlations between NMR data and dry matter content. This means that the way the spatial distribution of water in the material was determined did not cover the full information about dry matter content. Therefore, the image analysis of MR-images requires more attention in the future.     

Water dynamics in white bread and starch gels as affected by water and gluten content

Three levels of water (180, 220 and 260 g/345 g flour) and three levels of gluten (giving 11.2, 13.0 and 14.2 g protein/100 g flour) were used to study the effects of water and gluten on water mobility in white bread using ¹H nuclear magnetic resonance (NMR) relaxometry. Changes in the transverse relaxation time (T₂) were related to water mobility. The three water levels resulted in three different moisture contents in the finished bread (0.55-0.77 g H₂O/g solids). Distributed exponential analysis showed two distinct regions of T₂ (30-600 μs and 1-60 ms), associated with multiple domains of water in the bread crumb. There was no significant difference in peak T₂ values with different gluten content, but significant differences were observed with different moisture content. The results suggested that the mobility of water associated with starch decreased dramatically because of gelatinization. To further investigate the effect of gluten on starch gelatinization, NMR measurements were made directly on model systems containing starch and various gluten amounts. The starch-gluten gels had higher T₂ values than pure starch gels, indicating less swelling of starch granules and absorption of water. This was attributed to less water available to starch in the presence of gluten.     

蒸煮时间对滩羊肉蒸煮损失、嫩度及水分分布的影响

为了探究蒸煮时间对滩羊肉蒸煮损失、嫩度及水分分布的影响。对不同蒸煮时间处理下的滩羊肉蒸煮损失、剪切力进行测定,并结合低场核磁技术研究滩羊肉水分子T_2弛豫特性以及各指标间的相关性。结果表明:随着蒸煮时间的延长,滩羊肉的蒸煮损失和剪切力先增大后减小(P0.05)。LF-NMR检测到了4个明显的水分群,代表肉中的弱结合水(T_(21))、强结合水(T_(22))、不易流动水(T_(23))和自由水(T_(24))4种存在状态。延长蒸煮时间,弛豫时间先变长后缩短,T_(23)含量先减少后增加,T24含量先增加后减少。核磁成像结果显示,蒸煮时间的延长会使图像的亮度先增强后减弱。相关性分析结果表明,蒸煮损失与T_(23)、T_(24)呈显著正相关(P0.05),相关系数分别为0.966,0.994;剪切力与T_(23)呈显著正相关(P0.05),相关系数为0.846,与T23峰面积比例呈显著负相关(P0.05),相关系数为-0.504。说明不同蒸煮时间处理下的滩羊肉的蒸煮损失、嫩度及水分分布均具有显著的变化,且各指标之间具有显著的相关性。     

Protein denaturation and water-protein interactions as affected by low temperature long time treatment of porcine longissimus dorsi

The relationship between water–protein interactions and heat-induced protein denaturation in low temperature long time (LTLT) treated pork Longissimus dorsi was investigated by combining low-field NMR T₂ relaxometry with DSC measurements and measures of shrinkage of porcine Longissimus dorsi heated to 53 °C, 55 °C, 57 °C and 59 °C for either 3 or 20 h. Water within the myofibrils, measured by NMR T₂₁ relaxation times, was affected by both temperature and holding time during LTLT treatment between 53 °C and 59 °C. The changes in NMR T₂₁ relaxation times were associated with decreased fiber diameter and increased cooking loss, revealing a relationship between transverse shrinkage, water–protein interactions and cooking loss. DSC measurements revealed a concomitant decrease in ΔH_68 °C, which suggests impact of collagen denaturation on the retention of water within the meat during LTLT treatment. Furthermore, a decrease in ΔH_75 °C suggested that prolonged cooking (20 h) resulted in actin denaturation leading to decreased T₂₁ relaxation times and higher cooking loss.     

Direct measurement of phase transitions in milk fat during cooling of cream—a low-field NMR approach

Low-field ¹H nuclear magnetic resonance (NMR) transverse relaxation (T₂) was measured during cooling (31–4 °C) of cream with low or high content of long-chain fatty acids (FA). Distributed analysis of the T₂ relaxation data revealed marked differences in the T₂ relaxation characteristics of the liquid fat population (10–100 ms), which was ascribed to differences in the size of the fat globules in the two types of cream. Principal component analysis (PCA) of the obtained T₂ relaxation decay data showed clear shifts in NMR relaxation behavior at 17 and 22 °C for cream with low or high content of long-chain FA, respectively. Differential scanning calorimetric measurements on the creams revealed that these shifts took place at the onset of crystallization of fat. Consequently, the present study demonstrated that low-field NMR can be used to measure phase transitions in cream.     

Effect of gluten content on recrystallisation kinetics and water mobility in wheat starch gels

The effect of gluten on starch retrogradation at 5 °C was studied using ¹H NMR relaxometry. Gels were made from gluten and starch at 27.8 and 38.5% total solids and with gluten comprising either 10, 15 or 20% of the solids. Changes in the transverse relaxation time constant (T₂) were related to water mobility. Mono‐exponential analysis of relaxation curves showed that, in general, gluten retarded starch retrogradation. T₂ values in gluten gels also decreased during storage, but to a much lesser extent. Distributed exponential analysis showed that two distinct regions of T₂ were observed in all samples. During aging, the peak values of both regions shifted to lower values for all gels. Starch gel samples showed the most significant shift, and gluten gels showed the least. The three levels of gluten addition in starch/gluten gels produced similar shifts. For all samples the signal intensity of the less mobile region decreased more dramatically than that of the more mobile region during storage. It was suggested that gluten retards water loss in the granule remnants. Copyright © 2004 Society of Chemical Industry     

Water distribution in smoked salmon

Low‐field ¹H nuclear magnetic resonance (NMR) tranverse relaxations were measured on intact and minced muscle from raw and smoked salmon differing in size (small or large) and storage days after smoking (1, 11 and 20 days). Water distribution in the samples was calculated from the NMR signals by the use of multi‐way data analysis. For intact and minced salmon samples, three water ‘pools’ with T₂ values of 38, 50 and 182 ms and of 33, 50 and 183 ms, respectively, were identified. For both intact and minced samples, the size of water pool I and II were affected by storage time after smoking, whereas the size of pool III was affected by the size of the fish. Intact fillet samples taken near the skin and near the middle of the fillet differed with respect to shape of the NMR relaxation signal reflecting a non‐uniform distribution of the size of the three water pools in intact fillet. Copyright © 2006 Society of Chemical Industry     

Prediction of postharvest dry matter, soluble solids content, firmness and acidity in apples (cv. Elshof) using NMR and NIR spectroscopy: a comparative study

Dry matter (DM), soluble solids content (SSC), firmness and acidity by proton nuclear magnetic resonance (NMR) T₂ relaxometry and near infrared (NIR) spectroscopy were investigated on a total of 390 apples (cv. Elshof). The fruit came from four different pre- or postharvest treatments and covered a large range of DM (11.4–20.0 %) and SSC values (10.5–18.3 °Brix). NIR was superior in predicting DM (R ² = 0.82) and SSC (R ² = 0.80), compared to NMR (R ² = 0.50 and R ² = 0.58). However, NMR relaxometry was able to detect multiple water populations assigned to different water pools in the apples and variation in the water distribution between different pre- and postharvest treatments. Differences in the mobility of the vacuole water (population T₂₄) were consistent with changes in fruit firmness. In conclusion, even though NIR is superior in predicting DM and SSC, NMR provides useful information about the intrinsic water state and its distribution in the fruit.     

NaCl-induced changes in structure and water mobility in potato tissue as determined by CLSM and LF-NMR

This study for the first time reports the combined use of low-field nuclear magnetic resonance (LF-NMR) and confocal laser scanning microscopy (CLSM) in the study of the effects of salting on low and high dry matter (DM) potato tissue. The simultaneous use of CLSM and NMR resulted in important information in relation to the interpretation of the origin of the NMR water populations. Salting caused the raw potato cells to loose weight, which in the microscopic images was observed as loss of turgor pressure still further away from the edges of the samples with increased salting time. The total water loss after salting was lowest for high DM potatoes. The LF-NMR analyses revealed that this could be ascribed to a faster T₂ relaxation time of the cytoplasmatic and extracellular water and thus to water being more restricted in high DM potatoes. In addition, a tendency to a faster T₂ relaxation time was observed for both low and high DM potatoes with increasing salting time, which reveals a salt-induced restriction of the cytoplasmatic and extracellular water population. The paper illustrates the aptitude of NMR and CLSM to determine and elucidate structural changes and associated changes in water mobility in potato tissue.     

1H NMR studies of molecular mobility in wheat starch

Pulsed ¹H NMR was used to determine moisture content and molecular mobility of wheat starch suspensions at water activity (a_w) values between 0 and 0.93. Moisture determination by NMR agreed well with gravimetric determinations (R²=0.99). Transverse proton relaxation (T₂) of the systems were identified using single pulse and CPMG experiments. T₂ associated with starch molecules increased linearly from 7.1 to 10.1 μs as a_w increased from 0 to 0.93, due to plasticizing effects of water. T₂ associated with water molecules showed a linear increase up to a_w of 0.69. Results indicated that ¹H NMR could be employed to determine moisture content of starch suspension and to identify and quantify the molecular motion of starch chains and water molecules of starch suspensions at low moisture levels.     

Monitoring cheese ripening by single-sided nuclear magnetic resonance

The noninvasive, longitudinal study of products and food processing is of interest for the dairy industry. Here, we demonstrated that single-sided nuclear magnetic resonance (NMR) can be used for noninvasive monitoring of the cheese ripening process. The maturation of soft-ripened Camembert-like molded cheese samples was monitored for 20 d measuring 1-dimensional and 2-dimensional NMR relaxation and diffusion data at various depths, ranging from the hard surface layer to the soft center. Gelation and gel shrinkage were observed throughout ripening, and a complete loss of free water signal was observed at the cheese rind. Transversal (T₂) relaxation distributions include 3 components that evolve with ripening time and position, corresponding to water inside the casein gel network, water trapped in casein, and fat. Two-dimensional T₁-T₂ relaxation experiments provided enhanced resolution of the 3 components, allowing quantification of the relative proportions of each phase. Furthermore, diffusion (D)-T₂ relaxation correlation experiments revealed the bimodal size distribution of fat globules. The study demonstrated that single-sided NMR can provide spatially resolved signal intensity, relaxation, and diffusion parameters that reflect structural changes during the ripening process and can be exploited to understand and monitor the ripening of cheeses.     

Use of Magnetic Resonance Imaging for monitoring Parma dry-cured ham processing

Protocols were developed to apply Magnetic Resonance Imaging (MRI) to the dry-curing of Italian Parma ham. NMR relaxation analyses were performed on dry-cured hams at different processing stages to evaluate the ranges of variation of ¹H relaxation times T₁ and T₂ in representative ham muscle tissues, due to dehydration and salt uptake. MRI maps of the same ham sections were acquired, allowing T₁ and T₂ average values to be computed in selected Regions of Interest (ROI) inside muscle Semimembranosus, Semitendinosus and Biceps femoris. Chloride and moisture were determined by conventional chemical methods on the same ROIs, and MRI T₁ and T₁/T₂ ratio were selected in a model (R² = 0.90, P < 0.05) fitting the salt content of the analysed muscle cores. Short Time Inversion Recovery (STIR) sequences were also applied to green and cured hams, but on fresh samples only, a bright image, displaying a clear separation between lean and fat tissue, was obtained.     

Proton NMR relaxation study of swelling and gelatinisation process in rice starch–water samples

Proton transverse magnetization decay curves of rice flour starch–water samples were measured and analysed for the presence of four components in the relaxation curve. T₂ values were interpreted on the basis of the diffusive and chemical exchange model that provided evidence for extra granular bulk water and three more water populations whose relaxation rate is governed by diffusive and chemical exchange with starch components. The analysis of relaxation data provided information on dynamics of water molecules as well as on the size and dispersion of diffusive domains. Furthermore, by measuring solid to liquid ratio, transverse and longitudinal relaxation curves of starch–water mixtures at increasing temperatures – from 20 to 77 °C – swelling and gelatinisation processes were monitored.     

Original article: Characterisation of water species revealed in the drying operation of Todarodes pacificus Steenstrup using water proton NMR analysis

For the discrimination of water molecules during the squid‐drying process, the water distribution was characterised by water proton NMR and moisture diffusivity (De) analysis methods as a function of the water content (W₀). The proton NMR spectrum showed three peaks indicating three different species (species‐A, ‐B, and ‐C) distributed in the squid muscle, each of which had a characteristic behaviour of the relaxation time (T₂) as a function of the W₀. The 1/T₂ of species‐A was drastically varied at W₀ = 120%‐d.b., indicating two further categories, i.e., species‐A₁ and ‐A₂. Species‐A₁ is available at W₀ > 120%‐d.b. and was characterised as having De = 5.1 × 10^?10 m² s^?1, activation energy of moisture diffusivity (E_D) = 17 kJ mol^?1, and relaxation rate 1/T₂ = 74 s^?1, as evaluated by the proton NMR spectrum without depending on W₀. Species‐A₂ is available at W₀ < 120%‐d.b., indicating a distribution of De = 4.8 × 10^?10–1.7 × 10^?10 m² s^?1, E_D = 25–35 kJ mol^?1 and 1/T₂ = 1.8 × 10³–1.5 × 10² s^?1 with increasing W₀. Species‐A₁ and ‐A₂ were assigned as weakly restricted water and strongly restricted water, respectively.     

Water molecular dynamics during bread staling by Nuclear Magnetic Resonance

Bread staling is a complex phenomenon that originates from multiple physico-chemical events (amylopectin retrogradation, water loss and redistribution) that are not yet completely elucidated. Molecular properties of white bread loaves were characterized by multiple proton Nuclear Magnetic Resonance (NMR) techniques (proton FID, T₂ and T₁ relaxation time) over 14 days of storage. Changes at a molecular level (faster decay of proton FIDs and shifting of proton T₂ relaxation times distributions towards shorter times), indicating a proton mobility reduction of the bread matrix, were observed during storage. Multiple ¹H T₂ populations were observed and tentatively associated to water-gluten and water-starch domains. Proton T₁ of bread was for the first time measured at variable frequencies (Fast Field Cycling NMR) and found to be strongly dependent upon frequency and to decrease in bread during storage, especially at frequencies ≤ 0.2 MHz. An additional proton T₁ population, relaxing at 2 ms, was detected at 0.52 MHz only at early storage times and tentatively attributed to a water-gluten domain that lost mobility during storage.     

The effect of salt and fibre direction on water dynamics,distribution and mobility in pork muscle: A low field NMR study

The effect of salt concentration and fibre orientation on water within the meat matrix was investigated by low-field nuclear magnetic resonance (LF-NMR), water-binding capacity (WBC), diffusion studies and histological analysis. Pork M. longissimus thoracis et lumborum samples were cured with 5.7, 15.3 or 26.3% w/w NaCl at a parallel or perpendicular fibre direction. NMR transverse (T₂) relaxation identified three water components (T_2b, T₂₁ and T₂₂) which all exhibited characteristics correlated to WBC. Results indicated that T_2b increases with increasing NaCl concentration. Increasing intra-myofibrillar water and decreasing extra-myofibrillar water resulted in the highest WBC. Water diffused more quickly into the extra-myofibrillar space in samples cured at a parallel fibre direction. This water remained loosely bound in samples cured with the saturated solution (26.3% w/w NaCl) leading to decreased WBC. This study provides further information on water binding within the meat matrix by applying the results of LF-NMR to traditional water-binding theories.