Estimation of thermal diffusivity of foods using transfer functions

The effective thermal diffusivity of foodstuffs was estimated from time-temperature histories in the geometric center of samples exposed to heating and cooling processes.Transfer functions methodology was used as an alternative method to estimate the thermal diffusivity assuming that conduction was the main heat transfer mechanism. The samples were characterized as delayed first-order systems with unit gain, dead time (L) and time constant (τ).The results were compared with those obtained from the f_h value and with results reported in the literature.     

Performance evaluation of aluminum test cell designed for determining the heat resistance of bacterial spores in foods

Thermal inactivation kinetic studies are necessary to determine heat resistances of spores in the development of new thermal processes for low-acid shelf-stable products. Most currently available sample holders used for solid and semi solid samples in the kinetic studies take long time to reach the target sample temperature, hence fail to provide isothermal condition. In this research, novel aluminum test cells were developed to facilitate easy loading and unloading samples in a hermetically sealed 1 ml cavity to evaluate the heat resistance of bacterial spores when heated at temperatures above 100 °C. Design of the test cell was governed by minimum come-up time. A finite element model based on the commercial software ‘FEMLAB’ was used to simulate transient heat transfer and finalize the test cell dimensions. Performance of the new test cell was evaluated against capillary and aluminum thermal death time tube methods in characterizing the heat resistance of Clostridium sporogenes PA 3679 spores in a phosphate buffer and mashed potato at 121 °C. D₁₂₁ values of PA 3679 spores in both the phosphate buffer and mashed potato using the new test cells were not significantly different (P>0.05) from those by the capillary tube method. The results indicated that the new test cell is appropriate for studying the inactivation kinetics of bacterial spores in microbial validation of conventional and novel thermal processes for low-acid shelf-stable foods.     

Moisture diffusivity in pears: experimental determination and derivation of a mathematical prediction model

In the present work the dependence of moisture diffusivity on pear composition (in particular, water and sugar concentrations) was examined from the drying rate data measured experimentally. A model for predicting diffusivity that accounts for the effects of three variables (temperature, moisture content and sugar concentration) was then developed, and compared with the model presented by Zogzas and Maroulis (Drying Technology 1996;14:1543), which accounts only for the influence of temperature and moisture concentration. It was observed that the value for the activation energy predicted by the model that includes the influence of the sugar concentration is higher, showing that the amount of sugar present influences the diffusion of moisture through the pear. From the results obtained it was possible to conclude that at constant temperature the diffusivity is increased for higher water contents and lower sugar concentrations. Furthermore, it was possible to observe that higher temperatures have a much more pronounced influence on the diffusivity, compared with lower temperatures.     

Determination of expressions for the thermal diffusivity of canned foodstuffs by the inverse method and numerical simulations of heat penetration

This paper deals with the determination of expressions to estimate the thermal diffusivity of foodstuffs placed in cans during their pasteurisation. To this end, experimental data of the temperature in the geometric centre of the can were compared with data obtained by simulations of heat penetration. For these simulations, the diffusion equation was numerically solved by using the finite volume method, with fully implicit formulation. The numerical solution was coupled to an optimiser based on the inverse method, in order to fit this solution to the experimental data obtained during the pasteurisation of tomato puree and custard cream placed within sealed cans. In the literature, the assumption of constant thermal diffusivity during the pasteurisation is common but for the analysed cases such hypothesis resulted in biased fits. Better results were obtained supposing an increasing expression for the thermal diffusivity, as function of the local temperature. In this case, the errors of the experimental data about of the numerical solution can be considered randomly distributed.     

Development of a Versatile Database for Transport Properties of Foods

The requirement of reliable data on transport properties of foods has emerged in the last decade due to their importance in the design and simulation of food processing operations. An international effort to obtain and analyze the published data has been made recently, due to the lack of reliable theoretical predictive models. In this framework, a user-friendly database system for the storage and maintenance of the transport properties of foodstuffs is proposed and developed in a Microsoft Access environment. The system allows the end-user to input and retrieve data, as well as to view diagrams with single or comparative data and predict values via a fully featured user-friendly interface. Currently the database has registered over 250 articles, which correspond to approximately 1200 records for the thermal conductivity and 1780 records for moisture diffusivity. Due to its flexibility, the system can be used as an universal template for developing similar database systems.     

Development of a quantitative GC-FID method for the determination of sucrose mono- and diesters in foods

Sucrose esters (E 473) are emulsifiers used in foods to improve different technological properties. They should conform to the specifications laid down in Commission Regulation No. 231/2012 and be used at amounts not exceeding the maximal ones set by Commission Regulation No. 1129/2011. In order to be able to characterise commercial sucrose ester formulations and to evaluate whether they are used correctly by the food industry, a quantitative GC-FID method was developed. Standards of monoesters and diesters were isolated from commercial additive preparations because no commercial ones were available. Commercial sucrose monolaureate and in-house-synthesised sucrose diarachidonate were used as internal standards. The method showed limits of detection and quantification of 2.9 and 5.7 µg ml^–1 respectively for the monoesters and 42.8 and 129.7 µg ml^–1 respectively for the diesters. The analysed commercial additive formulations contained mainly mono- and diesters of palmitic and stearic acid with low amounts of free fatty acid and sucrose. Different food matrices were incurred with commercial sucrose esters formulations and recoveries ranged between 92% and 118% for the monoesters and between 77% and 120% for the diesters. Recovery of sucrose monoesters in cake was around 34% when no enzymatic treatment was applied, and about 64% when enzymatic treatment with Clara-Diastase was applied. This indicated that sucrose esters can interact strongly with the matrix during food production and that treatment with enzymes is essential to determine the esters’ content accurately in some classes of food products.     

Thermophysical properties and thermal behavior of leafy vegetables packaged in clamshells

Little is known about the thermophysical properties of fresh-cut lettuce other than heat of respiration. Empirical correlations based on food composition remain the only way to estimate the thermophysical properties of fresh-cut lettuce. The objectives of this study were (i) to determine the thermophysical properties of several baby-leaf lettuce and brassica greens and (ii) to verify the measured thermophysical properties by using them in a heat transfer model and comparing the predicted product temperatures with measured product temperatures in a simulated interruption of a cold chain. Density, leaf thickness, thermal conductivity, specific heat and water activity from nine varieties of baby-leaf lettuce and brassica greens were measured. A broken cold chain was simulated in a low temperature incubator set at 10 °C for a length of time before readjustment at 2 °C. Results showed that density (1078–1112 kg m⁻³), leaf thickness (0.18–0.54 mm), thermal conductivity (0.55–0.70 W (m °C)⁻¹) and specific heat (3.1–4.3 kJ (kg °C)⁻¹) varied significantly (P < 0.05) between varieties. However, no significant differences were observed for water activity (0.959 ± 0.006). Using thermophysical properties as input in the heat transfer model, experimental and calculated temperatures were well correlated (R² = 0.98) with a root mean square error of 0.57 °C over the 10–40 mg CO₂ (kg h)⁻¹ range of respiration rate. The measured thermophysical properties adequately predicted the temperature of the baby-leaf greens during simulated broken cold chains. A sensitivity analysis performed with the heat transfer model showed that the thermal conductivity, the specific heat and the density were relatively more important on the thermal behaviour of the baby-leaf greens than the heat of respiration.     

Comparison of heat transfer rates during thermal processing under end-over-end and axial modes of rotation

Thermal processing under conditions enabling container agitation are attractive to food processors since they provide higher rates of heat transfer and more uniform temperature due to product mixing. Two common types of agitations are end-over-end and axial agitation. While end-over-end agitation is common in batch retorts, axial agitation is predominant in continuous cookers in which the cans roll in a helical path along the retort shell. In this study, heat transfer rates to canned Newtonian fluids in end-over-end and axial modes of rotation were evaluated in the same retort. A single basket rotary retort was retrofitted to accommodate simultaneously both end-over-end and two types of axial modes of rotation, free axial and fixed axial. In the free axial rotation, the cans were allowed to rotate on their own axis as they rolled along the retort shell during the bottom third of the rotation, while in the fixed axial mode, the cans in the axial direction were held stationary in the rotating basket. The overall heat transfer coefficient U, heating rate index, lag factor and cook values associated with cans filled with Newtonian fluids were evaluated using a central composite rotatable design of experiments with glycerin at five concentration level (no glycerin to all glycerin), heating medium temperature (111.6-128.4 °C) and five basket-rotational speeds (4-20 rpm). Higher U, and lower heating rate index, lag factor and cook values were associated with higher rotational speed and retort temperatures for all modes of agitation. The heat transfer parameters associated with free axial rotation were significantly higher than in the end-over-end mode which was higher than in the fixed axial mode.     

An inverse lumped capacitance method for determination of heat transfer coefficients for industrial air blast chillers

An inverse model using lump body was developed to calculate heat transfer coefficients as a dynamic function of time. The model used sequential function specification algorithm to calculate surface heat flux from transient temperature measurements inside a lump body system. Transient temperature measurements were collected during cooling inside an industrial chiller at two different positions with different air velocities using four replicates for each position. The calculated surface heat flux was found to be very accurate as the maximum value of the root mean squares error (RMSE) for temperature is 0.045 °C, lower than the expected error form thermocouple measurements. The calculated heat flux was then used to calculate heat transfer coefficients as a dynamic function of cooling time followed by calculation of time average heat transfer coefficient using numerical integration. The approach developed here could be a pragmatic powerful dynamic method to model spatial variation of heat transfer coefficients for industrial chillers and freezers.     

Development of a quantitative GC-FID method for the determination of stearoyl-lactylates (E481/482) in foods

Sodium and calcium salts of stearoyl-lactylates (SLs) are food emulsifiers especially used in bread and bakery products to improve texture. They should be used at the lowest level at which the desired technological effect is achieved in a specific food category and at amounts not exceeding the maximums set by European Commission Regulation No. 1129/2011. In order to be able to evaluate whether these emulsifiers are used correctly but also to evaluate whether the commercial additive formulations comply with legislation, a quantitative GC-FID method was developed. An internal standard (nonadecanoyl-1-lactylate) was synthesized in-house and pure ester standards were isolated from commercial additive formulations. The method showed a limit of detection of 0.04 and a limit of quantification of 0.12 mg esters ml^?1. The commercial additive formulations analysed proved to be complex mixtures of free lactic and fatty acids together with only 50–60% esters. Besides SLs important amounts of palmitoyl-lactylates were present. Different food matrices (with low- and high-fat contents) were spiked with commercial SL formulations and recoveries ranged between 85% and 109%. Determination of SLs in commercial foods (such as bakery and bread) indicated that pre-treatment with amylase was essential to determine accurately the SL content due to the interaction of SL with the amylose.     

Development of a method for water determination in lactose

The determination of water content of lactose and the water activity, which is related to it, are important to assess its microbiological stability as well as its technical and commercial quality. Lactose is a very important product in the dairy sector, but also in the pharmaceutical domain.     

模拟移动床的简易参数设计方法

模拟移动床（SMB）色谱是一种高效的先进分离方法，在SMB分离操作设计中关键是其操作参数的确定。介绍了一种严密的、多功能的、通用的设计方法一安全因子法，就其原理进行了详细的阐述，并以此设计了一套SMB系统用于制糖工业的脱盐应用。     

An enzymatic method for the determination of fructans in foods and food products

 We report a new and non-equipment demanding method of measuring the content of fructans as well as the contents of free glucose, free fructose and sucrose in foods and food products enzymatically. This method comprises hydrolysis of fructans into d-glucose and d-fructose enzymatically and measurement of the released sugars enzymatically. Sucrose is hydrolysed by α-glucosidase instead of β-fructosidase, which is normally used. In addition, sucrose is measured in the form of d-fructose instead of the typical d-glucose form, and the fructanase used to hydrolyse the fructans has fewer side effects than the fructanase reported as normally used. The method is tested on ten standard substances and five fructan products, and nine foods and food products are also analysed. The enzymatic measurement of the released sugars is confirmed by measurements done by high performance anion exchange chromatography with pulsed amperemetric detection. Received: 1 March 1999 / Revised version: 23 April 1999     

Improved sample treatment and chromatographic method for the determination of isoflavones in supplemented foods

Isoflavones are a group of substances that belong to the family of phytoestrogens. These natural substances may offer several benefits to human health. One of the most important sources for human isoflavone intake is soy and soybean food derivatives. An improved sample treatment followed by a high performance liquid chromatographic method for the determination of isoflavones in supplemented milk and juices is proposed and compared to the AOAC official method. Detection limits found were between 0.2 and 0.3 mg L⁻¹ for daidzein and genistein respectively. Quantification limits found were between 0.7 mg L⁻¹ for daidzein to 1.0 mg L⁻¹ for genistin, while inter and intra-day variability was under 10% in all cases. Recoveries for spiked samples were over 90% and under 110%. The method was validated by comparison with the AOAC method and by recovery assay methodology.     

A rapid and simple method for the determination of glycogen in yeast

A rapid, simple, adaptable and widely applicable procedure for the determination of glycogen in yeast is described. The method exploits the known fact that an aqueous solution of iodine/potassium iodide will stain yeast cells. The intensity of staining has been shown to be proportional to the amount of glycogen present in samples of yeast representing four strains of Saccharomyces cerevisiae. Therefore levels of yeast glycogen can be determined quantitatively by measurement of absorbance, or approximated by visual examination of the yeast-iodine suspension. Such a convenient procedure should enable the brewer (i) to compensate for variations in the glycogen content between different batches of pitching yeast, thereby achieving greater reproducibility in fermentations, and (ii) to monitor changes in yeast composition during storage as a basis for developing improved yeast handling procedures.     

测汞仪固体直接进样测定保健食品中总汞的方法研究及方法比较

目的 研究测汞仪固体直接进样方法测定保健食品中总汞,通过与国标方法比较验证结果的可靠性.方法 采用测汞仪固体直接进样和原子荧光光谱法两种方法对保健食品中的总汞进行测定.结果 几种不同浓度加标的保健食品测汞仪测定的总汞结果与原子荧光法测定结果相符,回收率在88％～116％之间,RSD＜ 10％.结论 测汞仪直接固体进样测定保健食品中总汞的方法具有准确、快捷、运行成本低、无环境污染等优点,和现有国标原子荧光法测定同一样品,测定结果具有可比性,方法具有可推广性.     

Development of a sensitive polyclonal antibody-based competitive indirect ELISA for determination of citrinin in grain-based foods

ABSTRACT

A sensitive competitive indirect enzyme-linked immunosorbent assay (ciELISA) was developed for the detection and quantification of citrinin (CIT) in grain-based food samples. The limit of quantification (IC₂₀) of the established method was 0.10 ± 0.02 ng mL^?1, with the limit of detection (IC₁₀) being 0.04 ± 0.007 ng mL^?1 in wheat and corn flour matrices with a coefficient of variation (CV) less than 20%. The assay was very specific to CIT and showed no cross-reactivity with other mycotoxins (OTA, T-2 toxin, HT-2 toxin, DON, patulin and zearalenone). In spiked wheat and corn flours, the recoveries ranged from 86.6% to 115.6% with CVs of less than 20%. The effectiveness of this method was verified by participating in a proficiency test (PT) from the Food Analysis Performance Assessment Scheme (FAPAS) 17181 corn flour. A successful z-score (?0.6) for this PT sample showed that the present method is comparable to the instrumental methods used by other laboratories in the PT testing scheme. A small survey of grain-based foods was conducted using this method and CIT was detected in 43% of the samples up to a concentration of 17.7 ng g^?1. This method is suitable for sensitive and rapid quantitation of citrinin in wheat and corn matrices.