Rapid Inverse Method to Measure Thermal Diffusivity of Low‐Moisture Foods

Thermal diffusivity is an important transport property needed in modeling and computations of transient heat transfer in basic food processing operations. In addition, the prediction of nutritional and microbial changes occurring in food during thermal processing requires knowledge of thermal diffusivity of foods. The objectives of this study were to develop a new nonisothermal and nonlinear determination method of thermal diffusivity and to measure the thermal diffusivity of low‐moisture foods using that new method. Thermal diffusivities of 5 kinds of low‐moisture foods (almond meal, corn meal, wheat flour, chocolate fudge, and peanut butter) were estimated using an inverse technique. Samples were canned and heated at the surface in a water bath at about 70 °C. The 1‐dimensional transient heat conduction problem for radial coordinates was solved with a finite‐difference model. The thermal diffusivity of each of the 5 samples was determined by the ordinary least squares and sequential estimation methods, respectively. Predicted and observed temperature matched well, with maximum residuals of 0.9 °C. The thermal diffusivity values of the samples ranged from 9.8 × 10⁻⁸ to 1.3 × 10⁻⁷ m²/s. The advantages of this method are that the device and the estimation method are simple, inexpensive, rapid, and can handle large spatial temperature gradients, such as those experienced during heating of low‐moisture foods. The results obtained in this study will be useful in the design of equipment and in calculations for the thermal processing of low‐moisture foods.     

Water Diffusion from a Bacterial Cell in Low‐Moisture Foods

We used a Fick's unsteady state diffusion equation to estimate the time required for a single spherical shaped bacterium (assuming Enterococcus faecium as the target microorganism) in low‐moisture foods to equilibrate with the environment. We generated water sorption isotherms of freeze‐dried E. faecium. The water activity of bacterial cells at given water content increased considerably as temperature increased from 20 to 80 °C, as observed in the sorption isotherms of bacterial cells. When the water vapor diffusion coefficient was assumed as between 10^?12 and 10^?10 m²/s for bacterial cells, the predicted equilibration times (t_eq) ranged from 8.24×10^?4 to 8.24×10^?2 s. Considering a cell membrane barrier with a lower water diffusion coefficient (10^?15 m²/s) around the bacterial cell with a water diffusion coefficient of 10^?12 m²/s, the t_eq predicted using COMSOL Multiphysics program was 3.8×10^?1 s. This result suggests that a single bacterium equilibrates rapidly (within seconds) with change in environmental humidity and temperature.     

Computational Fluid Dynamics Approaches in Quality and Hygienic Production of Semisolid Low‐Moisture Foods: A Review of Critical Factors

Low‐moisture foods have been responsible for a number of salmonellosis outbreaks worldwide over the last few decades, with cross contamination from contaminated equipment being the most predominant source. To date, actions have been focused on stringent hygienic practices prior to production, namely periodical sanitization of the processing equipment and lines. Not only does optimum sanitization require in‐depth knowledge on the type and source of contaminants, but also the heat resistance of microorganisms is unique and often dependent on the heat transfer characteristics of the low‐moisture foods. Rheological properties, including viscosity, degree of turbulence, and flow characteristics (for example, Newtonian or non‐Newtonian) of both liquid and semisolid foods are critical factors impacting the flow behavior that consequently interferes heat transfer and related control elements. The demand for progressively more accurate prediction of complex fluid phenomena has called for the employment of computational fluid dynamics (CFD) to model mass and heat transfer during processing of various food products, ranging from drying to baking. With the aim of improving the quality and safety of low‐moisture foods, this article critically reviewed the published literature concerning microbial survival in semisolid low‐moisture foods, including chocolate, honey, and peanut butter. Critical rheological properties and state‐of‐the‐art CFD application relevant to quality production of those products were also addressed. It is anticipated that adequate prediction of specific transport properties during optimum sanitization through CFD could be used to solve current and future food safety challenges.     

等温水分活度及其对低水分食品中微生物抗热性的影响研究进展

水分活度（Water activity,a_w）是定义低水分食品（a_{w,25 ℃}<0.85）的关键指标,对低水分食品（LMFs）中微生物的抗热性起着决定性的作用。食品的水分活度受到食品组分、质构等的影响,并且与温度密切相关,在温度改变时呈现不同的变化趋势,是典型的温度依赖型指标。等温水分活度（a_w,T）反映了食品在高温灭菌过程中的实时水分活度,其准确测定或推算有助于学者量化低水分食品中微生物抗热性指标,进而设计、验证、完善相关热灭菌工艺。但商业化的水分活度仪只能测量低温区间（20~50 ℃）,人们对高温区间（>55 ℃）的水分活度还了解甚少。本文首先介绍了低水分食品微生物安全及其抗热性指标,然后对等温水分活度的定义、检测方法、模型预测、影响因素及其对微生物抗热性的影响研究进行综述。等温水分活度的认知与测定有利于食品企业降低低水分食品的微生物污染程度,保障消费者食品安全。     

Effect of Water Activity on Thermal Inactivation of Salmonella in Ground Turkey

Many factors, such as fat content and pH, are known to affect thermal inactivation of pathogens in meat products, and a few studies have suggested that the humidity of the cooking environment also affects thermal inactivation. However, the effect of process humidity has not been previously isolated from the effect of water activity on Salmonella inactivation. Therefore, the objective of this study was to directly test the effect of meat water activity on thermal inactivation of Salmonella. Ground turkey was dried to achieve water activities of 0.95 to 0.99, inoculated with an 8‐strain Salmonella cocktail, and heated isothermally (60 °C) in a water bath. The rate of thermal inactivation of Salmonella decreased 64% (P < 0.01) when decreasing meat water activity from 0.99 to 0.95. Inclusion of water activity improved the accuracy of a 1st‐order/Arrhenius‐type inactivation model from 1.94 log₁₀ to 0.8 log₁₀ (colony‐forming units [CFU]/g).     

Sodium Chloride Diffusion in Low‐Acid Foods during Thermal Processing and Storage

This study aimed at modeling sodium chloride (NaCl) diffusion in foods during thermal processing using analytical and numerical solutions and at investigating the changes in NaCl concentrations during storage after processing. Potato, radish, and salmon samples in 1% or 3% NaCl solutions were heated at 90, 105, or 121 °C for 5 to 240 min to simulate pasteurization and sterilization. Selected samples were stored at 4 or 22 °C for up to 28 d. Radish had the largest equilibrium NaCl concentrations and equilibrium distribution coefficients, but smallest effective diffusion coefficients, indicating that a greater amount of NaCl diffused into the radish at a slower rate. Effective diffusion coefficients determined using the analytical solution ranged from 0.2 × 10^–8 to 2.6 × 10^?8 m²/s. Numerical and analytical solutions showed good agreement with experimental data, with average coefficients of determination for samples in 1% NaCl at 121 °C of 0.98 and 0.95, respectively. During storage, food samples equilibrated to a similar NaCl concentration regardless of the thermal processing severity. The results suggest that sensory evaluation of multiphase (solid and liquid) products should occur at least 14 d after processing to allow enough time for the salt to equilibrate within the product.     

Correlation between Moisture and Water Activity of Honeys Harvested in Different Years

ABSTRACT: For the 1st time, a relationship has been investigated between moisture percentage (M) and water activity (aW) in honeys from 2 different harvesting years and 2 different climatic areas. Moisture was determined by measuring refractive index at 20°C, according to official methods. At the same time, aW of the same samples was assessed at exactly 20°C. Influence on the relationship of the crop's year, aging, and induced fine granulation was evaluated. The results show that, independent of all factors studied, an excellent and statistically significant linear correlation between both parameters [aW = 0.3114 + (0.0170 × M)] was found in all samples with a regression coefficient of 0.8809. The measurement of aW of honey is interesting but time-consuming. This study concludes that this parameter can be easily calculated from the moisture percentage, thus avoiding the use of expensive and slow equipment for aW measurement.     

Thermal Properties of Ration Components as Affected by Moisture Content and Water Activity During Freezing

ABSTRACT: Beef roast with vegetables is an example of a meal, ready‐to‐eat (MRE) ration entrée. It is a mixture of meat, potato, mushroom, and carrot with a gravy sauce. The thermal properties of each component were characterized in terms of freezing point, latent heat, freezable and unfreezable water contents, and enthalpy during freezing using differential scanning calorimetry. Freezing and thawing curves and the effect of freezing and thawing cycles on thermal properties were also evaluated. The freezing points of beef, potato, mushroom, and sauce were all in the range of −5.1 to −5.6 °C, but moisture content, water activity, latent heat, freezable and unfreezable water contents, and enthalpy varied among these components. Freezing temperature greatly affected the unfrozen water fraction. The unfreezable water content (unfrozen water fraction at −50 °C) of ration components was in the range of 8.2% to 9.7%. The freezing and thawing curves of vegetables with sauce differed from those of beef but took similar time to freeze or thaw. Freezing and thawing cycles did not greatly affect the thermal properties of each component. Freezing point and latent heat were reduced by decreasing moisture content and water activity of each component. Water activity was proportionally linear to freezing point at a_w > 0.88, and moisture content was proportionally linear to freezable water content in all ration components. Water was not available for freezing when moisture content was reduced to 28.8% or less. This study indicates that moisture content and water activity are critical factors affecting thermal behavior of ration components during freezing.     

Mitochondrial DNA Fragmentation as a Molecular Tool to Monitor Thermal Processing of Plant‐Derived,Low‐Acid Foods,and Biomaterials

Cycle threshold (Ct) increase, quantifying plant‐derived DNA fragmentation, was evaluated for its utility as a time‐temperature integrator. This novel approach to monitoring thermal processing of fresh, plant‐based foods represents a paradigm shift. Instead of using quantitative polymerase chain reaction (qPCR) to detect pathogens, identify adulterants, or authenticate ingredients, this rapid technique was used to quantify the fragmentation of an intrinsic plant mitochondrial DNA (mtDNA) gene over time‐temperature treatments. Universal primers were developed which amplified a mitochondrial gene common to plants (atp1). These consensus primers produced a robust qPCR signal in 10 vegetables, 6 fruits, 3 types of nuts, and a biofuel precursor. Using sweet potato (Ipomoea batatas) puree as a model low‐acid product and simple linear regression, Ct value was highly correlated to time‐temperature treatment (R² = 0.87); the logarithmic reduction (log CFU/mL) of the spore‐forming Clostridium botulinum surrogate, Geobacillus stearothermophilus (R² = 0.87); and cumulative F‐value (min) in a canned retort process (R² = 0.88), all comparisons conducted at 121 °C. D₁₂₁ and z‐values were determined for G. stearothermophilus ATCC 7953 and were 2.71 min and 11.0 °C, respectively. D₁₂₁ and z‐values for a 174‐bp universal plant amplicon were 11.3 min and 9.17 °C, respectively, for mtDNA from sweet potato puree. We present these data as proof‐of‐concept for a molecular tool that can be used as a rapid, presumptive method for monitoring thermal processing in low‐acid plant products.     

High‐Pressure Thermal Sterilization: Food Safety and Food Quality of Baby Food Puree

The benefits that high‐pressure thermal sterilization offers as an emerging technology could be used to produce a better overall food quality. Due to shorter dwell times and lower thermal load applied to the product in comparison to the thermal retorting, lower numbers and quantities of unwanted food processing contaminants (FPCs), for example, furan, acrylamide, HMF, and MCPD‐esters could be formed. Two spore strains were used to test the technique; Geobacillus stearothermophilus and Bacillus amyloliquefaciens, over the temperature range 90 to 121 °C at 600 MPa. The treatments were carried out in baby food puree and ACES‐buffer. The treatments at 90 and 105 °C showed that G. stearothermophilus is more pressure‐sensitive than B. amyloliquefaciens. The formation of FPCs was monitored during the sterilization process and compared to the amounts found in retorted samples of the same food. The amounts of furan could be reduced between 81% to 96% in comparison to retorting for the tested temperature pressure combination even at sterilization conditions of F₀‐value in 7 min.     

Microbial Safety and Shelf Life of UV‐C Treated Freshly Squeezed White Grape Juice

The effects of UV‐C irradiation on the inactivation of Escherichia coli K‐12 (ATCC 25253), a surrogate of E. coli O157:H7, and on the shelf life of freshly squeezed turbid white grape juice (FSWGJ) were investigated. FSWGJ samples were processed at 0.90 mL/s for 32 min by circulating 8 times in an annular flow UV system. The UV exposure time was 244 s per cycle. The population of E. coli K‐12 was reduced by 5.34 log cycles after exposure to a total UV dosage of 9.92 J/cm² (1.24 J/cm² per cycle) at 0.90 mL/s flow rate. The microbial shelf life of UV‐C treated FSWGJ was extended up to 14 d at 4 °C. UV exposure was not found to alter pH, total soluble solid, and titratable acidity of juice. There was a significant effect (P < 0.05) on turbidity, absorbance coefficient, color, and ascorbic acid content. Furthermore, all physicochemical properties were altered during refrigerated storage. The microbial shelf life of FSWGJ was doubled after UV‐C treatment, whereas the quality of juice was adversely affected similarly observed in the control samples.     

Formation of Food‐Grade Nanoemulsions Using Low‐Energy Preparation Methods: A Review of Available Methods

There is considerable interest in the production of emulsions and nanoemulsions using low‐energy methods due to the fact they are simple to implement and no expensive equipment is required. In this review, the principles of isothermal (spontaneous emulsification and emulsion phase inversion) and thermal (phase inversion temperature) low‐energy methods for nanoemulsion production are presented. The major factors influencing nanoemulsion formation using low‐energy methods and food‐grade components are reviewed: preparation conditions, oil type, surfactant type, surfactant‐to‐oil ratio, and cosolvent or cosurfactant addition. The advantages and disadvantages of different low‐energy and high‐energy methods for fabricating nanoemulsions are highlighted, and potential applications for these techniques are discussed.     

Efficacy of Combined Sous Vide‐Microwave Cooking for Foodborne Pathogen Inactivation in Ready‐to‐Eat Chicory Stems

There is a variety of different food processing methods, which can be used to prepare ready‐to‐eat foods. However, the need to preserve the freshness and nutritional qualities leads to the application of mild technologies which may be insufficient to inactivate microbial pathogens. In this work, fresh chicory stems were packed under a vacuum in films, which were transparent to microwaves. These were then exposed to microwaves for different periods of time. The application of sous vide microwave cooking (SV‐MW, 900 W, 2450 MHz), controlled naturally occurring mesophilic aerobic bacteria, yeasts and molds for up to 30 d when vacuum‐packed vegetables were stored at 4 °C. In addition, the process lethality of the SV‐MW 90 s cooking was experimentally validated. This treatment led to 6.07 ± 0.7 and 4.92 ± 0.65 log cfu/g reduction of Escherichia coli and Listeria monocytogenes inoculated over the chicory stems (100 g), respectively. With an initial load of 9 log cfu/g for both pathogens, less than 10 cfu/g of surviving cells were found after 90 s cooking. This shows that short‐time microwave cooking can be used to effectively pasteurize vacuum‐packed chicory stems, achieving >5 log cfu/g reduction of E. coli and L. monocytogenes.     

Influence of the Thermal Stabilization Process on the Volatile Profile of Canned Tomato‐Based Food

The literature contains several papers dealing with the volatile constituents contributing to the aroma of fresh and processed tomatoes. Along with the traditional tomato‐based products, tomato‐based pâtés, characterized by complex ingredient formulations, are commonly consumed as a seasoning for pasta, and as a dressing for meats, salads, and sandwiches. To the best of our knowledge, no investigations have been published on the influence of thermal stabilization treatments on the composition of volatile compounds in tomato‐based pâtés. To this aim, thermally stabilized and not stabilized tomato‐based pâtés were subjected to the analysis of volatile compounds. The results obtained highlighted the influence of the thermal stabilization process on the evolution of volatile composition tomato‐based pâtés. In particular, the terpenic compounds showed significant decreases after the thermal stabilization process treatment, due to their degradation and oxidation favored by high temperatures. The thermal stabilization caused, moreover, an increase in volatile compounds deriving from lipid oxidation and Maillard reaction, characterized by low‐sensory thresholds, and from the thermal degradation of carotenoids and fresh tomato‐derived compounds.     

Model for Combined Effects of Temperature, pH and Water Activity on Thermal Inactivation of Bacillus cereus Spores

Initially, the effect of water activity (a_w) on heat resistance of Bacillus cereus spores (decimal reduction time) was investigated. A linear relationship was found between log D and 1-a_w. The combined effects of temperature (85-105°C), pH (4.5-6.5) and water activity (0.80-1) were then studied. A four parameter model was fitted to the data. This model appeared to be parsimonious with each parameter having a biological significance. Interactions between factors were observed but they accounted for <2.4% of the total variation and they were not taken into account by the model.     

Meta‐Analysis for Correlating Structure of Bioactive Peptides in Foods of Animal Origin with Regard to Effect and Stability

Amino acid (AA) sequences of 807 bioactive peptides from foods of animal origin were examined in order to correlate peptide structure with activity (antihypertensive, antioxidative, immunomodulatory, antimicrobial, hypolipidemic, antithrombotic, and opioid) and stability in vivo. Food sources, such as milk, meat, eggs, and marine products, show different frequencies of bioactive peptides exhibiting specific effects. There is a correlation of peptide structure and effect, depending on type and position of AA. Opioid peptides contain a high percentage of aromatic AA residues, while antimicrobial peptides show an excess of positively charged AAs. AA residue position is significant, with those in the first and penultimate positions having the biggest effects on peptide activity. Peptides that have activity in vivo contain a high percentage (67%) of proline residues, but the positions of proline in the sequence depend on the length of the peptide. We also discuss the influence of processing on activity of these peptides, as well as methods for predicting release from the source protein and activity of peptides.     

Effect of Heat‐Moisture Treatment on Structural and Thermal Properties of Rice Starches Differing in Amylose Content

Starches from glutinous rice (1.4% amylose), Jasmine rice (15.0% amylose) and Chiang rice (20.2% amylose) were exposed to heat‐moisture treatment (HMT) at 100 °C for 16 h and at different moisture levels (18, 21, 24 and 27%). The effect of heat‐moisture treatment on structural and thermal properties of these three rice starches was investigated. The HMT did not change the size, shape and surface characteristics of rice starch granules. The A‐type crystalline pattern of rice starches remained unchanged after HMT. The relative crystallinity (RC) and the ratio of short‐range molecular order to amorphous (RSA) of heat‐moisture treated glutinous and Jasmine rice starches decreased with increasing moisture level of the treatments. In contrast, the RC of the treated Chiang rice starch remained practically unchanged. A peak of crystalline V‐amylose‐lipid complexes was clearly presented in all treated Chiang rice starches. The peak became progressively stronger with increasing moisture level of the treatment. Differential scanning calorimetry (DSC) of all treated rice starches showed a shift of the gelatinization temperature to higher values. Increasing moisture level of the treatments increased the onset gelatinization temperature (T_o) but decreased the gelatinization enthalpy (ΔH) of rice starches. A broad gelatinization temperature range (T_c‐T_o) with a biphasic endotherm was found for all treated Chiang rice starches and Jasmine rice starch after HMT₂₇ (HMT at 27% moisture level). Additionally the (T_c‐T_o) of treated Chiang rice starches increased linearly with increasing moisture level of the treatments.