Influence of Water Activity on Thermal Resistance of Microorganisms in Low‐Moisture Foods: A Review

A number of recent outbreaks related to pathogens in low‐moisture foods have created urgency for studies to understand the possible causes and identify potential treatments to improve low‐moisture food safety. Thermal processing holds the potential to eliminate pathogens such as Salmonella in low‐moisture foods. Water activity (a_w) has been recognized as one of the primary factors influencing the thermal resistance of pathogens in low‐moisture foods. But most of the reported studies relate thermal resistance of pathogens to a_w of low‐moisture foods at room temperature. Water activity is a thermodynamic property that varies significantly with temperature and the direction of variation is dependent on the product component. Accurate methods to determine a_w at elevated temperatures are needed in related research activities and industrial operations. Adequate design of commercial thermal treatments to control target pathogens in low‐moisture products requires knowledge on how a_w values change in different foods at elevated temperatures. This paper presents an overview of the factors influencing the thermal resistance of pathogens in low‐moisture foods. This review focuses on understanding the influence of water activity and its variation at thermal processing temperature on thermal resistance of pathogens in different low‐moisture matrices. It also discusses the research needs to relate thermal resistance of foodborne pathogens to a_w value in those foods at elevated temperatures.     

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.     

谷物湿热平衡新模型及热力学特性的研究

建立谷物平衡水分与相对湿度在不同温度下的关系模型,并利用小麦、玉米、水稻、大米的静态平衡试验数据进行拟合。结果表明,该方程适合描述小麦、玉米、水稻、大米的等温线,可直接求解出谷物的平衡水分。另外,根据所建立的模型,预测了不同温度条件下的安全储藏水分,并对谷物的热力学性能进行了分析:吸着等热值随着平衡水分值的减小而增加,解吸等热值增加更为显著;解吸和吸附过程中吸着等热值均高于水汽化潜热;平衡水分大于25%后,吸着等热值趋于稳定,与汽化潜热值接近;根据分析所得数据,拟合了小麦、稻谷、玉米解吸与吸附过程的吸着等热曲线回归方程,为以能量化的观念实施粮仓作业管理,实现高效、合理、节能储粮提供了有利依据。     

6种常用饲料原料平衡水分吸附等温线研究

通过研究6种常用饲料原料（小麦粉、米糠、棉粕、芝麻粕、玉米蛋白粉、DDGS）的平衡水分吸附等温线,评价了修正Halsey（MHAE）、修正 Henderson（MHE）、修正Chung-Pfost（MCPE）、修正3参数GAB（MGAB）、修正Oswin（MOE）等5种农产品常用的吸湿方程对这几种饲料原料平衡水分吸附等温线的拟合效果,发现小麦粉平衡水分吸附等温线：MGAB方程在15、35℃时拟合效果最好,MHE方程在25℃时拟合最好;米糠平衡水分吸附等温线： MOE方程在15、25、35℃时拟合效果均为最好;棉粕平衡水分吸附等温线：MOE方程在15、25℃时拟合效果最好,MGAB方程在35℃时拟合最好;芝麻粕平衡水分吸附等温线：MOE方程在15、35℃时拟合效果最好,MHAE方程在25℃时拟合最好;玉米蛋白粉平衡水分吸附等温线：MHE方程在15℃时拟合效果最好,MCPE方程在25℃时拟合最好,MOE方程在35℃时拟合最好;DDGS平衡水分吸附等温线：MGAB方程在15℃时拟合效果最好,MOE方程在25、35℃时拟合效果最好。并选择拟合最佳的方程计算出6种饲料原料的绝对安全水分和相对安全水分,以期为饲料贮藏和防霉工作提供数据指导。     

Moisture adsorption isotherms of Tarhana at 25 °C and 35 °C and the investigation of fitness of various isotherm equations to moisture sorption data of Tarhana

Moisture adsorption isotherms of Tarhana were determined at 25 °C and 35 °C. As the temperature increased, the adsorbed moisture content decreased at water activities between 0.2 and 0.9. The experimental sorption data obtained were applied to the Halsey, Harkins‐Jura, Smith, BET and Freundlich isotherm equations to test fitness of these equations to Tarhana. The order of best fit or sorption data obtained for Tarhana was Halsey > Harkins‐Jura > Smith > Henderson > BET > Freundlich. The obtained sorption data fitted the Halsey, Harkins‐Jura and Smith isotherm equations quite well in the 0.2–0.9 water activity range; However, the best fit was obtained in the Freundlich, BET and Henderson isotherm equations in this water activity range. © 2000 Society of Chemical Industry     

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.     

Equilibrium Water Content and the State of Water in Dehydrated White Cabbage

The water sorption isotherms for freeze-dried cabbage were determined at 15°C and 23°C, and the state of the water was studied using differential scanning calorimetry (DSC). The water sorption capacity of the freeze-dried cabbage depended on the temperature. The effects of free sugars were clear in both adsorption and desorption. The thermal behavior of freeze-dried cabbage was highly moisture dependent. In the capillary condensation range (>0.9 a_w) solution of low molecular components of the freeze-dried cabbage affected its freezing behavior.     

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.     

Effect of fermented soy product on the fecal microbiota of rats fed on a beef‐based animal diet

BACKGROUND: The aim of this study was to determine whether the consumption of soy product fermented with Enterococcus faecium CRL 183, would modify the fecal microbiota of rats fed a diet containing red meat. The rats were placed in groups, distinguished by their diets. For 60 days, group I was given a standard casein‐based rodent feed and groups II–VI, the beef‐based feed. From the 30th day, groups III–VI also ingested the following products: group III, E. faecium‐fermented soy product; group IV, pure suspension of E. faecium; group V, sterilized fermented soy product; and group VI, unfermented soy product. RESULTS: Rats that ingested fermented soy product showed a slight increase in the numbers of lactobacilli (0.45 log CFU g^?1), as did the casein‐based diet group (0.47 log CFU g^?1). The fermented soy product did not cause any reduction in the number of enterobacteria or clostridia, but promoted a slight fall in the viable count of Bacteroides spp. (2.80 ± 0.20 to 2.34 ± 0.07 log CFU g^?1). CONCLUSIONS: The results indicate that the ingestion of this fermented soy product did not lead to significant changes in the fecal microbiota of the rats fed on a beef‐based diet. Copyright © 2009 Society of Chemical Industry     

Moisture‐Absorption and Water Dynamics in the Powder of Egg Albumen Peptide,Met‐Pro‐Asp‐Ala‐His‐Leu

Moisture absorbed into the powder of Met‐Pro‐Asp‐Ala‐His‐Leu (MPDAHL)—a novel egg albumen antioxidant peptide—profoundly affects its properties. In this study, we elucidated water dynamics in MPDAHL using DVS, DSC, and low‐field ¹H NMR. Based on the DVS data, we found that MPDAHL sorption kinetics obey a parallel exponential model. DSC results indicated that both water and heating could change the microstructure of MPDAHL. The T₂ parameters of NMR reflected the different phases of moisture absorption revealed that there were 4 categories of water with different states or mobility in the MPDAHL during the moisture absorption process. The fastest fraction T_2b mainly dominated the hygroscopicity of MPDAHL and the absorbed water significantly changed the proton distribution and structure of MPDAHL. Thus, this study shows that DVS, DSC, and low‐field ¹H NMR are effective methods for monitoring water mobility and distribution in synthetic peptides. It can be used to improve the quality assurance of functional peptides.     

Moisture sorption isotherm and xerophilic moulds associated with dried cocoyam chips in storage in Nigeria

Extended storage of cocoyams (Colocasia antiquorum and Colocasia esculenta) is achieved in parts of Nigeria by processing them into smoked and dried chips. In this study, cocoyam chips were collected from parts of Nigeria at the end of drying, at various periods of storage and from markets, and analysed for moisture content, moisture sorption characteristics and xerophilic moulds. Moisture content of chips ranged from 7.07 ± 1.1% for freshly dried samples to 16 ± 2.2% for samples stored up to 8 months. Six mould species from four groups of the genus Aspergillus (including five xerophiles) identified as Aspergillus fumigatus, Eurotium repens, Eurotium amstelodami, Eurotium chevalieri and Aspergillus flavus and Aspergillus niger and Mucor sp. were isolated. The variety of moulds increased with storage and moisture content of samples. Moisture sorption in dried chips showed type II sigmoidal behaviour. Wood smoke significantly protected chips from mould colonization, but in cooperation with reduced water activity. Simple and inexpensive storage of freshly dried samples in airtight bags led to prolonged storage of up to 14 months without deterioration.     

Estimating Water Activity in Systems Containing Multiple Solutes Based on Solute Properties

A water activity (a_w) model for multicomponent systems expressed as a linear first order sum of molalities and requiring only data from binary solutions was tested. The model was developed based on a linear relationship between a_w and molality for each solute. Water activity predictions were validated and compared with Ross’equation using a_w data reported of different complex electrolyte and nonelectrolyte solutions. The equation proved to have a good predictive application, was extremely simple and exhibited comparable accuracy to Ross’equation in the mixtures considered.     

Water sorption and the plasticization effect in wafers

Wafers are low moisture food products whose crunchiness or crispness is considered a primary textural attribute, highly affected by the product's physical state (glassy or rubbery). The water activity–water content–glass transition relationships for commercial wafers are reported, using Gordon and Taylor's equation to model the water plasticization effect and also Brunnaver‐Emmett‐Teller (BET) and Guggenheim‐Anderson‐DeBoer (GAB) sorption models. BET monolayer moisture content was 6.2% and the moisture limit to fit this model was about 11.5%. Critical water activity and critical water content for the glass transition of the product at 20 °C were 0.591 and 0.118 (mass fraction) respectively. Abrupt changes in the mechanical product properties, evaluated from a three‐point bend test, could be observed at these limits. Moisture levels between 6 and 11% give rise to a glassy state matrix in the product and so to acceptable product crispness. Below this range, the glassy matrix seems to turn fragile and, above this range, the product becomes rubbery.     

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。     

Crunchiness Loss and Moisture Toughening in Puffed Cereals and Snacks

Upon moisture uptake, dry cellular cereals and snacks loose their brittleness and become soggy. This familiar phenomenon is manifested in smoothing their compressive force–displacement curves. These curves’ degree of jaggedness, expressed by their apparent fractal dimension, can serve as an instrumental measure of the particles’ crunchiness. The relationship between the apparent fractal dimension and moisture content or water activity has a characteristic sigmoid shape. The relationship between the sensorily perceived crunchiness and moisture also has a sigmoid shape whose inflection point lies at about the same location. The transition between the brittle and soggy states, however, appears sharper in the apparent fractal dimension compared with moisture plot. Less familiar is the observation that at moderate levels of moisture content, while the particles’ crunchiness is being lost, their stiffness actually rises, a phenomenon that can be dubbed “moisture toughening.” We show this phenomenon in commercial Peanut Butter Crunch® (sweet starch‐based cereal), Cheese Balls (salty starch‐based snack), and Pork Rind also known as “Chicharon” (salty deep‐fried pork skin), 3 crunchy foods that have very different chemical composition. We also show that in the first 2 foods, moisture toughening was perceived sensorily as increased “hardness.” We have concluded that the partial plasticization, which caused the brittleness loss, also inhibited failure propagation, which allowed the solid matrix to sustain higher stresses. This can explain other published reports of the phenomenon in different foods and model systems.     

Isolation and characterization of lactic acid bacteria from jiang‐sun (fermented bamboo shoots), a traditional fermented food in Taiwan

BACKGROUND: In this study, lactic acid bacteria (LAB) were isolated, characterized and identified from jiang‐sun (fermented bamboo shoots; a traditional fermented food in Taiwan). Samples were collected at seven time intervals from a fixed fermenting bucket during the fermentation process of jiang‐sun and its initial ingredients, dochi (fermented soybeans) and bamboo shoots. RESULTS: A total of 57 LAB cultures were isolated; 42 cultures were isolated from jiang‐sun samples and 15 cultures were isolated from dochi and bamboo shoots. These isolates were characterized phenotypically and then divided into three groups (A–C) by restriction fragment length polymorphism analysis and sequencing of 16S ribosomal DNA. Alteration of microbial populations during the fermentation process was observed. While various LAB were found in the dochi and bamboo shoots, it was mostly replaced by Lactobacillus plantarum after 1 day of fermentation. Furthermore, the antibacterial activities of the isolates were determined, and one Enterococcus faecium strain showed inhibitory activity against all the indicator strains. CONCLUSION: Results suggest that L. plantarum is the main LAB present during the fermentation of jiang‐sun. To our knowledge, this is the first report describing the distribution and varieties of LAB that exist in the jiang‐sun fermentation process. Copyright © 2010 Society of Chemical Industry     

Changes in maize starch water sorption isotherms caused by high pressure

The effect of high pressure (300 MPa, for up to 60‐min duration) on water sorption (adsorption and desorption) properties of maize starch suspensions was studied. Increase in the pressurisation time (5–60 min) augmented linearly the water‐holding capacity of starch by 2.7‐fold. Adsorption and desorption isotherms moved to higher moisture contents with the increment of pressurisation time. These movements caused the hysteresis effect extent to decrease/increase with the pressurisation time for lower/higher water activity values. While no particular tendency was found for the effect of pressurisation time on energy‐related constants, Cb (BET) and Cg (GAB), the K value decreases linearly, being always higher for sorption. The monolayer value obtained using the BET model was lower compared with the GAB model, increasing linearly in both cases with the pressurisation time. The results obtained in this study are of possible technological importance for food products containing starch.