Food structure: Its formation and relationships with other properties

Food materials are complex in nature as it has heterogeneous, amorphous, hygroscopic and porous properties. During processing, microstructure of food materials changes which significantly affects other properties of food. An appropriate understanding of the microstructure of the raw food material and its evolution during processing is critical in order to understand and accurately describe dehydration processes and quality anticipation. This review critically assesses the factors that influence the modification of microstructure in the course of drying of fruits and vegetables. The effect of simultaneous heat and mass transfer on microstructure in various drying methods is investigated. Effects of changes in microstructure on other functional properties of dried foods are discussed. After an extensive review of the literature, it is found that development of food structure significantly depends on fresh food properties and process parameters. Also, modification of microstructure influences the other properties of final product. An enhanced understanding of the relationships between food microstructure, drying process parameters and final product quality will facilitate the energy efficient optimum design of the food processor in order to achieve high-quality food.     

Shrinkage of Food Materials During Drying: Current Status and Challenges

The structural heterogeneities of fruits and vegetables intensify the complexity to comprehend the interrelated physicochemical changes that occur during drying. Shrinkage of food materials during drying is a common physical phenomenon which affects the textural quality and taste of the dried product. The shrinkage of food material depends on many factors including material characteristics, microstructure, mechanical properties, and process conditions. Understanding the effect of these influencing factors on deformation of fruits and vegetables during drying is crucial to obtain better‐quality product. The majority of the previous studies regarding shrinkage are either experimental or empirical; however, such studies cannot provide a realistic understanding of the physical phenomena behind the material shrinkage. In contrast, theoretical modeling can provide better insights into the shrinkage that accompanies simultaneous heat and mass transfer during drying. However, limited studies have been conducted on the theoretical modeling of shrinkage of fruits and vegetables. Therefore, the main aim of this paper is to critically review the existing theoretical shrinkage models and present a framework for a theoretical model for the shrinkage mechanism. This paper also describes the effect of different drying conditions on material shrinkage. Discussions on how the diverse characteristics of fruits and vegetables affect shrinkage propagation is presented. Moreover, a comprehensive review of formulation techniques of shrinking models and their results are also presented. Finally, the challenges in developing a physics‐based shrinkage model are discussed.     

Water transport in starchy foods: Experimental and mathematical aspects

BackgroundThe availability and movement of water inside the food materials play essential roles for food stability by affecting their physical and chemical properties, and microbiological activity. Understanding the moisture sorption behavior is a necessary step to control food properties. Food processing unit operations like drying and cooking influence the behavior of starch since such systems trigger swelling or shrinkage as a result of moisture sorption or desorption mechanisms. Also, these processes alter many aspects of starch-containing foods such as acceptability, nutritional value, quality, and shelf-life.Scope and approachTherefore, understanding the water transport in starchy foods and the changes occurring in functional properties of starch has a great importance to describe and model their sorption and drying behavior. First, the primary mechanisms occurring during water transport such as moisture sorption, swelling, gelatinization, and glass transition are discussed using experimental results presented in the literature. Additionally, the hybrid mixture theory (HMT) and its potential for predicting transport mechanisms in starchy foods is discussed.Key findings and conclusionsIn addition to experimental considerations, the mathematical modeling provides complementary information to predict the heat and fluid transfer. The hybrid mixture theory based multiscale models are able to describe the physico-chemical changes and general transport mechanisms occurring within a porous food matrix. This theory can also be used to predict the quality changes in food products during processing.     

Simulation of food drying processes by Computational Fluid Dynamics (CFD); recent advances and approaches

BackgroundUnderstanding the mechanisms underlying the drying processes has a critical role in dehydration of food and agricultural products. Advanced computer modeling and simulation techniques can help in developing new dryers, modification of current systems, energy saving and process optimization. Also the most important parameter during the drying food products is food quality (moisture content, crack formation, case hardening, etc.) which can be enhanced through using appropriate modeling. Computational Fluid Dynamics (CFD) is a well-known modeling technique which has received more attention in the food industry in the recent years. Hydrodynamics of fluid flow, heat and mass transfer during drying can be predicted using CFD.Scope and ApproachThis article reviews fundamentals, merits and shortcomings of CFD in the drying process modeling with a special focus on dehydration of food products. Since the drying is a growing unit operation, there is an emphasis on investigation of CFD utilization in modeling emerging drying processes of food products such as microwave assisted drying, infrared and superheated steam drying besides conventional convective drying systems notably in recent 5 years.Key Findings and ConclusionsCFD has been considered as a promising method which could help developing the design of new dryers, enhancing current dryers and the most important aspect of utilization of this method in the food industry research and development is “food quality” improvement.     

Structure evolution of pullulan–alginate edible films during drying studied by low‐field NMR

The evolutions of spatial structure for pullulan, alginate, and pullulan–alginate blend aqueous solutions during drying were investigated by low‐field nuclear magnetic resonance. For pullulan solutions, during the first 1,320 min of drying, NMR signal intensity in profiles did not vary with position. Thereafter, the apparent shift of the air–pullulan interface began at 1,440 min, indicating the onset of progressive shrinkage of pullulan samples. Similar shrinkage phenomena were observed for alginate and pullulan–alginate blend solutions. In contrast, spatial structural heterogeneity was observed for alginate solution during drying time from 1,200 to 1,560 min, due to the formation of a skin layer near the air–alginate interface. Based on the change of polymer solutions during drying, two‐stage evaporation process was detected in the aqueous pullulan, alginate, and their blend solutions. Moreover, the evaporation rate of water during the second stage of drying was significantly lower than that of the first stage.

Practical applications

Edible films have emerged as an alternative to synthetic petroleum‐based polymers for food packaging. The evolutions of spatial structure of pullulan–alginate solutions during drying were studied by monitoring of their NMR profiles. Information shown in this study would provide some scientific basis for studying film‐forming mechanism of edible films and their applications in the food field.     

Status of Physics‐Based Models in the Design of Food Products,Processes, and Equipment

This article is part of a collection entitled “Models for Safety, Quality, and Competitiveness of the Food Processing Sector,” published in Comprehensive Reviews in Food Science and Food Safety. It has been peer‐reviewed and was written as a follow‐up of a pre‐IFT workshop, partially funded by the USDA NRI grant 2005‐35503‐16208. ABSTRACT: Modeling, in particular physics‐based modeling, can be an important tool to food product, process, and equipment designers by reducing the amount of experimentation (thus reducing the time and expenses involved) and by providing a level of insight that is often not possible experimentally. Food processes involve unique physics and challenges compared to other types of materials processing such as polymers and ceramics. These include complex multiphase transport and multiphysics that are difficult to implement in the available software, and often drastic changes in material properties during processing for which data are unavailable. Such unique and challenging features have made it difficult to embrace modeling as a tool in the food industry. This article discusses, in the context of design use of models, the nature and the state of modeling of food processes, emphasizing the more complex scenarios in both modeling and material properties needed for the models.     

Microwave-convective drying of food materials: A critical review

Microwave convective drying (MCD) is gaining increasing interest due to its unique volumetric heating capability and ability to significantly reduce drying time and improve food quality. The main objective of this paper is to discuss, critically analyze and evaluate the recent advances in MCD and suggest the future directions in this field. The main focus of this paper is the mathematical modeling and experimental investigations in microwave convective drying of food materials. Recent developments in mathematical modeling of MCD is discussed and existing experimental setup and their advantages and disadvantages are discussed and analysed. Long drying time is a concern in food industries. Reductions in drying time by applying MCD compared to convection drying are calculated and discussed. It was apparent that the proper integration of mathematical modeling and experimental technique is the best way to maximize the advantages of this drying method. Although a plethora of research is being carried out on this topic, there is still need for research to develop fundamental modeling to optimize the process parameters and scale up this technology for the industrial application. Overall, the review provides an in-depth insight into the latest development of MCD and its mathematical modeling approaches and will hopefully serve to inspire future work in the field.     

Invited review: Modelling quality changes of fruits and vegetables during drying: a review

Fruits and vegetables have received much attention as these materials have been reported to contain various phytochemicals, which are claimed to exert many health benefits. When extraction of bioactive compounds cannot be performed on fresh fruits and vegetables, drying needs to be conducted to keep the materials for later use. Dried fruits and vegetables have also been regarded as alternative fat‐free snacks and received more attention from the food industry during the past decade. This implies that not only nutritional changes, but also other changes including physical and microstructural changes are of importance and need to be optimised, preferably through the use of various modelling approaches. The objective of this article is to provide a brief review of some advances in modelling quality changes of fruits and vegetables during drying. These include modelling of nutritional, colour and selected physical changes. Approaches to monitoring and modelling microstructural changes are also mentioned.     

Status of Observational Models Used in Design and Control of Products and Processes

This article is part of a collection entitled “Models for Safety, Quality, and Competitiveness of the Food Processing Sector,” published in Comprehensive Reviews in Food Science and Food Safety. It has been peer‐reviewed and was written as a follow‐up of a pre‐IFT workshop, partially funded by the USDA NRI grant 2005‐35503‐16208. ABSTRACT: Modeling techniques can play a vital role in developing and characterizing food products and processes. Physical, chemical, and biological changes that take place during food and bioproduct processing are very complex and experimental investigation may not always be possible due to time, cost, effort, and skills needed. In some cases even experiments are not feasible to conduct. Often it is difficult to visualize the complex behavior of a data set. In addition, modeling is a must for process design, optimization, and control. With the rapid development of computer technology over the past few years, more and more food scientists have begun to use computer‐aided modeling techniques. Observation‐based modeling methods can be very useful where time and resources do not allow complete physics‐based understanding of the process. This review discusses the state of selected observation‐based modeling techniques in the context of industrial food processing.     

Thermal energy storage based solar drying systems: A review

Solar dryer based on thermal energy storage materials is quite effective for continuously drying agriculture and food products at steady state in the temperature range (40 °C–60 °C). Such dryers have globally become a potential viable substitute to the solar dryers based on fossil fuel, due to the utilization of clean energy resources and cost-effectiveness. Storage materials utilized in these dryers can store energy during the sunshine hour and deliver the stored energy during off-sunshine. It reduces the existing load on the gap between energy demand and supply, hence plays a vital role in energy sustainability. A number of studies have been done in last few decades for drying agriculture and food products with a solar dryer based on thermal energy storage concept. This paper mainly presents a review on the important contributions made so far in the field of solar drying systems based on the thermal energy storage medium, with a focus on recent updates in thermal energy storage technology available in terms of materials capable of storing heat as sensible and latent heat.Industrial relevance

• 1.Solar energy defuses in the nature and provides low grade heat. This characteristic of solar energy is good for drying at low temperature, high flow rate with low temperature rise.
• 2.Worldwide, there is huge demand of efficient solar dryers utilized by various food and agricultural products.
• 3.Solar energy based solar dryers are not only cost efficient but also can be used across the globe, specially contributing to cleaner energy resources.
• 4.Novel system designs and techniques related to solar dryers are being explored to increase the efficiency and performance of solar dryers, which could in turn be much more commercially utilized by the end user.
• 5.Phase change materials can store energy during sunshine hours and retrieve during off-sunshine hours for drying purpose, hence solar dryers embedded with such materials could be quite useful.

     

Predictive modeling of microbial single cells: A review

In practice, food products tend to be contaminated with food-borne pathogens at a low inoculum level. However, the huge potential risk cannot be ignored because microbes may initiate high-speed growth suitable conditions during the food chain, such as transportation or storage. Thus, it is important to perform predictive modeling of microbial single cells. Several key aspects of microbial single-cell modeling are covered in this review. First, based on previous studies, the techniques of microbial single-cell data acquisition and growth data collection are presented in detail. In addition, the sources of microbial single-cell variability are also summarized. Due to model microbial growth, traditional deterministic mathematical models have been developed. However, most models fail to make accurate predictions at low cell numbers or at the single-cell level due to high cell-to-cell heterogeneity. Stochastic models have been a subject of great interest; and these models take into consideration the variability in microbial single-cell behavior.     

Review of recent applications and research progress in hybrid and combined microwave-assisted drying of food products: Quality properties

Abstract

The growing concerns over product quality have increased demand for high quality dried food products and encouraged researchers to explore and producers of such products to implement novel microwave (MW)-assisted drying methods. This paper presents a critical review of the key principles and drawbacks of MW-assisted drying as well as needs for future research. In this article, recent research into application of microwaves as an alternative heat source, applications and progress in hybrid MW-assisted drying that rely on various drying media and combined two or three stages of MW-assisted drying for the preservation of food products is reviewed critically. The effect of different MW-assisted drying methods, conditions and initial pretreatments on the thermophysical properties, color, nutritional value and rehydration potential of dried food products is discussed in detail along with the discussion on how the material properties evolve and change in structure, color, and composition during MW-assisted drying and recent attempts at mathematical modeling of these changes made for different fruits and vegetables. It should be noted that most of the published results were obtained in laboratory-scale dryers. Pilot-scale testing is needed to bridge the gap between laboratory research and industrial applications to fulfill the potential for novel hybrid and combined MW-assisted drying methods and to expand their role in food processing.     

Applications of macro–micro region concept in the state diagram and critical temperature concepts in determining the food stability

Food stability determination from a scientific basis rather than empiricism is a challenge to food scientists and engineers. Over the years, water activity and glass transition concepts are the two most successful theoretical foundations developed in determining the food stability. The macro–micro region concept in the state diagram and critical temperature concept are further extension of the water activity and glass transition concepts. This paper presents the applications of critical temperature concept in determining food stability during storage. In addition, it presents the applications of macro–micro region concept in analyzing air- and freeze-drying processes, maintaining rice quality using different drying strategies, analyzing structural changes during baking process, and determining chemical stability during storage.     

Foam drying in the food industry

Foam‐drying foods is now an established unit operation in the food industry. The three basic foam drying variants, vacuum puff, foam‐mat and foam spray drying are reviewed. Where applicable, the process parameters are related to other drying methods and to generalized drying theory. The foam‐drying methods are described in relation to equipment, process parameters, type of food products, product characteristics and economics.     

Understanding the heating characteristics in microwave heating moving and deforming foods by a hybrid double-layer ALE/implicit algorithm

Multiphysics modeling is an essential tool to understand the heating characteristics of microwave processing food products. However, when the heated sample is moving and deforming during the heating process (e.g. the long-term microwave drying process), the huge mesh distortions caused by the sample's motion and deformation will make the simulation impossible. To solve this problem, a hybrid double-layer Arbitrary-Lagrange-Eulerian (ALE)/implicit algorithm is proposed in this paper, where the sample's motion and deformation is tracked by a double-layer ALE framework and convert to time-varying implicit variables. By doing so, the influence of the sample motion and deformation on the electromagnetic field distribution will be characterized by the evolution of time-varying implicit variables, rather than by explicit modeling, which reduces the mesh distortion and makes the calculation possible. A two-dimensional and a three-dimensional model of microwave cavities containing a rotating and deforming food are given to describe the algorithm in detail. Experiments for the three-dimensional model are also conducted to validate the proposed method. Results show that the shrinkage of the food may have a negative impact on the heating performance (especially energy efficiency) and corresponding remedies need to be performed for long-term microwave heating processes.     

2‐Furoylmethyl amino acids,hydroxymethylfurfural, carbohydrates and β‐carotene as quality markers of dehydrated carrots

BACKGROUND: Processing of vegetables in the food industry usually includes dehydration as a preservation process. Industrial convective air drying of carrot can involve steam blanching of the raw product after peeling and cutting, and different stages of dehydration (first space, second space and final drying). Although the shelf‐life of carrot is significantly extended, important changes in its chemical composition can take place during dehydration since high temperatures and long times are used. This research is a preliminary study to evaluate the usefulness of β‐carotene, carbohydrates, hydroxymethylfurfural (HMF) and 2‐furoylmethyl amino acids (2‐FM‐AA) as quality markers of dehydrated carrots. RESULTS: A considerable decrease in β‐carotene and reducing carbohydrates was observed during dehydration. HMF, absent in raw carrots, increased during the whole drying process and the highest formation was found during the steam blanching stage. 2‐FM‐AA of lysine, arginine, γ‐aminobutyric acid and alanine were progressively originated up to the second space and decreased during the final drying. CONCLUSION: The combined use of HMF and 2‐FM‐AA seems to be advantageous for the assessment of the optimal processing conditions to obtain high‐quality dehydrated carrots. Copyright © 2008 Society of Chemical Industry     

Modeling the Thin‐Layer Drying of Fruits and Vegetables: A Review

The drying of fruits and vegetables is a complex operation that demands much energy and time. In practice, the drying of fruits and vegetables increases product shelf‐life and reduces the bulk and weight of the product, thus simplifying transport. Occasionally, drying may lead to a great decrease in the volume of the product, leading to a decrease in storage space requirements. Studies have shown that dependence purely on experimental drying practices, without mathematical considerations of the drying kinetics, can significantly affect the efficiency of dryers, increase the cost of production, and reduce the quality of the dried product. Thus, the use of mathematical models in estimating the drying kinetics, the behavior, and the energy needed in the drying of agricultural and food products becomes indispensable. This paper presents a comprehensive review of modeling thin‐layer drying of fruits and vegetables with particular focus on thin‐layer theories, models, and applications since the year 2005. The thin‐layer drying behavior of fruits and vegetables is also highlighted. The most frequently used of the newly developed mathematical models for thin‐layer drying of fruits and vegetables in the last 10 years are shown. Subsequently, the equations and various conditions used in the estimation of the effective moisture diffusivity, shrinkage effects, and minimum energy requirement are displayed. The authors hope that this review will be of use for future research in terms of modeling, analysis, design, and the optimization of the drying process of fruits and vegetables.