Transport phenomena and their effect on microstructure of frozen fruits and vegetables

In situ investigation of ice fractions and water states during partial freezing of pork loins and shrimps

Ice fractions and water states in partially frozen muscle foods greatly affect their quality. In the study, a variable temperature nuclear magnetic resonance (VT-NMR) with a liquid nitrogen temperature control system was employed to in situ investigate the relationship between ice fractions and temperatures and changes in water states during partial freezing and thawing of pork and shrimp. Results indicated that changes in ice fractions ranging from -2 ∼ -20 °C could be divided into 3 stages including slow increase, random leap and remarkable leap. More serious damages to the structures related to immobile water occurred in shrimp than in pork, and partial freezing also caused deterioration in muscle fibres related to free water. Additionally, -2 ∼ -3 °C and - 3.5 °C were the appropriate partial freezing temperatures for pork and shrimp, respectively. Therefore, the VT-NMR method possessed great potential for fundamental studies and applications of partial freezing of muscle foods.

Influence of different pre-cooling methods on the freshness preservation of bok choi(Brassica rapa var. chinensis)

Several pre-cooling methods for bok choi were used, such as natural convection pre-cooling (NCPC), strong wind pre-cooling (SWPC), vacuum pre-cooling (VPC), cold water pre-cooling (CWPC), electrolyzed water pre-cooling (EWPC), and fluid ice pre-cooling (FIPC), in order to determine the most suitable precooling method. It was found that VPC reduced the respiration rate, inhibited the increase of malondialdehyde (MDA) and relative electrolyte leakage, and significantly decreased the total bacterial count. This may be due to the rapid decompression process during vacuum pre-cooling, which disrupts the microbial structure and has a certain sterilizing effect. Bok choi pre-cooled by VPC had the best color, hardness value, chlorophyll, titratable acid (TA) content, vitamin C (VC) content, total phenolic (TP) content, soluble sugar content, superoxide dismutase (SOD) activity, ascorbate peroxidase (APX) activity, and catalase (CAT) activity. Therefore, the most suitable pre-cooling method for bok choi among the above pre-cooling methods was the VPC method.

Tunable thermoresponsive hydrogels for temperature regulation and warning in fruit and vegetables preservation

Fresh fruit and vegetables usually suffer from quality deterioration when exposed to inappropriate temperatures. Common energy-input temperature regulation is widely applied but there remain challenges of increasing energy consumption. Passive temperature management regulates the heat transfer without energy consumption, showing a sustainable strategy for food preservation. Here, thermoresponsive hydrogels were constructed by incorporating NaCl and sodium dodecyl sulfate (SDS) micelles into a poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAM-co-AM)) network. Due to the excellent mechanical properties and reversible thermochromism at 14 °C and 37 °C, Gel-8 wt%-NaCl could inhibit temperature rise and avoid sunburn damage to peppers under direct sunlight by blocking the input of solar energy and accelerating moisture evaporation. Additionally, hydrogels could act as a feasible sensor by providing real-time visual warnings for inappropriate temperatures during banana storage. Based on the self-adaptive thermoresponsive behaviour, the prepared hydrogels showed effective performance of temperature regulation and quality preservation of fruit and vegetables.

In-situ investigation of supercooling behaviour during high-pressure shift freezing of pure water and sucrose solution

Supercooling is a main controllable factor for the fundamental understanding the high-pressure shift freezing (HPSF). In the study, a self-developed device based on the diamond anvil cell (DAC) and confocal Raman microscopy was utilized to realize an in-situ investigation of supercooling behaviour during HPSF of the pure water and sucrose solution. The spectra were used to determine the freezing point which is shown as a spectral phase marker (SD). The hydrogen bond strengths of water and sucrose solution under supercooling states were estimated by peak position and peak area ratio of sub-peaks. The results showed that the OH stretching bands had redshift under supercooling states. Moreover, the addition of sucrose molecules could strengthen the hydrogen bonding strength of water molecules under supercooling states. Thus, the DAC combined with Raman spectroscopy could be considered a novel strategy for a deep understanding of the supercooling behaviour during HPSF.

Deep eutectic solvents (DESs) films based on gelatin as active packaging for moisture regulation in fruit preservation

To develop a novel active packaging for fruit preservation, two different deep eutectic solvents (DESs) comprising choline chloride, betaine and glycerol [ChCl:Gly (1:2) and Be:Gly (1:2)] were prepared and the corresponding DESs-based films (DES@Gel) using gelatin as polymer matrix were fabricated. DES@Gel showed smoother morphologies and better optical and mechanical properties as compared with Gel. Moisture sorption isotherm curves, the enhancement of water vapour permeability (WVP) and the excellent moisture absorption-desorption cyclist performance illustrated the moisture regulation hypothesis mechanism of DES@Gel. Furthermore, cherry tomato preservation experiment was carried out and the groups treated with DES@Gel showed better performances. The moisture regulation property of DES@Gel could broaden new avenues for active packaging.

Minimizing polyphenols and enzymes degradation using hydrogel packaging with combined evaporative and daytime radiative cooling functions during ambient transportation

Polyphenols and enzymes usually suffer from degradation during transportation due to the lack of a cold chain system in developing countries and regions. In this study, anthocyanin and trypsin were selected as examples of polyphenols and enzymes and investigated for minimizing their degradation during ambient transportation using hydrogel packaging with combined evaporative and daytime radiative cooling functions. A polyacrylamide/polyvinyl alcohol contained with nanoparticles (NPs@PAAm/PVA) hydrogel packaging was thus developed. The NPs@PAAm/PVA packaging exhibited desirable swelling behaviour, high solar reflectance, and strong atmospheric emissivity to synergistically achieve evaporative and daytime radiative cooling. The indoor experiments indicated that the vial with NPs@PAAm/PVA packaging realised sub-ambient temperatures under different working temperatures and humidities, and the field tests indicated that the vial with NPs@PAAm/PVA packaging could effectively preserve the anthocyanin and trypsin without degradation caused by strong sunlight and high temperature. Consequently, the NPs@PAAm/PVA packaging with evaporative and daytime radiative cooling effects has promising prospects for anthocyanin and trypsin transportation in an energy-saving and sustainable manner.

Recent developments in freezing of fruits and vegetables: Striving for controlled ice nucleation and crystallization with enhanced freezing rates

Fruits and vegetables are rich in essential nutrients such as minerals, vitamins, and antioxidants; however, they have short shelf life. Freezing is a superior method of preservation compared to other techniques with respect to nutrient retention and maintenance of sensory attributes. However, several physical and textural quality changes associated with freezing and thawing pose a serious problem to the quality of frozen products. Some of the disadvantages associated with the currently employed methods for freezing fruits and vegetables include low rates of heat exchange in blast freezers, shape limitation in plate freezers, high cost of operation in cryogenic freezing, and freezing solution dilution in immersion freezing. Therefore, novel freezing technologies have been developed to achieve controlled ice nucleation and crystallization, enhanced freezing rate, decreased phase transition time, and maintained temperature stability. This review discusses some of the most recent approaches employed in freezing and points to their adoption for maintaining the quality of fruits and vegetables with extended storage.

Study on the ice crystals growth under pectin gels with different crosslinking strengths by modulating the degree of amidation in HG domain

The ice crystal morphology formed under a series of amidated pectin gels with various crosslink strengths were investigated. The results showed that as the degree of amidation (DA) increased, pectin chains exhibited shorter homogalacturonan (HG) regions. Highly amidated pectin exhibited a faster gelation rate and a stronger gel micro-network via hydrogen bonds. Based on cryogenic scanning electron microscopy (cryo-SEM), smaller ice crystals were formed in frozen gel with low DA, suggesting that a weaker cross-linked gel micro-network was more effective at inhibiting crystallization. After sublimation, lyophilized gel scaffolds with high crosslink strength displayed less number of pores, high porosity, lower specific surface area, and greater mechanical strength. This study is expected to confirm that the microstructure and mechanical properties of freeze-dried pectin porous materials could be regulated by changing the crosslink strength of pectin chains, which is achieved by increasing the degree of amidation in the HG domains.

Ca2+ATPase enzyme activities and lipid and protein oxidations of grass carp (Ctenopharyngodon idellus) stored at 4 °C for 30 min under electromagnetic fields

This work studied the effects of electromagnetic fields (EMF) with frequencies between 100 and 400 Hz and a fixed strength of 12 mT on cold storage of grass carp at 4 °C for 30 min, and Ca²⁺ATPase enzyme activities, and lipid and protein oxidations in samples were measured to assess changes in intracellular Ca²⁺ concentration and oxidative stability. Results showed higher Ca²⁺ATPase activities in samples treated with EMF frequencies. Significant (p < 0.05) decreases occurred in protein oxidation for samples treated between 100 and 300 Hz, but an increase was observed for treatment with 400 Hz. However, the lipid oxidation increased for samples treated up to 200 Hz and decreased with further increase in frequency to 300 and 400 Hz. Nuclear magnetic resonance analysis showed that exposure to different frequencies of EMF could reduce the association of water molecules with protein for both bound and immobilized water. Overall, treatments of EMF between 100 and 400 Hz could improve grass carp quality during cold storage.

Aegle marmelos (L.) Correa: An Underutilized Fruit with High Nutraceutical Values: A Review

Aegle marmelos (L.) Correa (Bael) fruit, a member of the Rutaceae family, is a major cultivated fruit plant in tropical and subtropical regions in countries of southeast Asia. Bael fruit has been a major topic for studies in recent years mainly due to its high nutritional (carbohydrates, proteins, minerals, and vitamins) value and presence of various phytochemicals, which attributed to its high medicinal value. These phytochemicals include various compounds, e.g., alkaloids, flavonoids, and phenolic acids (protocatechuic acid, gallic, and ellagic acid). The fruit extract of bael has been also an important study area for its pharmacological activities, including antidiarrheal, antioxidant, antidiabetic, hepatoprotective, radioprotective, anticancer, antiulcer properties. The current review mainly highlighted the nutritional and pharmacological activities of bael fruit. The nutritional profile and phytochemical profile were discussed in the review, along with their concentration in the fruit. Moreover, the experiments carried out in vivo and in vitro of bael fruit extracts with respect to their pharmacological activities were also discussed in the article. The recent literature based on nutritional and pharmacological values of bael fruit showed its high potential as a food and pharmaceutical product. Despite having high nutritional and pharmacological value, research related to molecular mechanisms of bael fruit is still limited, and clinical trials are needed to ensure its safety as a product in the food and pharma industries.

Development of natural deep eutectic solvents (NADESs) as anti-freezing agents for the frozen food industry: Water-tailoring effects, anti-freezing mechanisms and applications

Nature-inspired natural deep eutectic solvents (NADESs) as anti-freezing agents including Pro:Glc (5:3), Pro:Glc (1:1), Pro:Sor (1:1), and Urea:Glc:CaCl₂ (3:6:1) were prepared. Viscosity (η), conductivity (σ), activation energy of viscous flow (E_η) and conduction (E_ᴧ), transverse relaxation time (T₂), thermal behaviours, and anti-freezing capacities of the NADESs were investigated. A critical T₂ of 24.60 ms for η changes was obtained, and the relationship between η and T₂ was determined as lnη = -1.398lnT₂ + 10.688. Differentialscanningcalorimetry and low-field nuclear magnetic resonance analyses indicated NADESs could hinder the molecular motion as temperature decreased through enhancing the hydrogen-bonding strength, endowing them with excellent anti-freezing capacity. NADESs showed varied E_η (41.58 ∼ 45.72 kJ mol^-1) and E_ᴧ (48.31 ∼ 63.08 kJ mol^-1), of which Pro:Sor (1:1) possessed the greatest ones, showing its greatest temperature sensitivity and best anti-frosting capacity. Applications in frozen chicken breast further announced the potentials of NADESs as anti-freezing agents for the industry.

In situ investigation of cellular water transport and morphological changes during vacuum cooling of steamed breads

Dynamic investigation of the effects of vacuum cooling on cellular water transport and structural changes of steamed bread was carried out using transverse relaxation times (T₂) and proton density-weighted images in a nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI) analyser, respectively. Initially, both steamed bread at room temperature of 25 °C and freshly steamed bread at 85 °C had three peaks of T₂₁, T₂₂, and T₂₃, respectively representing the tightly bound water, loosely bound water, and free water, while an additional peak T₂₄, was observed in freshly steamed bread at 85 °C. After vacuum cooling, freshly steamed bread at 85 °C had a higher mass loss of 10.29% due to its high initial temperature, and both samples were clearly discriminated with PCA of 88.2%, indicating that the initial food condition affected the vacuum cooling process. Lastly, the NMR/MRI technique and correlations were accurate (R²> 0.98), thus suitable for model validation at microscale and macroscale.

Finite Element Method for Freezing and Thawing Industrial Food Processes

Freezing is a well-established preservation method used to maintain the freshness of perishable food products during storage, transportation and retail distribution; however, food freezing is a complex process involving simultaneous heat and mass transfer and a progression of physical and chemical changes. This could affect the quality of the frozen product and increase the percentage of drip loss (loss in flavor and sensory properties) during thawing. Numerical modeling can be used to monitor and control quality changes during the freezing and thawing processes. This technique provides accurate predictions and visual information that could greatly improve quality control and be used to develop advanced cold storage and transport technologies. Finite element modeling (FEM) has become a widely applied numerical tool in industrial food applications, particularly in freezing and thawing processes. We review the recent studies on applying FEM in the food industry, emphasizing the freezing and thawing processes. Challenges and problems in these two main parts of the food industry are also discussed. To control ice crystallization and avoid cellular structure damage during freezing, including physicochemical and microbiological changes occurring during thawing, both traditional and novel technologies applied to freezing and thawing need to be optimized. Mere experimental designs cannot elucidate the optimum freezing, frozen storage, and thawing conditions. Moreover, these experimental procedures can be expensive and time-consuming. This review demonstrates that the FEM technique helps solve mass and heat transfer equations for any geometry and boundary conditions. This study offers promising insight into the use of FEM for the accurate prediction of key information pertaining to food processes.

The Formation and Control of Ice Crystal and Its Impact on the Quality of Frozen Aquatic Products: A Review

Although freezing has been used to delay the deterioration of product quality and extend its shelf life, the formation of ice crystals inevitably destroys product quality. This comprehensive review describes detailed information on the effects of ice crystals on aquatic products during freezing storage. The affecting factors (including nucleation temperature, freezing point, freezing rate, and temperature fluctuation) on the size, number, distribution, and shape of ice crystals are also elaborated in detail. Meanwhile, the corresponding technologies to control ice crystals have been developed based on these affecting factors to control the formation of ice crystals by inhibiting or inducing ice crystallization. In addition, the effects of ice crystals on the water, texture, and protein of aquatic products are comprehensively discussed, and the paper tries to describe their underlying mechanisms. This review can provide an understanding of ice crystallization in the aquatic products during freezing and contribute more clues for maintaining frozen food quality.

Phytohormones in postharvest storage of fruit and vegetables: mechanisms and applications

As a ubiquitous and essential part of phytophysiology, phytohormones have attracted tremendous attention for effective regulation of development and senescence of agricultural products. However, the postharvest mechanisms of phytohormones have not been thoroughly understood. This review provides an overview of common phytohormones for extending the shelf life of fruit and vegetables. The modulation principles are discussed in detail based on defence gene expression activation, sensitivity of senescence-related phytohormones inhibition, antioxidant enzymes activity stimulation, and cell membrane integrity maintenance. The applications of jasmonates, salicylic acids, cytokinins, gibberellins, polyamines, and brassinosteroids in preserving fruit and vegetables based on defence signaling network stimulation, senescence-related phytohormones expression or sensitivity repression, as well as antioxidant system enhancement and cell membrane integrity sustentation are introduced. The challenges and problems to be solved are discussed, and new trends of expanding lifespan by combining phytohormones with other treatments are also suggested. Although phytohormones have been demonstrated to have promising efforts in maintaining agricultural products, more novel and effective combination treatments should be developed to complement each other.

Pressure-related cooling and freezing techniques for the food industry: fundamentals and applications

Cooling and freezing are two widely used methods for food preservation. Conventional cooling and freezing techniques are usually with low efficiency and prone to damage foodstuffs. In order to increase cooling and freezing efficiencies and ensure better food quality, many efforts have been performed. As effective solutions, pressure-related techniques such as vacuum cooling (VC), vacuum film cooling (VFC), vacuum spray cooling (VSC), pressure shift freezing (PSF) and isochoric freezing (ICF) have attracted a lot of interests. The current review intends to provide an overview of pressure-related cooling and freezing techniques for the food industry. In the review, the fundamentals including principles, experimental systems, thermodynamic and kinetic mechanisms and their relevant mathematical models are presented, latest applications of these techniques in the food industry are summarized, and future trends concerning technological development and industrialization are highlighted. Pressure plays an important role in improving the cooling and freezing processes and ensuring food qualities, and mathematical modeling is an effective tool for understanding the thermodynamic and kinetic mechanisms of these processes. However, the latest researches showed that despite many merits of these pressure-related processes, limitations still exist in applying some of the techniques in the food industry. For achieving technological development and industrialization of the pressure-related processes, further researches should focus on improving model performance, integrating multiple technologies, and cost control.