Effect of temperature fluctuations on ice-crystal growth in frozen potatoes during storage

Influence of pre-treatment methods on quality indicators and mineral composition of plant milk from different sources of raw materials

BACKGROUND

Pre-treatment of plant materials is essential in producing plant-based products and can affect their various organoleptic and physicochemical characteristics. This work aimed to study the effect of pre-treatment of vegetable raw materials, namely ultrasonic processing and freezing of raw materials under various low-temperature conditions, to obtain multiple types of vegetable milk and determine their characteristics.

RESULTS

It is shown that by applying a certain kind of pre-treatment of vegetable raw materials it is possible to adjust organoleptic parameters and the content of solids, protein, fat, carbohydrates, fiber and mineral composition of various types of vegetable milk from soy, rice, oats, wheat, peas, buckwheat, pumpkin seeds and lentils. Ultrasound pre-treatment allows increasing of polyphenol content by an average of 15-20% for all types of plant milk, except for lentil milk. The results showed that ultrasound treatment for 3 min had the most significant effect on the overall acceptability for lentils, pumpkin, rice and pea milk. Pre-freezing at a temperature regime of -17 and -85 °C contributed to an increase in Fe, K, Zn, Ca, Mg, Si and P by an average of 30-100%, depending on the plant material.

CONCLUSION

Pre-treatment of vegetable raw materials, including freezing and ultrasonic treatment, can positively affect the macro- and micronutrient composition of plant milk. However, the effect may vary depending on the type of raw material and processing conditions. © 2023 Society of Chemical Industry.

Assessment of freeze damage in tuber starch with electrical impedance spectroscopy and thermodynamic, rheological, spectrographic techniques

In this study, we aimed to use electrical impedance spectroscopy (EIS) to assess the freeze-damage level of starches from potato tubers treated with multiple freezing-thawing (FT) cycles. The results showed that the relationship between the physicochemical properties of starches and the impedance characteristics of starch paste is temperature-dependent. As the temperature rises to 70-90 °C, the impedance modules show a significant correlation with the amylose and mineral contents, gelatinization and pasting properties, short-range ordered structure, relative crystallinity, and damage level within the range of 10-1 MHz (p < 0.01). This could be because FT leads to a reduction in amylose and ion content. Compared to a high level of freeze-damaged starch (FDS), a low level of FDS has less amylopectin and more amylose. Additionally, the ions could be typically evenly distributed throughout the unbranched linear amylose structure in starch paste. At the peak gelatinization temperature, the starch paste made from a low level of FDS exhibits a weakened network structure, allowing more unbound water for ion movement and enhancing electric conduction. In conclusion, EIS can predict the damage level and properties of FDS, which can benefit the frozen starchy food industry.

3D Characterization of Sponge Cake as Affected by Freezing Conditions Using Synchrotron X-ray Microtomography at Negative Temperature

In this study, the microstructural evolution of a non-reactive porous model food (sponge cake) during freezing was investigated. Sponge cake samples were frozen at two different rates: slow freezing (0.3 °C min⁻¹) and fast freezing (17.2 °C min⁻¹). Synchrotron X-ray microtomography (µ-CT) and cryo-scanning electron microscopy (Cryo-SEM) were used to visualize and analyze the microstructure features. The samples were scanned before and after freezing using a specific thermostated cell (CellStat) combined with the synchrotron beamline. Cryo-SEM and 3D µ-CT image visualization allowed a qualitative analysis of the ice formation and location in the porous structure. An image analysis method based on grey level was used to segment the three phases of the frozen samples: air, ice and starch. Volume fractions of each phase, ice local thickness and shape characterization were determined and discussed according to the freezing rates.

Impact of temperature fluctuations on quality changes of frozen green beans and carrots during storage

The quality of vegetables during frozen storage and distribution chain is affected by fluctuating temperature regimes. The temperature variations influence ice-water displacement due to ice crystal growth and ice-sublimation. Hence, the description of quality changes of frozen vegetables during temperature fluctuations is indispensable in the frozen food industry. In this context, frozen carrots and green beans were stored under four different temperatures: -8 °C ± 3 °C, -12 °C ± 3 °C, -18 °C ± 3 °C and -23 °C ± 3 °C for 12 months. In each storage condition, two different partitions were created to achieve different amplitudes of temperature fluctuations, namely low (±0.3 °C) and large (±2 °C). The evolution of frost forming and drip loss in green beans and carrots were analysed in addition to the changes of ascorbic acid in green beans. The results indicated that high mean storage temperature and large amplitude of fluctuation significantly affect the quality indicators. The quality data for drip loss and ascorbic acid were fitted to a first-order kinetic model. An Arrhenius model was applied to describe the temperature dependency by incorporating the temperature fluctuation scenarios. A simplified physical model was used to simulate frost formation during frozen storage in green beans and carrots. Finally, the models were validated using the data collected at -18 °C and -12 °C with low and large amplitudes of fluctuation.

Understanding the influence of pullulan on the quality changes, water mobility, structural properties and thermal properties of frozen cooked noodles

Pullulan is widely applied in the food industry due to its unique physicochemical properties, but little information is known about its effects on the quality of frozen cooked noodles (FCNs), nor the underlying mechanism. In this study, the addition of 0.3% and 0.5% pullulan resulted in better texture and cooking properties, and minor chrominance differences, and it significantly (P < 0.05) decreased the freezable water content and retarded the water migration. Pullulan inhibited the depolymerization of the glutenin macropolymer during 0-8 weeks of frozen storage. Meanwhile, pullulan caused slightly decreased α-helixes and increased β-turns, as well as decreased degradation temperature, further suggesting that pullulan influenced the gluten network. A more compact microstructure was shown in the pullulan-fortified FCNs. This study provides a theoretical basis for the positive effects of pullulan on the quality of FCNs from the perspectives of water state and protein structure.

Effect of soluble soybean polysaccharides on freeze-denaturation and structure of myofibrillar protein of bighead carp surimi with liquid nitrogen freezing

This paper investigated the synergistic effect of 3% soluble soybean polysaccharides (SSPS) and liquid nitrogen freezing (-80 °C) on the freezing process and protein denaturation of bighead carp surimi. Freezing curve showed that liquid nitrogen freezing could significantly minimize the elapsed time of maximum-ice-crystal formation zone. Both liquid nitrogen freezing and SSPS were useful in preventing protein denaturation of surimi during 12-week frozen storage. Protein denaturation results indicated that SSPS-LNfreezing surimi1 had the highest protein solubility, Ca2+-ATPase activity and total sulfhydryl content. SDS-PAGE indicated that SSPS and liquid nitrogen freezing could effectively inhibit the decrease of myosin heavy chain concentration after 12 weeks of frozen storage. Raman spectra showed that tryptophan and tyrosine were exposed to polar microenvironment, the ɑ-helix and β-sheet turned into random coil and β-turn, and the conformation of disulfide bond changed from trans-gauche-trans (t-g-t) to gauche-gauche-trans (g-g-t). Either SSPS or liquid nitrogen freezing could mitigate these changes during frozen storage and a synergistic effect emerged on preventing myofibrillar protein denaturation and protein structure change. The combination of SSPS with liquid nitrogen freezing could be applied to freeze bighead carp surimi.

Effect of Temperature Cycling on Ostwald Ripening

We study the effect of temperature cycling on the rate of Ostwald ripening (or coarsening) of spherical particles dispersed in a binary solution. A widespread view, which states a temperature cycle generally enhances the rate of Ostwald ripening by first dissolving the smallest particles (heating) and then regrowing the dissolved amount of material on the remaining particles (cooling), is shown to be inadequate as it does not include transient effects. On the basis of a simulation method that assumes mass transfer as the limiting growth mechanism, we show that each temperature cycle is followed by a significant relaxation of the particle-size distribution, during which the number of particles remains constant, and the average particle size decreases. The relaxation is shown to be crucial to obtain a linear scaling of the average particle radius cubed with the number of cycles applied (or time), which is the behavior generally observed for the evolution of ice crystals in cycling experiments on frozen aqueous solutions or frozen foods. We show the experimentally observed increase in the proportionality constant (or "coarsening rate") as compared to isothermal ripening, or the increase of the coarsening rate with increasing cycle frequency, can be reproduced convincingly only if some (transient) ripening is allowed to take place at the elevated temperature of each cycle. Our results thus suggest the effect of temperature cycling on Ostwald ripening is governed by a dissolution-ripening-regrowth-relaxation mechanism.

Predicting the Kinetics of Ice Recrystallization in Aqueous Sugar Solutions

The quality of stored frozen products such as foods and biomaterials generally degrades in time due to the growth of large ice crystals by recrystallization. While there is ample experimental evidence that recrystallization within such products (or model systems thereof) is often dominated by diffusion-limited Ostwald ripening, the application of Ostwald-ripening theories to predict measured recrystallization rates has only met with limited success. For a model system of polycrystalline ice within an aqueous solution of sugars, we here show recrystallization rates can be predicted on the basis of Ostwald ripening theory, provided (1) the theory accounts for the fact the solution can be nonideal, nondilute and of different density than the crystals, (2) the effect of ice-phase volume fraction on the diffusional flux of water between crystals is accurately described, and (3) all relevant material properties (involving binary Fick diffusion coefficients, the thermodynamic factor of the solution, and the surface energy of ice) are carefully estimated. To enable calculation of material properties, we derive an alternative formulation of Ostwald ripening in terms of the Maxwell-Stefan instead of the Fick approach to diffusion. First, this leads to a cancellation of the thermodynamic factor (a measure for the nonideality of a solution), which is a notoriously difficult property to obtain. Second, we show that Maxwell-Stefan diffusion coefficients can to a reasonable approximation be related to self-diffusion coefficients, which are relatively easy to measure or predict in comparison to Fick diffusion coefficients. Our approach is validated for a binary system of water and sucrose, for which we show predicted recrystallization rates of ice compare well to experimental results, with relative deviations of at most a factor of 2.