State/phase transitions induced by ice recrystallization and its influence on the mechanical properties of potatoes (Solanum tuberosum L.) var. Russet Brown

Impact of washing and freezing on nutritional composition, bioactive compounds, antioxidant activity and microstructure of mango peels

Mango peels are widely produced and highly perishable. Disinfectant washing and freezing are among the most used methods to preserve foods. However, their impact on products' properties is conditioned by the foods' features. This study evaluated for the first time the phytochemical composition, antioxidant activity, and microstructure of mango peels washed with peracetic acid (27 mg/mL for 19 min) and frozen at -20 °C for 30 days. Washing decreased the content of vitamin C (-7%), penta-O-galloyl-β-d-glucose (-23 %), catechin (-30 %), and lutein (-24 %), but the antioxidant activity was preserved. Freezing changed mango peels' microstructure, increased free phenolic compounds, namely acid gallic (+36 %) and catechin (+51 %), but reduced bound phenolic compounds (-12 % to -87 %), bound phenolic compounds' antioxidant activity (-51 % to -72 %), and violaxanthin (-51 %). Both methods were considered adequate to conserve mango peels since fiber and the main bioactive compounds (free mangiferin, free gallic acid, and β-carotene) remained unchanged or increased.

Experimental study on a novel vacuum sublimation-rehydration thawing of frozen potatoes

To realize a quick thawing of frozen potatoes, the experimental investigation of thawing performance was conducted by using a novel vacuum sublimation-rehydration thawing (VSRT) in this study. Frozen diced potatoes (20 mm × 20 mm × 20 mm) with a total mass of 1.5 kg were selected as the thawing object. The center temperature of the frozen diced potato was raised from -18°C to 5°C to assess the beginning and end of thawing. The effects of sublimation time, heating plate temperature, and rehydration temperature on thawing time of frozen potatoes were experimentally studied. The VSRT and vacuum steam thawing (VST) were compared in terms of thawing time, hardness, and specific energy consumption. The results showed that the conditions of sublimation time of 25 min, heating plate temperature of 30°C, and rehydration temperature of 100°C could effectively shorten the thawing time of VSRT for thawing frozen potatoes. The thawing time of VSRT was only 49% of that of VST. Compared to the hardness of frozen potatoes thawed by VST, the hardness of frozen potatoes thawed by VSRT was closer to that of blanched (unfrozen) potatoes. The specific energy consumption of VSRT was lower than that of VST. PRACTICAL APPLICATION: The quality of frozen potatoes is directly affected by the thawing method used. A novel vacuum sublimation-rehydration thawing was conducted in this study, which can provide a new idea for a reasonable, effective, and quick thawing method for frozen potatoes.

Glass transitions in frozen systems as influenced by molecular weight of food components

Freezing is a frequently used way to expand the storage life of foods with high water content. Under suitable cooling rates, frozen systems attain a condition of maximum freeze concentration, which is characterized by the glass transition temperature (T_g '), end point of freezing or onset of melting (T_m '), and concentration of solids (X_s ') in the maximum-freeze-concentrated matrix. The value of T_g ', T_m ', and X_s ' depends on the chemical composition of frozen system. Below T_g ', the rates of deteriorative reactions are significantly reduced. In this article, the data for T_g ', T_m ', and X_s ' of different frozen systems including sugars, starches, proteins, and food are collected and compiled. The trends in T_g ' and T_m ' data of food are investigated using molecular weight (MW) of food components. The T_g ' and T_m ' of most starches (increased by 2.46% to 87.3% and 10.8% to 85.0%) and some protein-rich foods (increased by 5.00% to 53.4% and 25.0% to 52.9%) were higher than the maximum values of sugar-rich foods. Both T_g ' and T_m ' values increased with increasing MW of solids in frozen food, reaching an asymptotic value. Moreover, there were exponential relationships between T_g ' or T_m ' values and MW for sugar and starch-rich foods taken together. Some studies found that frozen storage below T_g ' maintains the higher quality of food that was achieved by fast freezing. However, other studies found that there was no significant difference in the quality of frozen foods between storage temperature below and above T_g '. Therefore, storage below T_g ' is not the only factor for predicting the stability of frozen foods.

High-efficiency separation of hemicellulose from bamboo by one-step freeze-thaw-assisted alkali treatment

The selectivity of alkali treatment (AT) for hemicellulose separation is reduced due to the alkali solubility of lignin. It was improved using freeze-thaw-assisted alkaline treatment (FT/AT). In this study, bamboo hemicellulose was separated via a one-step freeze-thaw-assisted alkali treatment (OFT/AT). The effects of freezing temperature, freezing time, alkali concentration, and treatment time on bamboo components were studied. The separation yield of hemicellulose was 73.26%, compared to 64.00% using conventional FT/AT. The separation of lignin and cellulose was inhibited as alkali concentration decreased from 7.0% to 5.0%. The extraction yield of hemicellulose increased from 46.35% to 56.12%. Structural analysis of extracted hemicellulose revealed the effective inhibition of the breakage of the xylose backbone and arabinose side chain of hemicellulose. This indicated that the molecular structure of extracted hemicellulose was relatively complete. It provides theoretical support for the efficient separation of hemicellulose by AT.

Hybrid mixture theory-based modeling of transport of fluids, species, and heat in food biopolymers subjected to freeze-thaw cycles

A hybrid mixture theory (HMT)-based unsaturated transport (pores not saturated with liquid) model was applied to a food matrix subjected to freezing and freeze-thaw cycles. The model can explain the fluid, species, and heat transport, ice formation, thermomechanical changes, and the freezing point depression occurring inside food biopolymers during freezing. Volume changes during freezing were calculated using the stresses due to pore pressure and the phase-change based mechanical strain. The Eulerian-Lagrangian transformation was performed for solving the equations using a finite element mesh in Lagrangian coordinates. The predicted temperature profiles for constant and fluctuating freezing temperature conditions showed agreement with experimental data with reasonable accuracy (RMSE = 2.86°C and 2.23°C, respectively). The multiscale transport model coupled with a physical chemistry-based relation was able to predict solute concentration and the freezing point depression in potatoes with greater accuracy than an empirical equation published in the literature. Sudden temperature fluctuations representing the opening and closing of a freezer door were investigated using this solution scheme, and conditions causing less damage to the food were identified. PRACTICAL APPLICATION: Food materials are subjected to freeze-thaw cycles during storage, shipping, and distribution to the consumers. The study uses numerical modeling and experimental validation to elucidate the principles affecting ice formation, solute migration, and temperature changes. Outcomes will allow processors to improve the quality of frozen foods with improved design of freezing operation, and storage and distribution strategies.