Experimental determination of the amount of ice instantaneously formed in high-pressure shift freezing

The main aim of this paper was to experimentally determine the amount of ice instantaneously formed after expansion during high-pressure shift freezing (HPSF). To do that, an easy, quick and cheap procedure was developed able to obtain accurate ice ratio estimations from temperature data of the sample after the pressure release. Very different pressure and temperature conditions, both near and far from the melting curve liquid–ice I, were chosen to produce different amounts of ice after expansion in HPSF experiments. Sucrose solutions of several concentrations (10%, 20%, 30% and 40% w/w) were also employed to study the influence of the water content of the sample. Good agreement was found between experimental estimations and theoretical predictions from the heat balance model for all the pressure and temperature conditions tested. The results showed that increasing pressure at a given temperature before expansion is an effective and fast way to increase the amount of ice produced.     

A novel technique to control ice cream freezing by electrical characteristics analysis

The freezing process is very important in ice cream production affecting quality, taste and yield of the finished product. Batch freezer machines use different control techniques to tightly control the freezing process, based on monitoring of the temperature and/or the viscosity (i.e. consistency) of the product. Temperature control, however, features low sensitivity and need calibration for different product compositions, while product viscosity is essentially inferred from dasher motor load, sensitive to power line fluctuations, volume of product and dasher motor characteristics. In this context, this paper presents a novel technique based on measurements of the product electrical characteristics, tightly linked to temperature and viscosity. The experimental results presented in this work clearly indicate that the proposed technique provides a suitable, nondestructive tool to monitor ice cream quality product that overcomes the drawbacks of the standard methods, thus representing an advance in the state of the art for freezing control.     

Effects of freezing on cell structure of fresh cellular food materials: A review

BackgroundFresh cellular food materials including fruits and vegetables and animal tissues normally consist of fine organized cellular structures. Freezing is a common method to preserve the quality and safety of these cellular foods. However, the formation of ice crystals during food freezing may cause damage to the food microstructure, leading to the deterioration of food quality after thawing.Scope and approachThis review offers current knowledge on freezing damage to cell structure of fresh cellular food materials. Effects of cell structure and water distribution on the texture and sensory properties of fresh cellular foods are presented. Mechanisms of cell structure damages caused by freezing are discussed. Novel methods to control the formation of ice crystals and preserve cell structures are also provided.Key findings and conclusionsThe quality of cellular foods after frozen-thawed is highly correlated with the integrity and viability of tissue cells. The formation of ice crystals, water migration and the inherent characteristics of cell structure are regarded as the main factors affecting the cell structure during freezing. For obtaining better quality of frozen products, further investigation and understanding on freezing damage to cell structure of fresh cellular foods is necessary. It is hoped that the current review will provide more information on improving frozen food quality for the frozen food industry.     

Undercooling associated with slow freezing and its influence on the microstructure and properties of rice starch gels

It is well accepted that high undercooling or supercooling usually produces numerous small ice crystals. This paper shows that if heat transfer is not rapid enough, high undercooling causes non-homogenous sized ice crystals. Three freezing regimes (i.e. fast, slow and slow with undercooling) were used in this study. Fast freezing produced numerous homogeneously small ice crystals embedded in a thin rice starch gel matrix. This microstructure caused low % syneresis and hardness versus slow freezing’s rather homogenous distribution of fewer large ice crystals embedded in a thicker gel matrix resulting in high % syneresis and hardness. However, slow freezing with undercooling produced non-homogenous clusters involving small and large ice crystals embedded in a very thick gel matrix. Starch retrogradation before ice formation played an important role in this frozen structure. The information gained from this study enhances understanding of the behavior of starch-based food during freezing and storage.     

Ultrasound assisted nucleation of some liquid and solid model foods during freezing

Power ultrasound has been proven to be useful in promoting the nucleation of ice in water-based solutions, and different mechanisms have been proposed to describe this phenomenon. In the present work, the use of ultrasound waves to induce dynamic nucleation in deionised water, sucrose solution, and agar gel samples was studied, and the mechanism of ultrasound assisted nucleation was discussed. The samples were frozen in an ethylene glycol–water mixture (− 20 °C) in an ultrasonic bath system after putting them into tubing vials. Ultrasound irradiation (25 kHz, 0.25 W cm⁻¹) was applied continuously for 1, 3, 5, 10 or 15 s at different sample's temperatures in the range of 0 °C to − 5 °C. The nucleation temperatures of the water, sucrose solution and agar gel samples without ultrasound irradiation, occurred stochastically at − 7.4 ± 2.4 °C, −10.6 ± 1.7 °C and − 7.5 ± 0.92 °C, respectively and followed normal distributions. Unlike agar gel samples, the nucleation temperatures of water and sucrose were induced by applying ultrasound for 5 s at different temperatures after a short delay, and linear relationships between the ultrasound irradiation temperatures and the nucleation temperatures were observed. Evaluation of the effect of different durations of ultrasound application on agar gels indicated that 1 s was not long enough to induce nucleation, 3 s was optimal, 5 s and 10 s produced heat and inhibited nucleation, and 15 s did not exhibit significant differences from 3 s and 10 s. It was concluded that longer irradiation durations (especially 5 s and 10 s) caused the sample to heat up, which interrupted or delayed the nucleation. Ultrasound irradiation for 3 s induced nucleation in agar gel samples at different temperatures resulting in a linear relationship between irradiation and nucleation temperatures. The observations indicated that the Hickling's theory, according to which vigorous collapses of bubbles are the only driving mechanism of nucleation, is not adequate to describe the ultrasound assisted nucleation. The results, however, were in agreement with results of some other researchers suggesting that secondary phenomena such as flow streams are also important for the initiation of nucleation. In conclusion, the use of ultrasound as a means to control the crystallisation process offers promising application in food freezing, though further investigations are needed for understanding the mechanisms, especially in solid foods.     

Influence of commercial freezing and storage on vitamin C content of some vegetables

Vitamin C levels of commercially frozen okra, potatoes, green beans, broccoli, spinach and peas, including the impact of processing and storage, were studied. Depending on the vegetable type, prefreezing operations caused a 19.1–51.5% decrease in the initial vitamin C levels. The freezing process alone did not influence the vitamin levels except in the cases of green beans and spinach. Total losses (%) were between 27.6 and 57.9 for the vegetables at the end of commercial frozen storage (6 months). All the data obtained from this study confirmed that, depending on the vegetable type, prefreezing operations have a major impact on the vitamin C contents and this influence persists in the frozen storage.     

Impact of pretreatment and freezing conditions on the microstructure of frozen carrots: Quantification and relation to texture loss

The textural quality of carrots subjected to pretreatments affecting the pectin structure in combination with different freezing conditions was studied. Carrot samples frozen under different conditions were extensively studied by light microscopy quantifying the freezing damage based on the analysis of different parameters (number, area, perimeter, and shape factor of tissue particles) associated with carrot tissue damage. The reduced texture loss of rapidly or cryogenically frozen carrots, compared to slowly frozen samples, was associated with the reduction in cell wall damage in the carrot tissue. In case no pretreatment was used, carrot texture was only slightly improved by using high-pressure shift freezing instead of slow freezing. Detailed analysis of the different steps involved showed that severe tissue damage occurred during the completion of the high-pressure freezing process at atmospheric pressure. However, tissue damage, and thus texture loss, of high-pressure frozen carrots could be minimized by applying pretreatments consisting of a thermal treatment at 60 °C and a high-pressure treatment at 300 MPa and 60 °C.     

Study on the influence of freezing rate on lipid oxidation in fish (salmon) and chicken breast muscles

Chicken breast and salmon muscle were frozen either slowly or rapidly at –25°C, and then stored at –5°C for up to 83 and 47 days respectively. At intervals samples were thawed and TBA values measured on either muscle distillate or centrifugal exudates. TBA values increased slightly but significantly with storage time, but freezing rate had no effect.     

Effects of the methods of pre-treatment before freezing on the retention of chlorophylls in frozen leaf vegetables prepared for consumption

The investigation included kale, New Zealand spinach and spinach. The evaluation covered the raw material; the raw material after blanching; the raw material after cooking; and frozen products prepared for consumption after 0, 4, 8 and 12 months of refrigerated storage. Both the traditional method of freezing (blanching before freezing) and the modified method of freezing (cooked before freezing) were used in the experiment, as well as two storage temperatures, T = −20 °C and T = −30 °C. The content of chlorophylls in fresh kale was four times that in New Zealand spinach and 1.5 times that in spinach. With the exception of New Zealand spinach, blanching and cooking significantly reduced the content of chlorophylls. In kale products prepared for consumption, the content of chlorophylls decreased in each successive stage of the investigation. In products of New Zealand spinach and spinach, the losses were usually not significant. After 12 months of refrigerated storage, frozen kale products prepared for consumption retained 52–65% of total chlorophylls compared with the content in the raw material; products of New Zealand spinach and spinach retained 66–71%. In kale and New Zealand spinach, the content of chlorophyll a decreased more rapidly than that of chlorophyll b, while in spinach the converse was true. The kale products obtained using the modified method contained more chlorophylls, while in the two spinach species their content was lower. The lower storage temperature resulted in a higher retention of chlorophylls in vegetables.     

Effect of different freezing processes on the microstructure of Atlantic salmon (Salmo salar) fillets

Salmon fillets were frozen either by pressure shift freezing (PSF, 200 MPa, − 18 °C or 100 MPa, − 10 °C) or by air-blast freezing (ABF, − 30 °C, 1 m/s or 4 m/s) or direct-contact freezing, and then stored at − 20 °C for 6 months. The influence of these treatments on the microstructure of Salmon fillets was studied. The equivalent diameter of the intracellular ice crystals were 14.69 ± 4.11, 5.52 ± 2.11, and 30.65 ± 6.31 μm for the samples subjected respectively to PSF at 100, 200 MPa and ABF (− 30 °C, 4 m/s) after 2 days of storage. Smaller and more regular intracellular ice crystals were observed in fillets frozen by PSF (200 MPa) compared with PSF (100 MPa), ABF and direct-contact frozen ones. Significant differences were observed between the size of the ice crystals obtained after conventional freezing process and PSF. Large and extracellular ice crystals were observed in fillets frozen by ABF (1 m/s) and direct-contact frozen. Minimal changes in the size of ice crystals were observed during a 3 months storage.

Industrial relevance

This paper compares different freezing methods and subsequent frozen storage with respect to their effect on microstructures of salmon fillets. Pressure shift freezing at 200 MPa was superior to conventional freezing regarding small and regular ice crystal formation. Interestingly, during frozen storage for up to 3 months the high quality product obtained via pressure freezing at 200 MPa could be retained. For longer storage periods lower pressures (100 MPa) seem sufficient to achieve stable ice crystals.     

黑龙江4种市售稻米的基础特性研究

目的对葵花阳光、五常稻花香、松花江和糙米4种黑龙江市售稻米的基础特性进行研究。方法采用布拉本德黏度仪、X-射线衍射仪、差示扫描量热仪(differential scanning calorimeter,DSC)和扫描电子显微镜等对稻米粉的基础成分、糊化特性、晶体结构、热力学特性和微观结构及其相关性进行分析。结果 4种市售黑龙江稻米总淀粉含量具有一定差异性,糙米样品直链淀粉含量低,脂肪和灰分含量高,蛋白含量较高;糊化起始温度、终止温度和峰值黏度具有一定差异性,糊化时间差异性较大;DSC图谱均呈现吸热单峰趋势,4个样品的TO、TP、TC、ΔH具有一定差异性;不同稻米的X-射线图显示,其晶区均呈现A-型和V-型衍射结构;稻米的糊化峰值黏度、ΔH、结晶度与其直链淀粉、脂肪和蛋白含量呈显著负相关;电镜观察显示,稻米粉中的淀粉块表面的蛋白、脂肪滴清晰可见,淀粉粒径均在6~10μm之间。结论 4种市售黑龙江稻米在基础特性方面存在一定差异性和相关性。     

Recent advances in the improvement of freezing time and physicochemical quality of frozen fruits and vegetables by ultrasound application

Freezing is widely used for the preservation of fruits and vegetables (FVs). Whereas, the conventional freezing method presents low process efficiency in frozen FVs. Ultrasound application is used as a supplementary technology to improve the freezing process of FVs. This reviewed paper shows the cavitation effect of ultrasound and the effect of ultrasound application on freezing time and the physicochemical quality of frozen FVs. The cavitation effect of ultrasound in the freezing process was described in detail. Compared with conventional freezing, ultrasound application in freezing of FVs can shorten the freezing time, and improve physicochemical quality including drip loss, colour, firmness, chemical compositions (ascorbic acid, total phenolic and anthocyanin), as well as microstructure. Therefore, the results of these studies illustrated that ultrasound is a potential technology to enhance freezing efficiency and retain the quality of FVs during freezing.     

The impact of home freezing on the sensory characteristics of ready-to-use leafy vegetables

BACKGROUND: Owing to the increasing trend of consumption of ready‐to‐use leafy vegetables, the necessity of determining the best conditions for their frozen storage and the considerable impact of freezing on their sensory attributes, research was carried out to determine the best freezing temperature and storage time for a mixture of Allium ampeloprasum, Lepidium sativum and Stureia hortensis. RESULTS: The results for freezing temperature at three different storage times showed that colour and overall acceptability at ? 18 °C were always ranked first (P < 0.05), while taste at ? 18 °C was ranked first on days 120 and 150. The results for frozen storage time at three different temperatures indicated that colour, taste and acceptability were not significantly different. CONCLUSION: Overall, the results of this research indicated that the sensory attributes of leafy vegetables during 180 days of frozen storage were affected mainly by freezing temperature rather than frozen storage time. Copyright © 2010 Society of Chemical Industry     

反复煎炸对4种食用植物油品质的影响

以特级初榨橄榄油、混合橄榄油、一级大豆油、一级葵花籽油作为煎炸油,以薯条、鸡翅为煎炸原料,对4种食用植物油在重复煎炸过程中发生的品质变化进行观察。方法 测定酸价、羰基价和极性组分,并进行感官评价。结果 4种食用植物油的酸价、羰基价变化与煎炸次数有关;各种食用植物油的品质变化有差别,且煎炸荤食时的品质变化比煎炸素食时的品质变化大。结论 4种食用植物油在反复煎炸后品质均有劣变趋势,但特级初榨橄榄油总的变化率处于较低水平,可见特级初榨橄榄油是煎炸油的上佳选择。     

烹调对蔬菜中烟酸含量的影响

为研究不同烹调方法对蔬菜中烟酸含量的影响，对7种蔬菜(白萝卜、胡萝卜、豇豆、扁豆、茄子、土豆、菠菜)采用6种不同烹调方法(炒、炖、腌、蒸、焯、炸)进行处理，用微生物法测定其鲜样和烹调后样品的烟酸含量。以烹调至可食用状态为标准进行比较。蔬菜经6种不同烹调方法处理后，其烟酸保存率大部分在60％以上。经过炒、炖处理后的蔬菜中烟酸保存率较高；经过焯和大量失去水分的腌，蔬菜中烟酸保存率较低。其中炸扁豆、炒胡萝卜、炒茄子烟酸含量与其鲜样相比差异无显性意义(P＞0．05)，其它烹调后样品烟酸含量与未处理的鲜样比较差异均有显性意义(P＜0．01)。本研究结果对食品工业加工方法具有指导意义和参考价值。     

High-pressure shift freezing: recrystallization during storage

High-pressure shift freezing has been proposed as a method to produce frozen food with smaller ice crystal size and, consequently, with reduced tissular damage and higher overall quality. The fate of this initially improved crystal size distribution, decisive for the long-term value of this procedure, is unclear. The recrystallization behaviour of partially frozen aqueous solutions, as food models, is here compared with that of similar classically frozen samples. A microscopic observation cell has been specially designed for this purpose. The temporal evolution of high-pressure shift frozen ice crystals has been fitted to different mechanism models and is found to be similar within experimental error to that of classically frozen samples. However, differences in the shape evolution of crystals have been detected, which can be ascribed to small differences in the initial distribution. The implications of these observations for the long-term storage of frozen food are discussed.     

Influence of the frozen storage on some characteristics of ripened Manchego-type cheese manufactured with a powdered vegetable coagulant and rennet

A study was made of the effect of freezing and nine months’ frozen storage on the chemical, microbiological and sensorial characteristics of fully ripened Manchego-type cheese. Chemical components, water activity and nitrogen compounds were not altered by freezing and frozen storage. Soluble and non-protein nitrogen were significantly higher in cheeses produced with the powdered vegetable coagulant (PVC) compared to those made with rennet. Proteolysis continued slowly during frozen storage, with higher amino acid and ammonia nitrogen rates at the end of the storage period. Micrococci (P-value <0.05, in cheese made with PVC) and staphylococci counts tended to decrease during frozen storage; however total viable, lactic acid bacteria and mold-yeasts counts were similar throughout the nine months. Changes in sensorial parameters were observed (P-value <0.05) throughout freezing and frozen storage, affecting the creaminess of cheeses made with chymosin and the taste intensity and salty taste of cheese obtained with vegetable coagulant. Cheeses obtained with PVC proved to have a more acid taste and greater taste intensity (P < 0.01) as well as displaying a more bitter taste, greater creaminess and lower degree of hardness (P-value <0.001) than those produced with rennet.     

Effect of freezing on the viscoelastic behaviour of whey protein concentrate suspensions

The effect of freezing on viscoelastic behaviour of whey protein concentrate (WPC) suspensions was studied. Suspensions with total protein content of 5% and 9% w/v were prepared from a commercial WPC (unheated suspensions). A group of unheated suspensions was treated at two temperatures (72.5 and 77.5 °C) during selected times to obtain 60% of soluble protein aggregates (heat-treated suspensions). Unheated suspensions and heat-treated suspensions were frozen at −25 °C (frozen unheated and frozen heat-treated suspensions). Frequency sweeps (0.01–10 Hz) were performed in the region of linear viscoelasticity at 10, 20, 30, 40, and 50 °C. Mechanical spectra of all studied suspensions at 20 °C were similar to viscoelastic fluids and complex viscosity increased with the frequency (ω). Elastic (G′) and viscous (G″) moduli were modelled using power law equations (G′ = aω^x, G″ = bω^y), using fitted parameters a, x, b, and y for statistical analysis. Exponent y was the most influenced by freezing, indicating the existence of a higher degree of arrangement in frozen unheated suspensions and a lower degree of arrangement in frozen heat-treated suspensions. Only characteristic relaxation times (inverse of the crossover frequency) of suspensions with 5% w/v of total protein content were significantly influenced by freezing. Time–temperature superposition was satisfactory applied in unheated whey protein concentrate suspensions only in the range of high temperatures (30–50 °C). However, this principle failed over the complete temperature range in most of the frozen suspensions. It is possible that freezing produced an increase in the susceptibility to morphological changes with temperature during the rheological measurements.     

高效液相色谱-柱后衍生法测定植物油脂中 4种黄曲霉毒素

目的建立一种可同时测定植物油脂中黄曲霉毒素G2、G1、B2、B1残留量的分析方法。方法将待测样品经前期溶剂提取、免疫亲和柱净化处理后,通过高效液相色谱-柱后衍生-荧光检测器进行测定。结果黄曲霉素G2、B2在0.1~2 ng/m L范围内表现出良好的线性关系,而黄曲霉素G1、B1在1~20 ng/m L范围内r0.9995,其线性关系良好。在添加浓度为0.1~20μg/kg的范围内,平均回收率在75.0%~125.7%之间。结论该方法快速简便准确,可作为植物油脂中黄曲霉毒素的残留量测定方法。     

Using power ultrasound to accelerate food freezing processes: Effects on freezing efficiency and food microstructure

Abstract

Freezing is an effective way of food preservation. However, traditional freezing methods have the disadvantages of low freezing efficiency and generation of large ice crystals, leading to possible damage of food quality. Power ultrasound assisted freezing as a novel technique can effectively reduce the adverse effects during freezing process. This paper gives an overview on recent researches of power ultrasound technique to accelerate the food freezing processes and illustrates the main principles of power ultrasound assisted freezing. The effects of power ultrasound on liquid food, model solid food as well as fruit and vegetables are discussed, respectively, from the aspects of increasing freezing rate and improving microstructure. It is shown that ultrasound assisted freezing can effectively improve the freezing efficiency and promote the formation of small and evenly distributed ice crystals, resulting in better food quality. Different inherent properties of food samples affect the effectiveness of ultrasound application and optimum ultrasound parameters depend on the nature of the samples. The application of ultrasound to the food industry is more likely on certain types of food products and more efforts are still needed to realize the industrial translation of laboratory results.