USE OF VACUUM IMPREGNATION TO DEVELOP HIGH QUALITY AND NUTRITIONALLY FORTIFIED FROZEN STRAWBERRIES

Strawberries (Totem) were vacuum impregnated (VI) before freezing with cryoprotectants of high fructose corn syrup (HFCS) or high methoxyl pectin (HMP) to improve quality, and with calcium and zinc salts to enhance the nutritional value of the product. The VI process consisted of a 15 min vacuum at 50 mm Hg and 30 min restoration at atmospheric pressure. VI pretreatment significantly increased the calcium and zinc content of frozen strawberries. VI with cryoprotectant improved the textural quality and reduced drip loss of frozen-thawed strawberries in comparison with untreated strawberries. Calcium in the VI solutions further increased the firmness of frozen-thawed strawberries, and zinc improved the color stability of the strawberries during the impregnation and freeze-thawing process.     

Vacuum Impregnation Pretreatment with Pectinmethylesterase to Improve Firmness of Pasteurized Fruits

ABSTRACT: Vacuum impregnation allows replacing the occluded air present in fruits by a solution. Apples, strawberries, and raspberries were infused in a solution of pectinmethylesterase (PME) and calcium chloride (CaCl₂) either by using this technology or by classical infusion. The vacuum pretreatment limited the loss in fruits firmness following pasteurization. The influence of PME and CaCl₂concentrations in the vacuum impregnation solutions on the calcium content, the PME activity and the firmness of pretreated strawberries was studied: the firmness of strawberries was maximal after impregnation in solutions containing a threshold concentration of PME and/or calcium. The synergistic action of PME and calcium used in the vacuum impregnation solution was significant.     

Enzyme infusion and thermal processing of strawberries: Pectin conversions related to firmness evolution

Strawberries were infused with fungal pectinmethylesterase (PME) and/or calcium chloride with the aim of minimising tissue damage during subsequent thermal processing (95 °C). Firmness measurements and micrographs provided information on the extent of tissue damage. These observations were linked to the chemical structure of pectin. When PME was infused in absence of Ca²⁺, the degree of methoxylation of pectin was lowered, but chains remained water soluble, indicating that they were not crosslinked. Thermal processing of PME-infused strawberries resulted in pectin solubilisation and depolymerisation which was reflected in pronounced firmness decrease and tissue damage, comparable to non-infused processed strawberries. On the other hand, when a combination of both PME and Ca²⁺ was infused, an important decrease in processing-related tissue damage was perceived. This can be explained by increased crosslinking of pectin chains with low degree of methoxylation, rendering them insoluble and less susceptible to thermal depolymerisation.     

不同预处理方式对速冻草莓品质的影响

确定了速冻草莓的最佳冻结时间,并研究了用不同预处理方式处理速冻草莓在冻藏期间的VC含量、pH值的变化,硬度与汁液流失率.速冻预处理溶液采用蔗糖和氯化钙溶液,通过改变浸蘸溶液的浓度和浸蘸时间来改变处理方式.经实验最终确定,最佳速冻时间为5 h,速冻草莓冻藏期间的VC含量逐渐降低,pH值呈上升趋势,经20%糖液处理的草莓色泽较好,用CaCl2溶液预处理草莓的方式可明显提高草莓硬度,减少其汁液流失率.     

Enzyme infusion prior to thermal/high pressure processing of strawberries: Mechanistic insight into firmness evolution

Strawberries were infused with fungal pectinmethylesterase (PME) and calcium chloride, followed by a thermal (70 °C–0.1 MPa), a high pressure (25 °C–550 MPa) or a combined thermal-high pressure (70 °C–550 MPa) process. Macroscopic (firmness) and microscopic characteristics were assessed to evaluate the texture of the fruits. In order to interpret the texture changes, the chemical structure of pectin was investigated. Processing of strawberries caused a decrease in firmness, which was limited by infusion of PME and calcium chloride, although the extent of beneficial effects depended on the type of processing. PME was able to decrease the degree of methoxylation of pectin, which was accompanied by an increased crosslinking of the chains. During high pressure or combined thermal-high pressure processing, the degree of methoxylation of pectin in infused strawberries was even further decreased, probably due to a higher activity of the fungal PME under high pressure. In case of the high pressure process, this was reflected in a very firm texture. However, the combined thermal-high pressure process caused more severe tissue damage, in spite of the advantageous pectin properties.Industrial relevanceDuring high pressure processing of strawberries many nutritional and sensorial characteristics are quite well preserved. Unfortunately, texture of strawberries deteriorates during such processes. This paper provides mechanistic insight into how infusion of fungal pectinmethylesterase and calcium ions in strawberries can preserve the firmness of these fruits during high pressure processing.     

Minimizing texture loss of frozen strawberries: effect of infusion with pectinmethylesterase and calcium combined with different freezing conditions and effect of subsequent storage/thawing conditions

Vacuum infusion (VI), freezing, frozen storage and thawing conditions were optimized in order to minimize the texture loss of frozen strawberries. Slow freezing caused severe loss in textural quality of the strawberries. This quality loss could not be prevented by the application of VI prior to slow freezing, or by the application of rapid, cryogenic or high-pressure shift freezing conditions on non-infused fruits. A remarkable texture improvement was noticed when infusion of pectinmethylesterase (PME) and calcium was combined with rapid or cryogenic freezing. The highly beneficial effect of PME/Ca-infusion followed by HPSF on the hardness retention of frozen strawberries was ascribed to the combined effect of the infused PME (53% reduction in degree of esterification (DE) of the strawberry pectin) and the high degree of supercooling during HPSF. During frozen storage, textural quality of PME/Ca-infused high-pressure frozen strawberries was maintained at temperatures below −8 °C, whereas the texture of PME/Ca-infused strawberries frozen under cryogenic freezing conditions was only preserved at temperatures below −18 °C. Thawing at room temperature seemed to be an appropriate method to thaw strawberries. Fast thawing by high-pressure induced thawing (HPIT) did not prevent textural quality loss of frozenstrawberries.     

Texture improvement of fresh and frozen mangoes with pectin methylesterase and calcium infusion

BACKGROUND: A major problem of mango products is texture loss. The effect of commercial pectin methylesterase (PME) and calcium infusion on improvement of the texture of both fresh and frozen‐thawed mango cubes was investigated in the present study. RESULTS: The weight gain and moisture content of mango samples were greater at relatively high vacuum level (10 kPa). The PME activity of samples infused with PME and calcium at 10 kPa increased fourfold in comparison with that of control and water‐infused samples. The combined effect of PME and calcium was found to improve the texture and microstructure of both fresh and frozen‐thawed mangoes. Fresh mangoes infused with PME and calcium at 10 kPa showed significantly higher firmness than control fresh samples. Frozen‐thawed mangoes infused with PME and calcium at 50 kPa and atmospheric pressure had superior texture and microstructure in comparison with control frozen‐thawed samples. CONCLUSION: The results of the present analysis allow for a better appreciation of the role of PME, calcium and appropriate infusion conditions in improving the texture of both fresh and frozen‐thawed mangoes. Copyright © 2012 Society of Chemical Industry     

Improving textural quality in frozen jalapeño pepper by low temperature blanching in calcium chloride solution

The effects of low temperature blanching in calcium chloride solution of jalapeño peppers prior to freezing, on firmness retention, pH, methanol, colour and calcium ions of the product, were evaluated by response surface methodology. Texture, methanol content and pH were affected by all of the variables studied (P = 0.05). The optimum response was obtained at temperatures of 63.3–66 °C, calcium chloride concentrations of 0.17–0.21 m , immersion time of 11.6–14.4 min, with a holding time after blanching of 56.6–66.1 min. Microscopic evaluation of the thawed pepper showed that blanching in CaCl₂ solution provided a protective effect in maintaining cell wall integrity.     

Texture improvement of fermented minced pepper under vacuum impregnation with pectin methylesterase and CaCl2 during fermentation

Fermented minced pepper (FMP) usually suffers from the deterioration of texture quality during fermentation, which can affect sensory and consumer acceptance. In this study, vacuum impregnation (VI) with CaCl₂, pectin methylesterase (PME) and CaCl₂ and PME (PME + CaCl₂ + VI) were compared to improve the texture quality of FMP. FMP treated with PME + CaCl₂ + VI showed the relatively intact cells structure after fermentation. In that case, its firmness maintained high level, while water-soluble pectin (WSP) content was minimum after fermentation. Meanwhile, the molar ratio of most monosaccharides of WSP in PME + CaCl₂ + VI treated FMP decreased, while rhamnose (Rha) molar ratio significantly (p < 0.05) increased after fermentation. The high Rha content represents the stability of rhamnogalacturonan-I linear skeleton of WSP. The negative effect on molecular weight of WSP was delayed by PME + CaCl₂ + VI treatment, and its peak area and value increased after fermentation. Atomic force microscope images indicated that PME + CaCl₂ + VI treatment could retain the long chain and branch structures, and inhibit the degradation of WSP net-like structure at some extent. Hence, PME + CaCl₂ + VI treatment was effective to improve the texture of FMP and inhibit the solubilisation of WSP via the formation of cross-linked pectin chains between Ca²⁺ and demethylesterified pectin.     

Freezing Efficiency and Quality Attributes as Affected by Voids in Plant Tissues During Ultrasound-Assisted Immersion Freezing

Voids, filled with air, in plant tissues, can attenuate ultrasound, resulting in weakening the effectiveness of ultrasound during immersion freezing. The effect of voids on ultrasound-assisted immersion freezing (UF) in selected plant tissues, apple, radish, and potato was investigated in the present study. The freezing time and quality attributes of firmness, drip loss, total calcium content, and total phenolic content were investigated in apple, radish, and potato samples treated by normal immersion freezing (IF) and UF. The results showed that the more the percentage voids in the plant tissues, the lower the effectiveness of the ultrasound treatment. The total freezing time reduction (%) due to UF was a power function of the volume of voids (%): y?=?0.018x^?1.057 (R²?=?0.994). Ultrasound at 0.62 W/cm² (28 kHz) resulted in the best firmness and lowest drip loss in potato, while no significant (p?>?0.05) differences in quality attributes were observed between IF and UF in apple samples. These findings indicated that UF was more effective in freezing fruit or vegetables with a highly dense structure.     

Thermal and Calcium Pretreatment Affects Texture, Pectinesterase and Pectic Substances of Frozen Sweet Cherries

Freezing causes loss of turgidity and firmness in sweet cherries. Thermal pretreatment at 50°C for 10 min followed by immersion in 100 mM CaCl₂ and thermal pretreatment at 70°C/2 min with or without immersion in 100 mM CaCl₂ prevented freezing-induced loss of firmness. Thermal pretreatments increased the pectin fraction soluble in EDTA, reduced the degree of pectin esterification, and increased both the concentration of divalent cations in the cell wall and the pectinesterase activity bounded to the cell wall. Immersion in CaCl₂ increased the concentration of Ca²⁺ cations in the cell wall and enhanced the effect of thermal pretreatments on pectinesterase activity.     

Effect of processing on the texture and structure of raspberry (cv. Heritage) and blackberry (cv. Thornfree)

This paper reports separate studies of the effect of pre-treatments (CaCl₂, low methoxyl pectin (LMP), and combined solutions) and the effect of freezing method (at four different rates) and thawing mode (at two different rates) on objective parameters, structure and sensory characteristics of fresh raspberries and blackberries. After that, the effect of a complete freezing process combining the best pre-treatments with the best freezing/thawing conditions found for each fruit was investigated. Kramer Shear Cell (KSC), back extrusion, compression and multiple penetration tests were used to measure fruit texture objectively. For calcium and LMP pre-treatments, which were applied separately, texture parameters were significantly higher in samples treated at the highest concentrations (100 mM of CaCl₂ for both fruits and 0.3 and 3% of LMP for raspberry and blackberry, respectively) compared to fresh controls. Blackberry structure was more susceptible than raspberry structure to the effect of pre-treatments. For the combined pre-treatments, the highest texture parameters were found in the samples treated with CaCl₂ (100 mM) and LMP (0.1%) in the case of raspberries and CaCl₂ (100 mM) and LMP (3%) in the case of blackberries. Combined pre-treatment did not increase firmness with respect to that of samples treated only with calcium, which indicates that CaCl₂ preserved the raspberry structure more efficiently during processing. Fruits frozen by forced convection with liquid nitrogen vapour at –40 °C were significantly firmer. Raspberries should be thawed at 5 °C, whereas blackberries may be thawed at room temperature. Sensory analysis showed that the blackberry structure was more resistant to freezing. In both fruits, over the complete process parameter values were again highest in the samples treated with 100 mM CaCl₂, applied either separately or in combination with LMP. In raspberry, panellists detected no significant differences between sensory texture parameters of the different samples, and in blackberry, panellists found no significant differences between any of the sensory characteristics. Multiple penetration maximum force (F _MP) was the parameter that best expressed product firmness for both fresh and frozen raspberries, whereas compression slope (S _C) best reflected changes in blackberries. SEM mainly corroborated results from objective texture parameters.     

Effects of calcium pre-treatments and freezing rates on fluid loss from plantain products

Ripe plantain slices treated with calcium chloride (0, 1360 or 2700 ppm Ca) solutions were packed one layer thick in flexible pouches prior to freezing in still air at −18 to −20°C, liquid immersion at −18 to −20°C, and cryogenically in liquid nitrogen. A new technique in which the extent of fluid loss from thawed plantain slices could be associated with tissue damage was used to assess quality of the thawed plantain slices. Plantain frozen in liquid nitrogen had the least fluid loss followed by liquid immersion and still air feezing. Treating with calcium ions prior to freezing significantly reduced fluid loss in all freezing methods used. In another experiment the treated plantain slices were frozen in still air −18 to −20°C followed by sensory evaluation of the thawed slices after frying in hot peanut oil. Calcium also significantly increased firmness in the treated slices compared to the control and there were no adverse effects on flavour and appearance. The techno-economic implications of the study are discussed.     

不同预处理方式对冻藏草莓品质的影响

为了解决速冻草莓在贮藏过程中品质快速下降的问题,采用糖溶液和氯化钙溶液对其进行处理,以改善其冻藏品质。以VC、汁液流失率、电导率、硬度、可溶性固形物和总酸含量为评价指标,考察不同预处理后在－18 ℃条件下冻藏0.5、1.0、2.0、3.0、4.0、5.0 个月时草莓的品质变化情况。结果表明：20%糖溶液处理20 min的处理组VC损失率最小,可溶性固形物含量、电导率变化最小,总酸含量降低得慢,且与其他处理组相比差异显著（P＜0.05）;3%氯化钙溶液处理15 min的处理组汁液流失率最小,硬度变化最小,且与其他处理组相比差异极显著（P＜0.01）。     

Quality attributes and cell wall properties of strawberries (Fragaria annanassa Duch.) under calcium chloride treatment

Effects of CaCl₂ (0%, 1% and 4%) treatment on quality attributes and cell wall pectins of strawberry fruits stored at 4 °C for 15 d were investigated. Strawberry firmness was not significantly affected by CaCl₂ treatment. Compared to the other groups, the 1% CaCl₂ group had better quality attributes, including decay rate, weight loss and soluble solids content. The treatment with 4% CaCl₂ inhibited weight loss but caused phytotoxicity. During storage, the chain widths and lengths of water-soluble pectin (WSP), chelate-soluble pectin (CSP) and sodium carbonate-soluble pectin (SSP) decreased. Strawberry softening seemed to be due to modifications of CSP and SSP, especially the side chains. CaCl₂ treatment significantly slowed the breakdown of CSP and SSP chains by strengthening the ionic crosslinkages among these pectin molecules. These results illustrate the fundamental CaCl₂ effects and will help improve the application of CaCl₂ to postharvest fruits.     

Prefreezing application of whey protein‐based edible coating to maintain quality attributes of strawberries

Prefreezing application of whey protein concentrate (WPC)‐based edible coating to maintain quality attributes of strawberries was studied. Beeswax (BW) was added to the solutions (0%, 20% and 40% with respect to the solids contained in the mixture WPC/glycerol). Coated and control fruits were frozen, stored at ?20 °C and thawed. After thawing, weight loss, firmness, microstructure and colour parameters were measured. Coating with 20% BW reduced strawberries weight loss after thawing (55%). Strawberries firmness was maintained equally in all groups analysed although a slight improvement at the cell microstructure alterations caused by the freezing process was observed in coated fruits. Strawberries brightness was similar in all groups. Colour parameter a* showed a tendency to decrease with the increasing BW concentration, and only b* of coated fruits was lower than controls. The application of whey protein coating could be an attractive treatment to maintain quality attributes of strawberries undergoing the freezing process.     

Calcium Uptake during Immersion of Strawberries in CaCl2 Solutions

The present study studied calcium uptake in strawberries during immersion in CaCl(2) solutions by analyzing total calcium and cell-wall calcium contents and the relationships between cell-wall calcium content, pectin content, and the degree of pectin esterification, considering different temperatures and concentrations of the immersion solution. In addition, calcium uptake at different zones of the strawberries was studied. Calcium uptake increased significantly when the concentration of the immersion solution increased from 1% to 10% (v/v), and calcium uptake was higher at 38 degrees C at some immersion times, but there were no significant differences between 1 and 24 degrees C. Most of the calcium incorporated by the strawberries was retained in the alcohol insoluble solids and it may be related to calcium bounded to the cell wall. Calcium content did not change during immersion in CaCl(2) solution in the internal or in the central zones; however, it increased significantly in the external zone.     

The effect of the pre-treatments and the long and short-term frozen storage on the quality of raspberry (cv. Heritage)

Results are presented of the effect of pre-treatments before freezing followed by long and short-term frozen storage (12 months at –18 °C and 24 days suffering temperature fluctuations between –18 °C and –12 °C) on quality parameters of raspberry. Pre-treatments were carried out with calcium, low methoxyl pectin, a combined solution, and results compared with untreated control fruits. Kramer shear, back extrusion, compression, and multiple penetration tests were used to measure rheological behavior. One-hundred mM CaCl₂ reduced the long and short-term frozen storage induced loss of firmness. For long-term storage at –18 °C, a softening of the tissue became evident between 3 and 12 months and at each date test the stored fruits were firmer than those without storage. For short-term storage with fluctuations, the loss of firmness was evident between 0 and 24 days, and at all the testing dates the stored fruits were softer than those without storage. Results evidenced a higher cell damage in the short-term frozen storage. Coefficients of softening per day suffering fluctuation were determined, the highest values being given by Kramer shear energy and back extrusion maximum force (>1%). Short-term frozen storage affected physical and physico-chemical characteristics, increasing the saturation (r) and the anthocyanins and decreasing the ascorbic acid of the raspberries. In both storage conditions, pre-treatments reduced the drip loss, which correlated best with the Kramer shear energy. Panelists detected mainly time effect on the sensory firmness. For long-term, sensory firmness and juiciness gave the highest correlations with back extrusion maximum force, while for the short-term, sensory firmness and drip loss gave the highest correlations with the Kramer shear energy. SEM revealed different degrees of mechanical damage to structure, which accounted for rheological behavior of the fruits.     

Use of pectinmethylesterase and calcium in osmotic dehydration and osmodehydrofreezing of strawberries

Strawberry samples of two varieties (Camarosa and Elsanta) were dehydrated using different osmotic solutions (60% glucose, fructose, sucrose and raftilose) and subsequently frozen by rapid and high-pressure shift freezing (HPSF). The effect of pectinmethylesterase (PME) and calcium (Ca⁺⁺) added to the osmotic solutions on several compositional parameters and the textural/structural quality of dehydrated and osmodehydrofrozen-then-thawed samples was studied. Due to the presence of PME and Ca⁺⁺ in the osmotic solutions, weight reduction upon dehydration was slightly decreased, which was correlated to a small positive effect on the net uptake of sugars and depression of the initial freezing point. Except for the Camarosa samples treated with sucrose, PME and Ca⁺⁺ in osmotic sugar solutions positively affected the relative hardness of dehydrated fruits, which was ascribed to the effect of PME and Ca⁺⁺ on the cell wall strength of the tissue. No cell wall damage and tissue particle alterations were observed upon dehydration. The effect of osmotic dehydration (OD) using different sugar solutions without PME and Ca⁺⁺ on the texture and structure of frozen-then-thawed samples was limited and sometimes negative. The added PME and Ca⁺⁺ however positively influenced the volume and shape of the thawed samples, which could be related to slightly higher relative hardness values and, for the Elsanta strawberry fruits, also to the reduced (up to 81%) drip loss upon thawing. Upon freezing the dehydrated fruits, no cell wall disruption was observed. Tissue distortion caused by freezing and indicated by a decrease in particle size, convexity and roundness, was compensated by the use of PME and Ca⁺⁺ during OD.     

Vacuum Infusion of Citrus Pectinmethylesterase and Calcium Effects on Firmness of Peaches

Vacuum infiltration of Majestic peaches (Prunus persica) for 1 hr with a solution of Marsh grapefruit pectinmethylesterase (PME) containing 100 mg/L CaCl₂ significantly increased firmness of canned peaches. Mean firmness of peaches infused 2 hr and thermally processed was 13.9 J/Kg as compared to 3.2 J/kg for noninfused, processed controls. The calcium content increased from 278 to 432 mg/kg during 2 hr infusion of blanched peaches. Specific activity of PME in peach halves increased more than 20 fold after infusion.