Higher molecular weight pectin inhibits ice crystal growth and its effect on the microstructural and physical properties of pectin cryogels.

Understanding the formation of ice crystals is essential for tailoring the microstructure and physical properties of cryogels. This study investigated the effects and mechanisms of pectin molecular weight (Mw) on impacting ice crystal formation. Pectin fractions various Mw (10.13-212.20 kDa) were prepared by hydrothermal method. The solution of high Mw pectin fractions exhibited higher contact angle, lower water freedom, and stronger adsorption of water molecules. The splat experiment and molecular dynamic (MD) results confirmed that higher Mw pectin have stronger ice crystal growth inhibition activity than lower Mw pectin. Furthermore, the pore size distribution of the cryogel increased from 98-203 µm to 105-267 µm as the molecular weight decreased from 212.2 kDa to 121.0 kDa. Additionally, in the higher Mw pectin cryogel, stronger mechanical strength was observed. These findings suggested that changing the molecular weight of pectin has the potential to regulate the ice crystal growth, microstructure and physical properties of frozen products.

Tailoring microstructure and mechanical properties of pectin cryogels by modulate intensity of ionic interconnection.

Porous morphology and mechanical properties determine the applications of cryogels. To understand the influence of the ionic network on the microstructure and mechanical properties of pectin cryogels, we prepared low-methoxyl pectin (LMP) cryogels with different Ca2+ concentrations (measured as R-value, ranging from 0 to 2) through freeze-drying (FD). Results showed that the R-values appeared to be crucial parameters that impact the pore morphology and mechanical characteristics of cryogels. It is achieved by altering the network stability and water state properties of the cryogel precursor. Cryogel precursors with a saturated R-value (R = 1) produced a low pore diameter (0.12 mm) microstructure, obtaining the highest crispness (15.00 ± 1.85) and hardness (maximum positive force and area measuring 2.36 ± 0.31 N and 12.30 ± 1.57 N·s respectively). Hardness showed a negative correlation with Ca2+ concentration when R ≤ 1 (-0.89), and a similar correlation with the porosity of the gel network when R ≥ 1 (-0.80). Given the impacts of crosslinking on the pore structure, it is confirmed that the pore diameter can be designed between 56.24 and 153.58 µm by controlling R-value in the range of 0-2.

New insight into pectic fractions of cell wall: Impact of extraction on pectin structure and in vitro gut fermentation.

Pectic polysaccharides modulate gut fermentation ability, which is determined by structural characteristics. In this work, apple pectins were extracted by HCl (HAEP), NaOH (AEP), cellulase (EAEP), and in parallel cell wall pectic fractions were sequentially extracted by water (WEP), chelator (CEP) and NaOH (NEP). The aim is to comprehensively compare the impact of extraction on pectin structure and gut fermentation behavior. Results showed that high content of galacturonic acid (90.65 mol%) and large molecular weight (675 kg/mol) were detected in the HAEP. Molecular morphology of the HAEP presented high linearity, while AEP, EAEP and WEP exhibited compact filamentous structures with highly branched patterns. The AEP was characterized by high yield (33.1 g/100 g d.b.), moderate molecular weight (304 kg/mol) and large extent of rhamnogalacturonan-I region (24.88 %) with low degree of branching (1.77). After in vitro simulated gut fermentation for 24 h, total content of short-chain fatty acid (SCFA) generated with the AEP supplement increased to 36.8 mmol/L, followed by EAEP, HAEP and WEP (25.2, 24.2 and 20.3 mmol/L, respectively). Meanwhile, WEP simultaneously produced the highest ammonia content (22.4 mmol/L). This investigation suggests that the fermentation of AEP produces more beneficial SCFA and less ammonia, thus indicating a better gut fermentation property.

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.

Effects of cell morphology on the textural attributes of fruit cubes in freeze-drying: Apples, strawberries, and mangoes as examples.

The influence of cell morphology on the textural characteristic of freeze-dried apple, strawberry, and mango cubes was evaluated. Corresponding restructured cube samples without intact cell morphology were prepared as controls. Results indicated that the presence of cell morphology strengthened the shrinkage and collapse of samples during freeze-drying, especially in mangoes due to the high content of sugar. Intact cell morphology was found in natural fruit cubes after freeze-drying by scanning electron microscopy (SEM) observation, making them exhibit a more regular microporous structure, further resulting in higher hardness than the restructured cubes. However, the intact cell morphology negatively affected the crispness of freeze-dried cubes since it enhanced structural collapse. The freeze-dried samples without cell morphology would destroy the cellulose structure and form a continuous open-pore structure under the concentration effect of ice crystals during freezing, which accelerates the escape of water molecules, increases the drying rate, and avoid collapse. Sensory experiments found that restructured cubes without intact cell morphology exhibited greater comprehensive acceptance, suggesting the potential application of cell morphology disruption in the future freeze-drying industry.

The role of fructose at a range of concentration on the texture and microstructure of freeze-dried pectin-cellulose matrix cryogel.

Freeze-dried (FD) fruit and vegetable materials with a large amount of sugar are unstable. With the aim to understand the structure formation of FD products, the effects of fructose content on the texture and microstructure of FD matrix were investigated by using pectin-cellulose cryogel model. Cryogels containing fructose of 0-40% were produced using freeze-drying at three different primary drying temperatures of -40, -20, and 20°C. The resultant cryogels were characterized by texture profile analyzer, scanning electron microscope, and µCT. Results indicated that at drying temperature of -40°C, increasing fructose concentration promoted the hardness of the cryogels, and cryogels of 16% fructose obtained maximum hardness. Excessive fructose (≥20%) weakened the described hardness, while exhibiting stronger springiness and resilience. The microstructure showed that dense pores and increased wall thickness due to fructose aggregation were critical factors responsible for increased hardness. The porous structure as well as relatively large pore size were necessary for crispness, in addition, rigid pore wall with certain strength were also required. At the drying temperature of 20°C, large hetero-cavities dominated the microstructure of cryogels with 30% and 40% fructose, caused by melting inside during FD process. In this situation, lower Tm (-15.48 and -20.37°C) were responsible for cryogels' melting In conclusion, if possible, regulating fructose content and state may enable the precision texture design of FD fruit and vegetable foods.

Influence of starch with different degrees and order of gelatinization on the microstructural and mechanical properties of pectin cryogels: A potential pore morphology regulator.

The applications of cryogels are defined by their porous morphology as well as mechanical properties. To achieve efficient regulation of porous properties for pectin cryogels, we selected starch as a potential polysaccharide regulator. Pectin/starch composite cryogels with different degrees of gelatinization were formulated, and two ways of starch gelatinization were considered: starch gelatinization occurred before or after pectin crosslinking during forming the hydrogel network. The results showed that high gelatinized starch (73.8 %-100.0 %) rendered pectin cryogels with denser pore morphology and higher mechanical strength. The pore diameter transferred from 160-200 µm to 40-60 µm with the degree of gelatinization, while the total porosity decreased by about 15 % and the specific surface area increased by about 100 m2/g. When starch gelatinization occurred before pectin crosslinking, the hydrogen bond interactions between gelatinized starch and pectin were formed to accelerate the gelation rate of the pectin Ca2+-dependent network. When gelatinization occurred after pectin crosslinking, the pre-formed pectin network delayed the breakdown of the starch crystalline structure during gelatinization. The qualitative regulation of the pore morphology in pectin cryogels by incorporating starches with varying degrees of gelatinization was confirmed.

Modulation of ice crystal formation behavior in pectin cryogel by xyloglucan: Effect on microstructural and mechanical properties.

To fully understand the role of xyloglucan (XyG) on the microstructural and mechanical properties of pectin cryogels, we formulated two types of low methoxyl pectin cryogels containing different XyG content (with or without an ionic network) using freeze-drying. The interaction between the pectin and XyG was explored, and the morphologies of ice crystals were characterized by the pore morphometric of freeze-dried scaffolds. Results showed that XyG and pectin could be cross-linked via hydrogen bonding interactions. When the ionic network was present, XyG accelerated pectin gelation but decreased the size of ice crystals during freezing, reducing the pore diameters of cryogels from 340 to 350 µm to 210-230 µm while shifting the structural thickness from 50 to 70 µm to 30-50 µm. When the ionic network was absent, the addition of XyG increased the pore size of the pectin cryogel from 30 to 50 µm to 70-90 µm, while rising the pore wall thickness from 50 to 70 µm to 70-90 µm. The ionic cross-links between the pectin chains gave freeze-dried scaffolds mechanical strength and crispiness, and the incorporation of XyG further strengthened the mechanical resistance of cryogels.

Cell wall polysaccharides and mono-/disaccharides as chemical determinants for the texture and hygroscopicity of freeze-dried fruit and vegetable cubes.

In order to select the critical factors on the texture and hygroscopic characteristics of freeze-dried fruit and vegetable cubes, the correlation analysis was performed on the major chemical compositions of 12 fresh materials, and the microstructure, texture, hygroscopicity of corresponding freeze-dried samples. The dry proportion of starch-rich materials, such as taro, was mainly composed of polysaccharides (0.76-0.89 g/g db), while the dry proportion of starch-poor materials such as apple was mainly composed of mono-/disaccharides (0.70-0.95/g db). Data from the microscopy showed that cell wall polysaccharides constituted the scaffold of freeze-dried cubes and natural starch granules attached to the scaffold as fillers. Both of them inhibited structural collapse. Mono-/disaccharides were accountable for the hardness and crispiness of freeze-dried cubes, however, excessive mono-/disaccharides could reduce the size of the pores and cause severe shrinkage. Polysaccharides reduced the hygroscopicity of freeze-dried cubes, on the contrary, mono-/disaccharides promoted hygroscopicity, especially fructose.

Pectin and homogalacturonan with small molecular mass modulate microbial community and generate high SCFAs via in vitro gut fermentation.

The intestinal fermentability of pectic polysaccharides is largely determined by its molecular size. In this study, fermentation properties of enzymatic-modified apple pectin (AP) and homogalacturonans (HG) with high, medium and low molecular weight (Mw) were evaluated by in vitro fermentation model, and their structural changes were also investigated. Results showed that Mw, monosaccharide contents and molecular linearity of the AP hydrolysates were reduced after microbial degradation. On the other hand, culture media supplemented with low-Mw AP (60,300 g/mol) and low-Mw HG (861 g/mol) exhibited lower pH (5.1 and 5.7, respectively) and produced higher total short-chain fatty acid contents (SCFA, 230.40 mmol/L and 187.19 mmol/L, respectively). However, reduced trends in abundance of the pectinolytic microorganisms Faecalibacterium and Eubacterium were showed as Mw of the HG decreased, whereas growth of the SCFA-producer genera Bifidobaacterium, Megasphaera and Allisonella were improved. This work confirmed that low-Mw pectin and homogalacturonan generated more beneficial metabolites, developing structure-microbiota-gut health relationship.

Systematic Review of Phenolic Compounds in Apple Fruits: Compositions, Distribution, Absorption, Metabolism, and Processing Stability.

As the most widely consumed fruit in the world, apple (Malus domestica Borkh.) fruits provide a high level of phenolics and have many beneficial effects on human health. The composition and content of phenolic compounds in natural apples differs according to the tissue types and cultivar varieties. The bioavailability of apple-derived phenolics, depending on the absorption and metabolism of phenolics during digestion, is the key determinant of their positive biological effects. Meanwhile, various processing technologies affect the composition and content of phenolic compounds in apple products, further affecting the bioavailability of apple phenolics. This review summarizes current understanding on the compositions, distribution, absorption, and metabolism of phenolic compounds in apple and their stability when subjected to common technologies during processing. We intend to provide an updated overview on apple phenolics and also suggest some perspectives for future research of apple phenolics.