Sofia K. Øiseth

Improved mechanical properties of retorted carrots by ultrasonic pre-treatments

The use of ultrasound pre-processing treatment, compared to blanching, to enhance mechanical properties of non-starchy cell wall materials was investigated using carrot as an example. The mechanical properties of carrot tissues were measured by compression and tensile testing after the pre-processing treatment prior to and after retorting. Carrot samples ultrasound treated for 10 min at 60 °C provided a higher mechanical strength (P<0.05) to the cell wall structure than blanching for the same time period. With the addition of 0.5% CaCl(2) in the pre-treatment solution, both blanching and ultrasound treatment showed synergistic effect on enhancing the mechanical properties of retorted carrot pieces. At a relatively short treatment time (10 min at 60 °C) with the use of 0.5% CaCl(2), ultrasound treatment achieved similar enhancement to the mechanical strength of retorted carrots to blanching for a much longer time period (i.e. 40 min). The mechanism involved appears to be related to the stress responses present in all living plant matter. However, there is a need to clarify the relative importance of the potential stress mechanisms in order to get a better understanding of the processing conditions likely to be most effective. The amount of ultrasound treatment required is likely to involve low treatment intensities and there are indications from the structural characterisation and mechanical property analyses that the plant cell wall tissues were more elastic than that accomplished using low temperature long time blanching.

Influence of boron on carrot cell wall structure and its resistance to fracture.

Plant cell wall structure integrity and associated tissue mechanical properties is one of key determinants for the perceived texture of plant-based foods. Carrots (Daucus carota) were used to investigate the effect of mineral supply of boron (B) and/or calcium (Ca), during plant growth, on the plant cell wall structure and mechanical properties of matured root tissues. Five commercial cultivars of carrots, Kuroda (orange), Dragon Purple, Kuttiger White, Yellow, and Nutri-Red, were cultivated under controlled glasshouse conditions over two seasons. Significant increases in the accumulation of B and Ca were found for all cultivars of carrots when additional B and Ca were included in the nutrient feeding solutions throughout the plant growth period. Elevated levels of B in carrot root tissue reduced the uptake of Ca and other mineral nutrients and enhanced plant cell wall structural integrity, its resistance to fracture, and the weight and size (both diameter and length) of carrots. Although higher amounts of Ca were accumulated in the plant materials, the additional supply of Ca did not have a significant effect on the mechanical properties of mature plant tissues or on the uptake of B by the plant. The results suggest that B cross-linking of pectin (rhamnogalacturonan II) has a greater influence on mature tissue mechanical properties than Ca cross-linking of pectin (homogalacturonan) when supplied during plant growth.

Why is Abalone So Chewy? Structural Characterization and Relationship to Textural Attributes

ABSTRACT Abalone is a highly regarded food in many cultures. It is consumed as a luxury food, valued for its unique sensory properties, which include both flavor and texture. The aim of this research was to understand the texture of abalone and to link textural attributes to the microstructure of the muscle tissue. Two different sources, and species, of abalone—wild (Haliotis rubra) and farmed (Haliotis laevigata)—were characterized structurally using light microscopy and confocal laser scanning microscopy. The structure at different length scales of the abalone foot muscle tissues was related to perceived texture by a trained sensory panel. The results of the microscopy work showed isotropic assemblies of interwoven muscle bundles with a diameter of approximately 20–40 µm. The muscle fibers consisted of bundles of aligned muscle fibrils, 2–4 µm in diameter, that were interconnected with anisotropic collagen. During steaming, the muscle fibers were observed to separate as a result of configurational changes of the protein. The sample from wild abalone was found by the sensory panel to be the most chewy, firm and springy. The size of the collagen-rich areas was linked to the texture perception, with the toughest pieces of meat displaying the largest collagen-rich areas. The size of the muscle fiber bundles also contributed to the perceived texture, in which samples containing larger bundles were perceived as more chewy than samples with fewer fibers per bundle.

Faster Fermentation of Cooked Carrot Cell Clusters Compared to Cell Wall Fragments in Vitro by Porcine Feces

Plant cell walls are the major structural component of fruits and vegetables, which break down to cell wall particles during ingestion (oral mastication) or food processing. The major health-promoting effect of cell walls occurs when they reach the colon and are fermented by the gut microbiota. In this study, the fermentation kinetics of carrot cell wall particle dispersions with different particle size and microstructure were investigated in vitro using porcine feces. The cumulative gas production and short-chain fatty acids (SCFAs) produced were measured at time intervals up to 48 h. The results show that larger cell clusters with an average particle size (d(0.5)) of 298 and 137 μm were more rapidly fermented and produced more SCFAs and gas than smaller single cells (75 μm) or cell fragments (50 μm), particularly between 8 and 20 h. Confocal microscopy suggests that the junctions between cells provides an environment that promotes bacterial growth, outweighing the greater specific surface area of smaller particles as a driver for more rapid fermentation. The study demonstrates that it may be possible, by controlling the size of cell wall particles, to design plant-based foods for fiber delivery and promotion of colon fermentation to maximize the potential for human health.

Visualization of carrageenan hydrogels by electron tomography

The visualization of hydrogels and other forms of hydrated, soft matter pose a significant challenge for studies by electron microscopy. The main challenges can be subdivided into: (1) accurate preservation of structure, (2) ensuring a sufficiently high signal-to-noise ratio, and (3) acquisition of comprehensive datasets. A shortcoming in any of these areas will lead to measurement uncertainty. We demonstrate the characteristic differences between the polymer networks formed by the potassium and sodium forms of κ-carrageenan, in 3D and at a resolution sufficient to resolve fiber bundles. Finally, we discuss the uncertainties involved in quantitative measurements obtainable with current methodologies as well as prospects for improvement.