Miriam N. Martino

Microstructural characterization of yam starch films

Yam starch films were produced by thermal gelatinization of starch suspensions using different starch and glycerol concentrations and were compared to control samples without glycerol. Films were characterized by polarized light microscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), X-ray diffraction, water vapor permeability (WVP) and water sorption isotherms. The polarized light microscopy and DSC data showed that starch gelatinization for film formation was complete. Plasticized films have a homogeneous structure as observed by SEM. At water activities >0.43, glycerol increased the equilibrium moisture content of the films due to its hydrophilic character. X-ray pattern of the yam films could be assigned to a B-type starch; during storage this pattern remained almost the same, however a slight recrystallization process could be observed. Amylopectin retrogradation was not observed by DSC with storage time of the films. Glass transition temperatures of films with glycerol were lower than those of control films as measured by DSC and TMA. WVP of yam starch films increased with the presence of glycerol.

Microstructural Characterization of Plasticized Starch‐Based Films

Edible films were developed using different starch sources (corn starch and amylomaize). Starch suspensions were cold gelatinized with NaOH; either glycerol or sorbitol were used as plasticizer. Films were characterized by Differential Scanning Calorimetry (DSC), X-ray diffraction, Scanning Electron Microscopy (SEM) and gas (CO2 and O2) permeabilities. SEM observations showed that plasticizer addition was necessary for film integrity. The evaluation of film formation by DSC indicated that cold gelatinization was the main factor of thermal transitions. Film crystallinity was analyzed by DSC and X-ray diffraction during storage. For all tested formulations, film crystallinity increased while gas permeability decreased during storage. Films containing glycerol or sorbitol showed a lower crystalline/amorphous ratio by X-ray diffraction and DSC than unplasticized films. Amylomaize films with higher crystalline/amorphous ratio gave lower gas permeabilities than the corresponding corn starch films; films containing sorbitol showed lower permeability values than those containing glycerol.

Size and location of ice crystals in pork frozen by high-pressure-assisted freezing as compared to classical methods

In high-pressure-assisted freezing, samples are cooled under pressure (200 MPa) to - 20 °C without ice formation then pressure is released (0.1 MPa) and the high super-cooling reached (approx. 20 °C), promotes uniform and rapid ice nucleation. The size and location of ice crystals in large meat pieces (Longissimus dorsi pork muscle) as a result of high-pressure-assisted freezing were compared to those obtained by air-blast and liquid N(2). Samples from the surface and centre of the frozen muscle were histologically analysed using an indirect technique (isothermal-freeze fixation). Air-blast and cryogenic fluid freezing, having thermal gradients, showed non-uniform ice crystal distributions. High-pressure-assisted frozen samples, both at the surface and at the central zones, showed similar, small-sized ice crystals. This technique is particularly useful for freezing large pieces of food when uniform ice crystal sizes are required.

Edible coatings from cellulose derivatives to reduce oil uptake in fried products

Abstract Methylcellulose (MC) and hydroxypropylmethylcellulose (HPMC) were used in coating formulations to reduce oil uptake in deep-fat frying potato strips and dough discs. MC coatings were more effective in reducing oil uptake than HPMC ones. The effect of plasticizer addition (sorbitol) was also evaluated. The best formulations were 1% MC and 0.5% sorbitol for fried potatoes and 1% MC and 0.75% sorbitol for dough discs. For these formulations, oil uptake reduction was 40.6 and 35.2% for potato strips and dough discs compared to the uncoated samples; the increase in water content was 6.3 and 25.7%, respectively. Non-significant differences in texture of coated and uncoated samples were observed. Although instrumental color differences were detected, all samples were accepted by the non-trained panel.

Starch-based coatings: effect on refrigerated strawberry (Fragaria ananassa) quality

Edible coatings can provide an alternative for extending post-harvest life of refrigerated fruit and vegetables. The influence of different starch-based coating formulations on quality attributes of strawberries stored at 0°C and a relative humidity of 84·8% was studied. Starch sources were classified according to the amylose content in starch, with medium amylose content (potato and corn) and high amylose content (amylomaize and amylose-rich product). Quality of fruits was evaluated by weight loss, firmness retention, microbial decay, surface colour development, titratable acidity and sugar content. The effects of starch amylose content and glycerol (plasticiser) concentration on coating properties were also analysed. The coatings reduced the number of infected fruits and extended storage life of strawberries by retarding senescence. The addition of glycerol improved coating performance, with 20 g litre−1 the most effective concentration. The starch source had a significant effect on surface colour development, weight loss and firmness retention. Coated strawberries produced the lowest ratios of chromaticity parameters (a/b, red/yellow) with regard to the control fruits, thus retarding senescence. High amylose content starches reduced weight loss, maintained firmness and reduced decay better than medium amylose content starches.

Composite starch-based coatings applied to strawberries (Fragaria ananassa).

Starch-based coatings were used to the extend storage life of strawberries (Fragaria ananassa) stored at 0 degree C and 84.8% relative humidity. Effects of coating formulation (including starch type, plasticizer, lipid and antimicrobial agent) were analysed with respect to fruit quality. Plasticizer addition was necessary for film and coating integrity to avoid pores and cracks. Plasticizer presence reduced weight losses and maintained surface colour of fruits. Amylomaize coatings showed lower water vapour and gas permeabilities and decreased weight losses for longer periods than corn starch ones. Coatings with sorbitol showed lower permeabilities than glycerol ones. Coatings with antimicrobial agents decreased microbial counts, extending storage life of coated fruits by 10 to 14 days in comparison to the control. The addition of 2 g/l sunflower oil to the formulations decreased the water vapour permeability of starch-based films, maintained the surface colour of coated fruits and controlled effectively fruit weight losses during storage. Lipid addition minimized the effects of starch and plasticizer types. Composite starch-based coatings showed selective gas permeability (CO2 higher than O2) which helps to delay senescence of fruits.

Chemical and Functional Characterization of Products Obtained from Yam Tubers

Aiming to find alternative uses of yam (Dioscorea alata) in the food industry, three products (flour, starch and mucilage) were obtained form yam tubers and characterized. Flours were obtained from peeled fresh tubers, dried at two different temperatures (30 and 55 °C) and then milled. Starch and mucilage were extracted from mashed yam before drying. Products were characterized by their chemical, structural, morphological, rheological and thermal properties. Flours showed differences in pasting, thermal and water absorption properties as a function of drying temperature. Starch contained 30% amylose. The analysis of monosaccharide composition of starch showed traces of mannose, xylose and rhamnose. The onset gelatinization temperature was 74.4 °C for starch, whereas those for flours varied between 71.5 and 74.4 °C. The relatively high gelatinization temperature and the absence of breakdown in the amylograph test evidenced the stable structure of starch when submitted to heat and shear. The mucilagenous material contained 55.36% protein and 43.05% starch, and also an acidic polysaccharide fraction linked to a neutral fraction. Flow curves of 1% mucilage dispersions showed a pseudoplastic behavior with high pH sensitivity. Results demonstrated that yam products have promising applications in the food industry.

Methylcellulose Coatings Applied to Reduce Oil Uptake in Fried Products

Reducing fat content of fried foods by application of coatings is an alternative solution to comply with both health concerns and consumer preferences. The objective of the present work was to analyse the effect of coating formulation on the quantity and composition of oil-uptake by potato strips and dough discs, and the quality of frying oil after several frying batches. The effect of coating composition showed that the most efficient formulations were 1% methylcellulose (MC) and 0.5% sorbitol for fried potatoes and 1% MC and 0.75% sorbitol for dough discs. The most effective coating formulations reduced oil uptake by 35-40%, depending on the product. The increase in water content was 6.3% and 25.7% for potato strips and dough discs, respectively. Although coatings were effective barriers to reduce oil uptake in fried products, they did not modify oil composition as detected by HPLC analysis. Oil stability of frying oils, evaluated by total polar compounds and acidity, was within the allowed limits established by the Argentine Food Code (acidity <0.6mg KOH/g oil).

Viscoelastic properties of frozen starch-triglycerides systems

Abstract Dynamic rheological methods are useful to determine the stability of starch-based products submitted to different processing conditions. A dynamic oscillatory test covering a wide range of strain values was used to analyse the behaviour of corn and waxy starch (7 and 10% w/w) pastes with and without triglycerides (5% w/w) or xanthan gum (0.03%). The effect of freezing rate on the structure stability was analysed using a slow freezing rate of 0.3 cm/h and a rapid one of 31 cm/h. After slow freezing, dynamic parameters (G′, G ∗ , tan δ) showed an increase in the rigidity of the pastes and a structural breakdown with a marked stress decay compared with the unfrozen or rapid frozen samples. The linear viscoelastic range of starch pastes decreased after rapid or slow freezing, showing that the paste structure cannot resist large deformations without undergoing irreversible breakdown. Pastes with lipids showed an increase of the linear viscoelastic range and higher values of G′ compared with the control without lipids. The presence of triglycerides in frozen starch pastes led to a decrease of the solid component G′ of the starch pastes, in addition, a less fluid character after structural breakdown was observed as well. A minor increase in rigidity after slow freezing was observed in pastes with sunflower oil, as compared with pastes containing a shortening with a more saturated fatty acid composition.

Ice recrystallization in a model system and in frozen muscle tissue

Recrystallization produces modifications on ice crystal sizes during storage and transport of frozen foods, reducing the advantages obtained by quick freezing and inducing physicochemical changes which alter their quality and shorten their shelf life. This process involves the growth of the larger crystals at the expense of the smaller ones, being the interfacial energy, the driving force of the phenomenon. In the present work recrystallization was analyzed using direct microscopic observation of ice crystals in a model solution (0.28 N NaCl) and indirect observation of frozen muscle tissue. The model solution allowed visualization of the interface behavior; from the analysis of the ice crystal frequency distributions, relationships between shape and size of the grains were established. A kinetic model based on the average system curvature was proposed obtaining a satisfactory fitness of the experimental data. Values of the kinetic constants determined at different temperatures allowed the estimation of the process activation energy. In muscle tissues isothermal freeze-substitution was used to observe the holes left by the ice in frozen semitendinous beef muscle stored at -5, -10, -15, and -20 degrees C during long periods of time. A different evolution of the mean ice crystal diameter was observed with respect to the model system. In meat samples, at long storage times, a limit diameter value was reached; this situation has been proved to be independent of temperature and initial size (freezing rate); a theoretical expression based on tissue characteristic parameters was proposed for its evaluation. Activation energy for recrystallization in muscle tissue was also determined, being comparable to values for protein denaturation and quality losses.