Harald Rohm

Improving the nutritional quality of microwave-vacuum dried strawberries: a preliminary study

Microwave-vacuum drying is an up-to-date technique for the conservation of fruits and vegetables, resulting in products with improved texture and colour. In order to evaluate the effects on the content of ascorbic acid, anthocyanins and phenolic compounds, and antioxidant capacity, three strawberry varieties were subjected to microwave-vacuum (MV) drying combined with convective pre- and post-drying, convective drying, and freeze-drying. The MV drying procedure comprised immersion in a pretreatment bath, pre-drying in a belt dryer to a moisture load of approx 45 g/100 g dry matter, and microwave treatment at 4 kPa for 8 min, leading to products with 5 g/100 g moisture. During post-drying, the moisture content was further reduced to approx 2 g/100 g. Convective drying and MV drying decreased the content of ascorbic acid to approx 40% of the initial value, phenolic compounds to approx 35%, and the antioxidative capacity to approx 60%. No reduction was observed in freeze-dried strawberries. Bypassing the pre-treatment bath, extending residence time in the pre-dryer to reduce temperature peaks, and reducing MV treatment time increased the recovery of ascorbic acid to approx 65%. Phenolic compounds remained stable, and the reduction of the antioxidative capacity was limited to 10% – 25%. Except ascorbic acid, the residual concentration of nutritionally relevant compounds after improving the processing conditions was close to the values as observed for freeze-dried products.

Colour of hard cheese

During ripening, body colour of Emmental cheeses was evaluated by tristimulus reflectance measurements. Tristimulus primaries were transformed to the Helmholtz system and into CIELAB-coordinates in order to obtain interpretable results. Apart from lightness decreasing with increasing maturation time, it was found by using both systems that, (1) colourfulness increased continuously up to a cheese age of approximately 10 weeks and then remained constant, and (2) the hue of the cheese body shifted towards a slightly more orange colour in the last stage of ripening. The yellowness index, which represents a one-dimensional measure calculated from tristimulus primaries, is proposed as a simple and adequate measure of cheese body colour, and interrelations with CIELAB-values commonly used in food science are outlined.

Integrating sensory evaluation in adaptive conjoint analysis to elaborate the conflicting influence of intrinsic and extrinsic attributes on food choice.

Sensory properties and packaging information are factors which considerably contribute to food choice. We present a new methodology in which sensory preference testing was integrated in adaptive conjoint analysis. By simultaneous variation of intrinsic and extrinsic attributes on identical levels, this procedure allows assessing the importance of attribute/level combinations on product selection. In a set-up with nine pair-wise comparisons and four subsequent calibration assessments, 101 young consumers evaluated vanilla yoghurt which was varied in fat content (four levels), sugar content (two levels) and flavour intensity (two levels); the same attribute/level combinations were also presented as extrinsic information. The results indicate that the evaluation of a particular attribute may largely diverge in intrinsic and in extrinsic processing. We noticed from our utility values that, for example, the acceptance of yoghurt increases with an increasing level of the actual fat content, whereas acceptance diminishes when a high fat content is labelled on the product. This article further implicates that neglecting these diverging relationships may lead to an over- or underestimation of the importance of an attribute for food choice.

Microflora of Austrian natural-set yogurt

Randomly selected Austrian natural-set yogurts were examined for storage-induced changes in the populations of viable yogurt starter organisms and microbial contaminants. Typically, numbers of Streptococcus thermophilus and Lactobacillus bulgaricus remained above 108 cfu/g in yogurt stored at 10°C until the sell-by-date (15-20 d after manufacture). Both elevated storage temperature and storage periods markedly reduced the survival of yogurt starter bacteria. Depending on the sample origin, the yeast populations increased from less than 10/g to above 106/g when yogurts were stored at 10°C until the sell-by-date. The isolated yeasts were identified as Candida inconspicua , C. intermedia , C. parapsilosis , C. rugosa , C. tropicalis , C. zeylanoides , Debaryomyces hansenii , Metschnikowia reukauffii , Pichia guilliermondii , Rhodotorula mucilaginosa , Torulaspora delbrueckii , Trichosporon beigelii , and Yarrowia lipolytica . While a few out of 233 isolates were found to assimilate lactose, none were able to ferment lactose.

A Combination of Fat Replacers Enables the Production of Fat-reduced Shortdough Biscuits with High-sensory quality

Because of the increasing number of overweight consumers, a decrease of the daily energy intake without changing the palatability of foods is of tremendous importance. This study was performed to replace 30% or 40% fat in the formulation with a single fat replacer (corn fibre, maltodextrin or lupine extract) or a binary combination thereof and to investigate its impact on product characteristics (moisture, volume, colour and texture) and sensory properties of the biscuits. The replacement of 30% baking fat resulted in increased moisture content after baking, volume increase was significantly lower than for reference biscuits, biscuit colour was affected and the firmness increased drastically. Forty percent fat replacement increased these differences. Quantitative descriptive analysis revealed deviations, which were in line with analytical results and which were highest when lupine extract was used as fat replacer. The deviation to a reference biscuit was subsequently reduced by using appropriate combinations of the investigated fat replacers. One of those combinations, with corn fibre and lupine extract in a ratio of 1:1 replacing 30% baking fat, was given priority in a consumer test with a total of 192 respondents.

Ultrasonic excitation affects friction interactions between food materials and cutting tools.

In the food industry, ultrasonic cutting is used to improve separation by a reduction of the cutting force. This reduction can be attributed to the modification of tool-workpiece interactions at the cutting edge and along the tool flanks because of the superposition of the cutting movement with ultrasonic vibration of the cutting tool. In this study, model experiments were used to analyze friction between the flanks of a cutting tool and the material to be cut. Friction force at a commercial cutting sonotrode was quantified using combined cutting-friction experiments, and sliding friction tests were carried out by adapting a standard draw-off assembly and using an ultrasonic welding sonotrode as sliding surface. The impact of material parameters, ultrasonic amplitude, and the texture of the contacting food surface on friction force was investigated. The results show that ultrasonic vibration significantly reduces the sliding friction force. While the amplitude showed no influence within the tested range, the texture of the contact surface of the food affects the intensity of ultrasonic transportation effects. These effects are a result of mechanical interactions and of changes in material properties of the contact layer, which are induced by the deformation of contact points, friction heating and absorption heating because of the dissipation of mechanical vibration energy.

Uniaxial compression of Swiss-type cheese at different strain rates

Abstract Rheological behaviour of 48 Swiss-type cheese samples in lubricated uniaxial compression was evaluated at three initial strain rates ranging from 4·76 × 10 −3 to 7·62 × 10 −2 s −1 . Modulus, apparent fracture stress and apparent fracture strain increased significantly with increasing strain rate, which is attributed to the viscoelastic nature of cheese. The influence of strain rate on apparent fracture strain strongly depended on the shape of the stress-strain curves. The relationship between the rheological parameters evaluated at different strain rates proved to be significant. The strain rate dependency of the apparent fracture strain is further explained by the contribution of elastic and viscous components, which was evaluated in relaxation experiments in the linear viscoelastic region.

Exopolysaccharide production by three different strains of Streptococcus thermophilus and its effect on physical properties of acidified milk

Physical and rheological properties of commercial low‐fat yogurt and of skim milk acidified with single strains of Streptococcus thermophilus were evaluated. Concentration of exopolysaccharides (EPS) varied from 40 to 168 mg glucose equivalents per liter, and EPS showed different degrees of ropiness. Further characterization of nonropy and ropy EPS from three S. thermophilus strains revealed intrinsic viscosity of 0.13–0.66 mL/mg, respectively, which had a significant impact on apparent viscosity and the ability of regaining structure after shearing. Compared with regular milk acidification, batch fermentation with supplemented skim milk at constant pH resulted in a strain‐dependent increase of EPS yield by a factor of 4–7.

The DONE framework: Creation, evaluation, and updating of an interdisciplinary, dynamic framework 2.0 of determinants of nutrition and eating

The question of which factors drive human eating and nutrition is a key issue in many branches of science. We describe the creation, evaluation, and updating of an interdisciplinary, interactive, and evolving “framework 2.0” of Determinants Of Nutrition and Eating (DONE). The DONE framework was created by an interdisciplinary workgroup in a multiphase, multimethod process. Modifiability, relationship strength, and population-level effect of the determinants were rated to identify areas of priority for research and interventions. External experts positively evaluated the usefulness, comprehensiveness, and quality of the DONE framework. An approach to continue updating the framework with the help of experts was piloted. The DONE framework can be freely accessed (http://uni-konstanz.de/DONE) and used in a highly flexible manner: determinants can be sorted, filtered and visualized for both very specific research questions as well as more general queries. The dynamic nature of the framework allows it to evolve as experts can continually add new determinants and ratings. We anticipate this framework will be useful for research prioritization and intervention development.

Growth and exopolysaccharide yield of Lactobacillus delbrueckii ssp. bulgaricus DSM 20081 in batch and continuous bioreactor experiments at constant pH

Some Lactobacillus delbrueckii ssp. bulgaricus strains are able to synthesize exopolysaccharides (EPS) and are therefore highly important for the dairy industry as starter cultures. The aim of this study was to investigate the nutritional requirements for growth and EPS production of Lactobacillus delbrueckii ssp. bulgaricus DSM 20081. A medium was developed from a semi-defined medium (SDM) in which glucose was replaced by lactose and different combinations of supplements (nucleobases, vitamins, salts, sodium formate and orotic acid) were added. Constant pH batch fermentation with the modified medium resulted in an EPS yield of approximately 210 mg glucose equivalents per liter medium. This was a 10-fold increase over flask cultivation of this strain in SDM. Although not affecting cell growth, the mixture of salts enhanced the EPS synthesis. Whereas EPS production was approximately 12 mg/g dry biomass without salt supplementation, a significantly higher yield (approximately 20 mg/g dry biomass) was observed after adding the salt mixture. In continuous fermentation, a maximal EPS concentration was obtained at a dilution rate of 0.31/h (80 mg EPS/L), which corresponded to a specific EPS production of 49 mg/g dry biomass.