Effect of Temperature on Firmness of Thermally Processed Fruits and Vegetables

The firmness of many commercially canned fruits and vegetables was measured by puncture, extrusion and Texture Press over the temperature range3–48°C. The firmness almost always decreased with increasing temperature and the firmness-temperature plots were rectilinear. The firmness-temperature coefficient (FT) is defined as the percent change in firmness per degree temperature increase. It ranged from close to zero to about -1.0% for most of the samples tested. The temperature should be kept within narrow limits when making firmness measurements on thermally processed fruits and vegetables, otherwise significant errors can be introduced.     

Low Temperature Blanching Affects Firmness and Rehydration of Dried Cauliflower Florets

Cauliflower florets were blanched at 55, 60, 65, and 70°C, held without cooling for 15, 30, 45, 60, and 90 min, blanched again at 100°and then dried in a hot air dehydrater. The coefficient for the rate of rehydration (?) was calculated using a diffusion model. Rehydrated samples were divided into two equal parts, one part was boiled in water and the other was uncooked. Firmness of rehydrated samples was measured by back extrusion. Blanching cauliflower between 55°and 70°before dehydration caused a substantial increase in extrusion forces after rehydration. Cooking the rehydrated cauliflower decreased firmness of all samples. However, the degree of softening caused by cooking was proportionally less for the low temperature blanch treatments than for the controls.     

GROWTH OF SACCHAROMYCES CEREVISIAE AND SACCHAROMYCES UVARUM IN A TEMPERATURE GRADIENT INCUBATOR

A temperature gradient incubator has been used to determine the effect of temperature on the growth of strains of Saccharomyces cerevisiae and Saccharomyces uvarum (including lager brewing yeasts formerly classified as Saccharomyces carlsbergensis). The maximum temperatures for growth (T_max) for all strains of S. cerevisiae examined were in the range 37.5°C-39.8°C and the optimum temperatures for the most rapid initial growth (T_opt) were in the range 30.0°C-35.0°C. Strains of S. uvarum, however, formed two distinct groups: Group A (including all brewing strains of S. uvarum tested) had T_max values 31.6°C-34.0°C and T_opt values 26.8°C-30.4°C; Group B had T_max values 38.2°C-40.0°C and T_opt values 30.0°C-34.6°C. It is proposed, therefore, that the species name S. carlsbergensis should be re-introduced and applied to those strains of S. uvarum (Group A) which have the lower T_max values. Minimum temperatures for growth (T_min) of the yeasts were not investigated as initial studies had shown that they could not be measured satisfactorily. Measurements of the generation times for one brewing strain of S. cerevisiae and one brewing strain of S. uvarum (Group A) over the temperature range 6.0°C-22.0°C have shown that there are significant differences between the yeasts at the lower end of the temperature range and that the relationship between generation time (GT) and temperature (T) for both yeasts closely follows the mathematical expression:      

Enzymatic Firming of Processed Red Pepper by Means of Exogenous Pectinesterase

Abstract

The objective of this investigation was to demonstrate that the firmness of a commercial vegetable product, diced and frozen red pepper (Capsicum annum var. Sendt), could be improved by the use of exogenous pectinesterase in an industrially relevant process. The diced pepper pieces 10?×?10?×?7?mm³ were infused under vacuum with a commercially available pectinesterase. The range of optimal process conditions was: 15–20°C, 45?min infusion time, a 10–25?mM CaCl₂ infusion brine, a w/w ratio of pepper fruit to infusion brine of 1.5:1, and an enzyme dosage of 30–60 pectinesterase units (PEU) per kg pepper fruit. The firmness as measured by back extrusion was improved by a factor of two to three. The effect of firming was robust and conserved after freezing and heating in a simulated household cooking process. The firming process seems easily adaptable to industrial conditions and may be applicable to other vegetable and fruit products.     

Influence of Annealing Temperature on Tg′ of Cooked Rice Stick Noodles

T_g′, a glass transition temperature under conditions of maximal freeze concentration, is important to the stability of frozen food. The procedures for using differential scanning calorimetry (DSC) to measure T_g′ have been reported under different experimental conditions. The aims of this study were to investigate the T_g′ of cooked rice stick noodles, and to determine the effect of the annealing temperature on its T_g′ value using DSC. Cooked rice stick noodles in aluminum DSC pans were scanned from ?60°C to 25°C at 5°C/min. to locate the apparent T_g′, which was ?5.3°C in the non-annealed state. When subjected to 4 different annealing temperatures of ?2, ?6, ?8 and ?10°C for 15 minutes, the T_g′ of cooked rice stick noodles was ?5.3, ?4.0, ?4.2, and ?4.9°C, respectively. The value of T_g′ was clearly observed in the annealed sample at ?6°C. The annealing process allowed time for the maximum formation of ice. This study showed that annealing at a temperature slightly below T_g′ gave a higher and more accurate value.     

Effect of pH and Ionic Strength on the Heat Stability of Rapeseed 12S Globulin (Cruciferin) by the ANS Fluorescence Method

The heat stability of rapeseed 12S globulin (cruciferin) was examined using 8-anilinonaphthalene-1-sulphonic acid (ANS) as a fluorescence probe. Heating cruciferin (0·06–0·3 mg ml⁻¹ in 10 mM glycyl–glycyl piperizine buffer, pH 7·0, with 0·1–1·0 M NaCl) for 20 min increased its hydrophobicity as monitored by ANS fluorescence measurements. The mid-point temperature for the heat effect (T_m) increased linearly with increasing solvent pH (T_m (°C)=4·16 pH+41 (μ=0.1)) or sodium chloride concentration (T_m (°C)=14·7 [NaCl]+71 (pH=7·0)). The range of T_m values for cruciferin was 45–96°C. At 20°C cruciferin was unstable at pH<3·0 but relatively stable under alkaline conditions (pH 8–10). Though possessing an oligomeric structure, cruciferin appears to heat denature in accordance with the two-stage deactivation model for simple globular proteins.     

Firmness Retention in Pickled Peppers as Affected by Calcium Chloride, Acetic Acid, and Pasteurization

Critical factors influencing firmness retention in pickled peppers were studied. The addition of CaCl, (0.2%, w/v, optimum) to whole, pickled‘Red Cherry’peppers increased firmness retention as determined by a puncture test using an Instron Universal Testing Machine. Pasteurization reduced firmness in the absence, but not in the presence, of added CaCl₂. CaCl₂ significantly (P ≤0.01) reduced softening during storage of‘Red Cherry’peppers at higher temperatures (36.7, 46.7°C), and resulted in a slight increase in firmness at 26.7°C. CaCI₂ did not significantly (P≥ 0.05) improve firmness retention in‘Jala-peno’peppers, but resulted in greater uniformity of firmness. CaCI, also improved firmness retention in pickled cucumbers. Firmness of unpasteurized peppers and cucumbers was not influenced significantly (P ≥0.05) by acetic acid concentrations of 2, 3 or 4%.     

Low‐field nuclear magnetic resonance analysis of the effects of heating temperature and time on braised beef

We investigated the characteristics of water mobility and distribution in Chinese braised beef after treatment at different temperatures for different times using low‐field nuclear magnetic resonance (LF‐NMR). The beef was heated at 45, 55, 65, 75, 85 or 95 °C for 30, 60, 90 and 120 min. Results showed that T₂ changed significantly with heating temperature. T₂₁ and A₂₁ decreased significantly with increasing temperature below 65 °C, with a steady phase from 75 to 95 °C, which agreed with cooking loss. Inversely, T₂₂ had no changes below 65 °C and changed apparently from 75 to 95 °C. The change in T₂₁ below 65 °C may be related to proteins denaturation and shrinkage and, above 65 °C, T₂₂ possibly induced by the dissolution of connective tissue. The characteristics of braised beef at 65 °C were different from those at other temperatures in T₂ distributions. The findings could provide a theoretical basis for the processing of Chinese braised beef.     

Canning Quality Evaluation of Pinto and Navy Beans

Thermal processing of pinto and navy beans at 121.1°C for 16 or 14 min in a still retort gave similar sterilization value (F_o= 10) as the processing at 115.6°C for 45 min. The 121.1°C/16 or 14 min process produced beans with greater firmness than the 115.6°C/45 min process. The addition of CaCl₂ and EDTA improved firmness and color of canned beans. Calcium chloride also reduced clumping and splitting of the canned beans. Sensory evaluation showed that the acceptability of canned beans was reduced when CaCl₂ was increased up to 10 mM. High correlation between firmness and soluble pectin in various bean cultivars implied that soluble pectin content could be used as a parameter for screening bean cultivars with desirable firmness.     

Effect of semolina and absorption level on extrusion of spaghetti containing non‐traditional ingredients

Specific mechanical energy (SME), mechanical energy, extrusion rate and temperature of extruded spaghetti were monitored to determine the effects of semolina, hydration level and non‐traditional ingredients on pasta extrusion using a semi‐commercial pasta press with a fixed screw speed of 25 rpm. SME transferred to the dough during extrusion and the temperature of extruded spaghetti were greater with strong than with weak gluten semolina and at low than at high absorption levels. When compared with semolina hydrated to 300 g kg^?1 absorption, SME transferred to the dough was 13 kJ kg^?1 lower for semolina mixed with buckwheat (Fagopyrum esculentum Moench.) bran flour, 47 kJ kg^?1 lower for semolina mixed with flaxseed (Linum usitativissimum L.) flour and 7 kJ kg^?1 lower for semolina mixed with wheat (Triticum turgidum var. durum L.) bran. Weak gluten semolina, high absorption levels and non‐traditional ingredients reduced the mechanical energy required for extrusion more than they reduced extrusion rate. The target temperature for extruded spaghetti was 45 °C. The temperature of extruded spaghetti containing flaxseed flour was below 45 °C whereas the temperature of spaghetti containing wheat bran was above 45 °C, regardless of semolina type or absorption level. Copyright © 2006 Society of Chemical Industry     

Salmonella Inactivation During Extrusion of an Oat Flour Model Food

Little research exists on Salmonella inactivation during extrusion processing, yet many outbreaks associated with low water activity foods since 2006 were linked to extruded foods. The aim of this research was to study Salmonella inactivation during extrusion of a model cereal product. Oat flour was inoculated with Salmonella enterica serovar Agona, an outbreak strain isolated from puffed cereals, and processed using a single‐screw extruder at a feed rate of 75 kg/h and a screw speed of 500 rpm. Extrudate samples were collected from the barrel outlet in sterile bags and immediately cooled in an ice–water bath. Populations were determined using standard plate count methods or a modified most probable number when populations were low. Reductions in population were determined and analyzed using a general linear model. The regression model obtained for the response surface tested was Log (N_R/N_O) = 20.50 + 0.82T ? 141.16a_w – 0.0039T² + 87.91a_w² (R² = 0.69). The model showed significant (p < 0.05) linear and quadratic effects of a_w and temperature and enabled an assessment of critical control parameters. Reductions of 0.67 ± 0.14 to 7.34 ± 0.02 log CFU/g were observed over ranges of a_w (0.72 to 0.96) and temperature (65 to 100 °C) tested. Processing conditions above 82 °C and 0.89 a_w achieved on average greater than a 5‐log reduction of Salmonella. Results indicate that extrusion is an effective means for reducing Salmonella as most processes commonly employed to produce cereals and other low water activity foods exceed these parameters. Thus, contamination of an extruded food product would most likely occur postprocessing as a result of environmental contamination or through the addition of coatings and flavorings.     

Optimization of a Diced Tomato Calcification Process

The effect of calcium concentration (0.05-1.45% CaCl₂), temperature of dipping solution (35-65°C), and contact time (0.5-3.5 min) on calcium uptake, firmness and pH of diced tomatoes was evaluated during a calcification process using response surface methods. Temperature had no significant effect on the process. Application of graphical optimization techniques revealed that processing in a solution of relatively low calcium concentration (?0.43% CaCl₂) at ambient temperature (? 35°C) for about 3.5 min would yield a product with Ca⁺⁺ content below the legal limit (<800 μ-g/g), improved firmness (shear force value > 20 N/g), and with pH low enough (<3.95) to eliminate any requirement for acidification treatment.     

RESEARCH NOTE LOSS OF COLOR AND FIRMNESS DURING THERMAL PROCESSING OF CANNED SNAP BEANS

Kinetic parameters were determined for the loss of color and firmness of snap beans during thermal processing. The D₂₅₀ and the z-values for the loss of color were found to be 12.6 min and 91°F, respectively. Firmness loss was determined as the peak back extrusion force. The D₂₅₀ and the z-values for the loss of firmness were 3.4 min and 106°F, respectively. The magnitudes of the kinetic parameters are in the range of values reported in the literature.     

Phase Transitions and Unfreezable Water Content of Carrots, Reindeer Meat and White Bread Studied using Differential Scanning Calorimetry

Differential scanning calorimetry was used to measure the phase transitions and unfreezable water of carrots, reindeer meat, and white bread. The incipient melting point (T_im), incipient intensive melting point (T′_im), the onset temperature of melting (T_m), latent heat of melting (ΔH_m), specific heat (C_p) and enthalpy (ΔH) were determined from the melting curves. T′_im, T_mΔH_m and ΔH and the unfreezable water were found to be functions of moisture. The T_im, temperatures were ? 39°C, ?33°C, ?40°C; T′_im, ?11.8°C, ?13.3°C ?17.3°C T_m, ? 3.4°C, ? 3.1°C, ? 12.2°C for carrot, reindeer meat, and white bread, respectively. The unfreezable water was 8.3% for carrots, 15.1% for reindeer meat, and 22.5% for white bread, determined from ΔH_m and 3.4%, 6.4% and 2.9%, determined from ΔH. The lowest water detectable from ΔH_m was 26.4% and from ΔH 3.6%.     

Effect of water content,temperature and storage on the glass transition,moisture sorption characteristics and stickiness of β‐casein 1

Moisture sorption isotherms at +4 °C and +22.5 °C were obtained for β‐casein after isolation and after 9 months of storage at ‐29 °C and +22.5 °C. Glass transition state diagrams (T_g vs. moisture) were determined for β‐casein after storage. The results showed that effects of storage temperature on moisture sorption isotherms were varied; however, at any a_w differences in moisture content were small (< 0.03g H₂O/g solids at high a_w). β‐casein stored at ‐29°C had lower m_o and T_g values than that of β‐casein stored at +22.5°C. The glass transition temperatures for β‐casein were above room temperature, even at a_w = 0.76. Onset of stickiness occurred above a_w = 0.76.     

Influence of milk proteins on the development of lactose-induced stickiness in dairy powders

The stickiness behaviour of a range of spray dried dairy powders differing in protein/lactose ratio was determined using a fluidised bed apparatus. Powders with higher protein/lactose ratios were less susceptible to sticking. Stickiness was related to both the glass transition temperature (T_g) and the temperature increment by which T_g must be exceeded before sticking occurred (T?T_g). T?T_g values of approximately 10, 22, 29, 45 and 90 °C were found for powders containing 15.5, 26.9, 39.5, 55.7 and 83.4% protein respectively. Composition had different effects on T_g and T?T_g. The rate at which water was sorbed and desorbed by powders increased with protein content. With increasing protein content, preferential sorption of water by non-amorphous constituents delayed the rate at which lactose underwent the requisite change from the ‘glassy’ to the ‘rubbery’ form in order that powder particles became sticky.     

Effect of Carbon Dioxide on the Thermodynamic State of Water in Collagen

Thermodynamic activities of polar sites of collagen in the presence of CO₂ were observed by inverse gas chromatographic techniques using water as a probe. The interactions between collagen and the water probe were evaluated by determining the specific retention volume (Vg°) and partition coefficient (Kp) at 25°C, 3°C, and 35°C. Thermodynamic parameters were determined from these data. CO₂ exhibited a significant effect on the water binding of collagen as shown by increased Vg° and Kp values as compared to N₂- and He-treated collagen. The thermodynamic parameters of partial molar Gibb's free energy (Δ_T°), partial molar enthalpy (Δ_T°) and partial molar entropy (Δ_T°) indicated CO₂ significantly increased the average energy of water binding by collagen.     

High-pressure homogenisation of raw bovine milk. Effects on fat globule size distribution and microbial inactivation

Whole raw milk was processed using a 15 L h⁻¹ homogeniser with a high-pressure (HP) valve immediately followed by a cooling heat exchanger. The influence of homogenisation pressure (100–300 MPa) and milk inlet temperature T_in (4°C, 14°C or 24°C) on milk temperature T₂ at the HP valve outlet, on fat globule size distribution and on the reduction of the endogenous flora were investigated. The T_in values of 4–24°C led to milk temperatures of 14–33°C before the HP valve, mainly because of compression heating. High T_in and/or homogenisation pressure decreased the fat globule size. At 200 MPa, the d_4.3 diameter of fat globules decreased from 3.8±0.2 (control milk) to 0.80±0.08 μm, 0.65±0.10 or 0.37±0.07 μm at T_in=4, 14°C or 24°C, respectively. A second homogenisation pass at 200 MPa (T_in=4°C, 14°C or 24°C) further decreased d_4.3 diameters to about 0.2 μm and narrowed the size distribution. At all T_in tested, an homogenisation pressure of 300 MPa induced clusters of fat globules, easily dissociated with SDS, and probably formed by sharing protein constituents adsorbed at the fat globule surface. The total endogenous flora of raw milk was reduced by more than 1 log cycle, provided homogenisation pressure was ⩾200 MPa at T_in=24°C (T₂∼60°C), 250 MPa at T_in=14°C (T₂∼62°C), or 300 MPa at T_in=4°C (T₂∼65°C). At all T_in tested, a second pass through the HP valve (200 MPa) doubled the inactivation ratio of the total flora. Microbial patterns of raw milk were also affected; Gram-negative bacteria were less resistant than Gram-positive bacteria.     

Thermal stability of processed PVC/bamboo blends: effect of compounding procedures

Polyvinyl chloride (PVC) was mixed with bamboo (Bambusa vulgaris) particle and additives by using PVC composite manufacturing system including initial dry blending with hot-cool mixing, followed by granulation via counter-rotating extrusion, and then consolidation by compression moulding to produce compression moulded board (CMB). The effects of different bamboo particle size (75 µm and 1 mm), bamboo particle loading (25 and 50% loading ratio), and differing processing lubricants content level (compositions 1 and 2) on the thermal stability of the composites were determined. Results show no significant trends in glass transition temperature (T_g) between dry blends, granules, and CMB, and between B. vulgaris particle loading at the respective processing steps. For samples with higher lubricant contents, the PVC T_g was observed to decrease up to 5?°C, possibly due to the reduced melt viscosity. The thermal decomposition temperature at 5% mass loss (T_?5%) appeared to decrease from dry blend to CMB due to sample degradation on further processing at higher temperatures. The use of 50% B. vulgaris particle loading also reduced the T_?5% values, assignable to bamboo particle degradation caused by the high processing temperatures. For oxidative induction time (OIT) testing, only granules and CMB from pure PVC composites system showed measurable oxidative times compared with OIT profiles of PVC/B. vulgaris composites system, suggestive of comparatively stabilized thermoplastic composites. This revealed that processing with bamboo particles does not contribute to degradation of PVC composites.     

Postharvest Ripening Characterization of Greenhouse Tomatoes

The determination of °Brix, pH, titratable acidity and antioxidant composition such as β-carotene, lycopene, and vitamin C was done in greenhouse “Savoura” tomatoes during maturation. The chromatic values L*, a*, b* were determined at the top surface of the tomato, and its strength-deformation curve was tested with a texturometer. Results pointed out that °Brix and titratable acidity did not change during postharvest, while β-carotene, lycopene, and vitamin C contents increased continuously toward the red stage of tomato. The colour values L*, a*, b*, and ratio a*/b* had a good correlation with the maturity stages. Also, a high correlation between lycopene content and (a*/b*) ratio was found, which was well represented with a second order polynomial function (r² = 0.95). This equation permits to appropriately estimate lycopene content of tomato as a function of its color, without any chemical analysis. During ripening, tomato texture changed from firm turgid to soft and the puncture deformation degree increased. Tomato firmness and chromatic values had a good correlation, and the lycopene content was a logarithm function of firmness (r² = 0.81). The strength in puncture test increased linearly with deformation until the bioyield point. The measured force was analyzed as the sum of the compression and the shear forces. The two force coefficients can be considered as tomato properties, independent of the puncture probe. These coefficients were found to decrease during tomato maturation.