Effect of cooling rate on the structure and mechanical properties of milk fat and lard

The mechanical and thermal properties of a fat crystal network are determined by factors such as chemical composition, solid fat content (SFC), and crystal habit (polymorphism and microstructure). Processing conditions affect the crystallization of fats, thus having an effect on their functional properties. The effects of cooling rate on the macroscopic properties of a fat crystal network were studied by crystallizing anhydrous milk fat (AMF) and lard either rapidly (5 °C/min, Newtonian cooling) or slowly (0.1 °C/min, stepwise cooling). AMF crystallized rapidly was harder than AMF crystallized slowly and had a higher SFC. Moreover, its solid state was in a more metastable polymorphic form. Upon slow crystallization, AMF had a lower SFC and its solid state was in a more stable polymorphic form. AMF crystallized under fast cooling yielded a higher Avrami constant (k), and a lower Avrami exponent (n) than AMF crystallized under slow cooling. The microstructure was also different between the two treatments. When crystallized rapidly, crystallites were numerous and small, while a smaller number of larger crystallites were observed when crystallized slowly. Similar results were observed for lard crystallized under the same conditions. The mechanical properties of both AMF and lard, expressed as hardness, depend on a variety of factors, which include the SFC, polymorphism of the solid state as well as the microstructure of the fat crystal network.     

Composition assessment of raw meat mixtures using ultrasonics

The use of ultrasonic velocity measurements to determine the composition of dry fermented sausages was assessed. Mixtures of ground lean and fatty tissues were prepared to cover a wide range of fat (2–90 wt.%), moisture (7–76 wt.%), and protein (2–21 wt.%) contents. The ultrasonic velocity in fat decreased on average 5.6 ms⁻¹ per °C increase in temperature, due to the negative temperature coefficient for fat and the fat melting, which is observed in (DSC) differential scanning calorimetry analysis. The ultrasonic velocity temperature dependence allowed the determining of the fat, moisture and protein+others content, by measuring the ultrasonic velocity in the mixtures at 4 and 25°C and using a semi-empirical equation. The explained variance was 99.6% for fat, 98.7% for moisture and 85.4% for protein+others. The results obtained show the feasibility of using ultrasonic velocity measurement to assess the composition of meat products such as dry fermented sausages, rapidly and non-destructively.     

Concentration Measurement by Acoustic Reflectance

ABSTRACT The speed of sound and the acoustic reflection coefficient of sucrose (0 to 60 wt%), glycerol (0 to 50 wt%), sodium chloride (0 to 27 wt%) solutions and tomato ketchup (0 to 100 wt%) dispersions were measured using a modified pulse-echo technique (2.25 MHz transducer, 20 °C). The density of all solutions is measured using a vibrating tube densitometer. All measured parameters are linear functions of concentration. Ultrasonic reflectance can provide equally precise measurements of concentration as conventional ultrasonic velocity measurements. For all samples except concentrated ketchup (> 50 wt%), measured reflectance is demonstrated to be a function of ultrasonic velocity and density.     

Comparison of pulsed NMR and ultrasonic velocity techniques for determining solid fat contents

An ultrasonic velocity technique has been compared with pulsed NMR'weight' and 'direct' methods of determining solid fat contents (SFC). Measurements were made at 18°C using samples 0–20% of rapidly cooled tristearin in paraffin oil, tristearin in sunflower oil and tripalmitin in paraffin oil. Correlations between the amount of triglyceride added and the SFC predicted by each of the techniques were always better than 0.995 ( n ± 13). The precisions of the ultrasonic technique (0.2%) and the weight method (0.3%) were significantly better than that of the direct method (0.7%). The ultrasonic technique may therefore offer a suitable alternative or adjunct to the established pulsed NMR technique for SFC determinations; it has considerably lower capital cost and a faster sampling rate.     

Cooking method and endpoint temperature effects on sensory and color characteristics of pumped pork loin chops

The objective of this study was to evaluate the effects of pumping, cooking method and endpoint temperature on quality of fresh pork chops. The longissimus was removed from 30 paired pork sides. One side was pumped to 110% of original weight (0.4% phosphate, 0.4% salt+added water). Chops were cut (2.5 cm thick) then fried or grilled to 70 or 80 °C. As endpoint temperature increased from 70 to 80 °C, pork flavor increased (P<0.05) from 95 to 104 (on a 150 point scale) for pumped samples but remained constant in unpumped samples. Juiciness decreased when unpumped samples cooked to 80 °C (62) rather than 70 °C (78). Warner–Bratzler shear values were lower (P<0.05) in pumped chops than in unpumped controls. Pork flavor was higher and off-flavor was lower in fried chops. Pumping loins appears to enhance sensory attributes of chops even when cooked to abusive endpoint temperatures with limited detrimental effects on physical characteristics.     

Effect of temperature and water activity on growth and ochratoxin A production by Australian Aspergillus carbonarius and A. niger isolates on a simulated grape juice medium

The effect of water activity (0.92, 0.95, 0.965 and 0.98) and temperature (15 °C, 25 °C, 30 °C and 35 °C) on growth rate and ochratoxin A (OA) production by five strains of Aspergillus carbonarius and two strains of A. niger isolated from Australian vineyards was characterised on a synthetic grape juice medium. Maximum growth for A. carbonarius occurred at ca 0.965 a_w and 30 °C, and for A. niger, at ca 0.98 a_w and 35 °C. The optimum temperature for OA production was 15 °C and little was produced above 25 °C. The optimum a_w for toxin production was 0.95–0.98 for A. carbonarius and 0.95 for A. niger. Toxin was produced in young colonies after and, typically, did not continue to accumulate the entire surface area of the plate was colonised. Rather, the amount decreased as colonies aged. Trends for growth and OA production were similar among Australian isolates and those from European grapes, as reported in the literature.     

Impact of sorbitol on the thermostability and heat-induced gelation of bovine serum albumin

The influence of sorbitol (0–40 wt.%) on the thermal denaturation and gelation of bovine serum albumin (BSA, pH 7.0) in aqueous solution has been studied. The effect of sorbitol on heat denaturation of 0.5 wt.% BSA solutions was measured using ultrasensitive differential scanning calorimetry. The unfolding process was irreversible and was characterized by the thermal denaturation temperature (T_m). As the sorbitol concentration increased from 0 to 40 wt.%, T_m increased from 73.0 to 80.9 °C. The rise in T_m was attributed to the increased thermal stability of the globular state of BSA relative to its native state. The dynamic shear rheology of 4 wt.% BSA solutions containing 200 mM NaCl was monitored as they were heated from 30 to 90 °C at 1.5 °C min⁻¹, held at 90 °C for 120 min, and then cooled back to 30 °C at −1.5 °C min⁻¹. Sorbitol increased the protein gelation temperature (ΔT_gel +10 °C for 40 wt.% sorbitol), decreased the isothermal gelation rate at 90 °C, but increased the final shear modulus of the gels cooled to 30 °C. The impact of sorbitol on gel characteristics was attributed to its ability to increase protein thermal stability, increase the attractive force between proteins and decrease the protein–protein collision frequency.     

Monitoring Tempered Dark Chocolate Using Ultrasonic Spectrometry

Ultrasonic spectrometry was used to distinguish between properly tempered and untempered commercial dark chocolate. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to characterize the polymorphic state of tempered and untempered chocolate, results that were correlated to those of ultrasonic spectrometry. Four different kinds of dark chocolate samples with different amounts of sugar (7.5–50 %), fat (30–50 %), and cocoa mass (70–90 %) were subjected to two different tempering protocols. The tempering was achieved using cocoa butter seeds under static cooling from 50 to 14 °C. The ultrasonic generator and analyzer SIA-7 (V.N. Instruments) was used to monitor the crystallization process. The instrument generated a chirp signal with a bandwidth of 0.5 to 3 MHz and was set to work in a four-pathway configuration with two transducers and a center frequency of 2.25 MHz. Spectrometric analysis was carried out with chocolate samples containing 2, 4, and 6 % solid fat content (SFC). The SFC was obtained from DSC measurements. Ultrasonic signals for attenuation, reflection, and velocity were compared between tempered and untempered chocolates. It was shown that seed-tempered chocolate with 30 % sugar and 47.5 % fat attenuated 2.5 MHz of ultrasonic waves by 0.8, 1.7, and 2.0 dB/cm at 2, 4, and 6 % SFC, respectively. On the other hand, untempered chocolate attenuated the ultrasound signal by 3.5, 3.6, and 4.3 dB/cm. Furthermore, it was found that ultrasound reflection signals were stronger and ultrasonic velocity was higher in chocolates with high sugar content.     

Ultrasound Doppler based in‐line viscosity and solid fat profile measurement of fat blends

This study reports the use of upgraded ultrasonic velocity profiling with pressure difference methodology; extended from previous work demonstrating true in‐line rheological and solid fat content (SFC) characterisation of complex opaque fat blends, subjected to scaled dynamic processing conditions. The experimental results have successfully confirmed previous non‐invasive, in‐line measurements for instantaneous velocity and rheological profiling of complex opaque fat blends [International Journal of Food Science and Technology 43 (2008) 2083]. A method for in‐line measurements under dynamic processing conditions to obtain the SFC of a fat blend was developed and successfully tested for a 30% palm stearin and 70% rapeseed oil system over a temperature range of 10–40 °C. These measurements correlated well with standard SFC values from pulsed‐nuclear magnetic resonance (p‐NMR) measurements deviating not more than +/– 2% SFC points from the standard p‐NMR values.     

Effect of temperature and growth media on the attachment of Listeria monocytogenes to stainless steel

Listeria monocytogenes ATCC 19111 cultivated in nutrient-rich medium (brain heart infusion, BHI) or starved in minimal medium (10% filter sterilized pond water and 90% sterilized distilled water) were investigated for their initial attachment to austenitic stainless steel No. 4 with satin finish at 4 °C, 20 °C, 30 °C, 37 °C, or 42 °C. A droplet (10 μl) containing  10⁷ CFU/ml of L. monocytogenes suspended in BHI or minimal medium was placed on the stainless steel surface. After holding in saturated humidity for 3 h at the desired temperature the surface was washed and prepared for scanning electron microscopy (SEM). Using SEM, attachment of L. monocytogenes was determined by counting cells remaining on the surface. When L. monocytogenes cultivated in BHI were used, with the exception of the number of attached cells being lower at 42 °C than at 37 °C and 30 °C, the number of attached cells increased with increasing temperature (P < 0.05). When L. monocytogenes starved in minimal medium were used, the number of attached cells also increased with increasing attachment temperature (P < 0.05), but the number of attached cells at 42 °C was lower than that at the other temperatures. The attachment of L. monocytogenes to stainless steel surface was greater when cultivated in rich medium of BHI vs starved in the minimal medium.     

Changes in fatty acids and microbial populations of pork inoculated with two biopreservative strains

Meat surface fermentation has been reported as an efficient method to reduce undesirable microbial population of this food commodity in tropical areas with high ambient temperatures and humidity. However, in order to efficiently apply this method, growth of biopreservative strains and changes in the meat substrate should be studied. Changes in long-chain fatty acids as well as pH and microbial growth (lactic acid bacteria and enterobacteria) were studied in pork inoculated with two biopreservative strains (Lactobacillus alimentarius and Staphylococcus carnosus) and stored at 4 and 20 °C. Both tested strains produced more than 6 mg lactic acid/g tissue, however, S. carnosus was more efficient in reducing enterobacteria populations at 20 °C. No significant increase in long-chain fatty acid concentration in samples stored at 4 °C was observed but there was a rapid increase of free fatty acids when stored at 20 °C.     

Modeling the combined effects of pH, temperature and ascorbic acid concentration on the heat resistance of Alicyclobacillus acidoterrestis

In this study, thermal inactivation parameters (D- and z-values) of Alicyclobacillus acidoterrestris spores in McIlvaine buffers at different pH, apple juice and apple nectar produced with and without ascorbic acid addition were determined. The effects of pH, temperature and ascorbic acid concentration on D-values of A. acidoterrestris spores were also investigated using response surface methodology. A second order polynomial equation was used to describe the relationship between pH, temperature, ascorbic acid concentration and the D-values of A. acidoterrestris spores. Temperature was the most important factor on D-values, and its effect was three times higher than those of pH. Although the statistically significant, heat resistance of A. acidoterrestris spores was not so influenced from the ascorbic acid within the concentration studied. D-values in apple juice and apple nectars were higher than those in buffers as heating medium at similar pH. The D-values ranged from 11.1 (90 °C) to 0.7 min (100 °C) in apple juice, 14.1 (90 °C) to 1.0 min (100 °C) in apple nectar produced with ascorbic acid addition, and 14.4 (90 °C) to 1.2 min (100 °C) in apple nectar produced without ascorbic acid addition. However, no significant difference in z-values was observed among spores in the juices and buffers at different pH, and it was between 8.2 and 9.2 °C. The results indicated that the spores of A. acidoterrestris may survive in fruit juices and nectars after pasteurization treatment commonly applied in the food industry.     

Modeling the effect of inoculum size and acid adaptation on growth/no growth interface of Escherichia coli O157:H7

The objective of this study was to model with logistic regression the growth/no growth interface of different initial inoculation levels (10¹, 10³ and 10⁵ CFU/ml; study 1), or nonadapted vs acid-adapted (study 2) Escherichia coli O157:H7 as influenced by pH, NaCl concentration and incubation temperature. Study 1 was conducted with a mixture of four E. coli O157:H7 strains grown (35 °C, 24 h) in tryptic soy broth (TSB). Study 2 was conducted with the same mixture of four E. coli O157:H7 strains grown (35 °C, 24 h) in glucose-free TSB with 1% added glucose (final pH 4.83), or in diluted lactic acid meat decontamination runoff fluids (washings; final pH 4.92), or nonadapted cultures prepared in glucose-free TSB (final pH 6.45), or in water washings (final pH 6.87). Parameters included incubation temperature (10–35 °C), pH (3.52–7.32), and NaCl concentration (0–10% w/v). Growth responses were evaluated for 60 days turbidimetrically (610 nm) every 5 days in 160 (study 1) and 360 (study 2) combinations in quadruplicate samples, with a microplate reader. The lower the initial inoculum the higher were the minimum pH and a_w values permitting growth. Differences in the pH and a_w growth limits among inoculum concentrations increased at 15 and 10 °C. Acid-adapted cultures were able to grow at lower pH than nonadapted cultures, while at temperatures below 25 °C, growth initiation of nonadapted cultures stopped at higher a_w compared to acid-adapted cultures for the whole pH range of 3.52 to 7.32. A comparison with available data indicated that our model for acid-adapted E. coli O157:H7 in different environments may provide representative growth probabilities covering both nonadapted and stress-adapted contaminants.     

Maintaining muscles at a high post-mortem temperature induces PSE-like meat in turkey

The aim of the present study was to validate an experimental model which surely generates pale, soft, exudative (PSE) turkey meat. Immediately after exsanguination, Pectoralis major (n=15) were kept at various temperatures (4, 20 or 40 °C) for 6 h. All the muscles were then stored at 4 °C for 9 days. They had the same rate of pH fall. L^* values were higher in the 40 °C treatment muscles than in the two other treatment muscles between 1 and 9 h. Drip loss of the 40 °C treatment muscles was higher than in the two other treatment muscles. However, thawing and cook loss were not significantly different between treatments. Cooked meat from the 40 °C treatment muscle was tougher than the two other treatment muscles. Napole yield was lower for these muscles. Myofibrillar protein extractability was lower in the 40 °C treatment muscle whatever the considered time. We showed that the 40 °C treatment muscles were similar to PSE muscles.     

Effects of pH and temperature on metmyoglobin solubility in a model system

From a series of experiments heating metmyoglobin solutions at pH 5.0 through 7.0, the effects of temperature and pH on the thermal stability of metmyoglobin were investigated. The percent metmyoglobin denatured at temperatures from 25 to 80 °C was determined. pHs lower than 6.5 caused metmyoglobin denaturation at various temperatures from 25 to 80 °C, but it was particularly apparent when pH was < 5.6. Thermal stability of metmyoglobin increased as pH increased. Metmyoglobin denaturation occurred at 55 °C at pH 5, however, denaturation did not occur until 60 °C at pHs from 5.3 to 7.0. A slower heating rate (0.9 °C/min) resulted in more metmyoglobin thermal denaturation than a faster heating rate (1.3 °C/min) when the temperature was above 55 to 60 °C. The denaturation caused by low pH alone was reversible, while that caused by high temperature was not. Techniques which increase muscle pH, such as the injection of sodium bicarbonate, could effectively improve the color condition of PSE meat.     

Temperature and water activity effects on growth and temporal deoxynivalenol production by two Argentinean strains of Fusarium graminearum on irradiated wheat grain

The ability of six strains of Pichia anomala, four strains of Pichia kluyveri and two strains of Hanseniaspora uvarum predominant during coffee processing to produce polygalacturonase (PG), pectin esterase (PE) and pectin lyase (PL) in yeast polygalacturonic acid medium (YPA) and in coffee broth (CB) was studied. For comparison, a reference strain of Kluyveromyces marxianus CCT 3172 isolated from cocoa and reported to produce high amount of PG was included.

Initial screening of PG activity using YPA medium showed that K. marxianus CCT 3172, P. anomala S16 and P. kluyveri S13Y4 had the strongest activity. Enzymatic assays showed that the four yeast species secreted PG, but none of the yeasts investigated was found to produce PE or PL. P. anomala S16 and P. kluyveri S13Y4 were found to produce higher amounts of PG when grown in CB than in YPA. When K. marxianus CCT 3172, P. anomala S16 and P. kluyveri S13Y4 were grown in YPA broth adjusted to pH of 3.0–8.0 and incubated at temperatures of 15–40 °C, the three yeast species secreted the highest amount of PG at pH 6.0 and at 30 °C. For PG secreted by K. marxianus CCT 3172 and P. anomala S16, the optimum pH and temperature for the enzymatic activity were 5.5 and 40 °C, respectively. On the other hand, PG produced by P. kluyveri S13Y4 showed the highest activity at pH 5.0 and 50 °C.

Significant differences in the extracellular activity of PG were found between the yeasts species as well as between strains within same species. High amounts of PG were produced by two strains of P. anomala and P. kluyveri. It is therefore likely that strains of those two species may be involved in the degradation of pectin during coffee fermentation.     

Ultrasonic velocity measurements on some liquids under thermal cycle: Ultrasonic velocity hysteresis

This study was performed to determine the existence of hysteresis during ultrasonic velocity measurements on methanol, ethanol, distilled water, processed water and glycerol under thermal cycle (5–50 °C). The ultrasonic velocity of the liquids were measured with 1.0 MHz transducers using a through transmission ultrasonic system. Statistical analyses results showed that there were ultrasonic velocity hystereses on the liquid samples under thermal cycle. The study of liquids undergoing thermal cycle is useful for determining the temperature effects on ultrasonic velocity and will help developing precise control applications in food industry by reducing the uncertainties in ultrasonic velocity measurements.     

Effect of season on color of Hanwoo (Korean native cattle) beef

A total of 1278 head of Hanwoo (Korean native cattle) slaughtered over four seasons were used to evaluate the effect of season on color characteristics of beef longissimus dorsi (LD) muscle. CIE L^*, a^*, b^*, C^* values and hue angle were significantly lower (P<0.05) in cattle slaughtered in the winter season. Meat color was darker in the winter than in the spring and autumn seasons. The L^* values among three average daily temperature (T_a) categories were different (P<0.05) in order of: [5 °CT_a<25 °C] > [T_a25 °C] > [T_a<5 °C], indicating that the meat color of cattle slaughtered at T_a<5 °C was darker. The a^*, b^*, C^* values and hue angle were significantly lower (P<0.05) in cattle slaughtered at T_a<5 °C. Season at slaughter is of great importance for meat color. Namely, meat color of Hanwoo beef was influenced by environmental temperature. Overall, cattle slaughtered in the winter season of T_a<5 °C produced beef with more undesirable meat color properties.     

Pressure-induced thermotolerance of Lactobacillus rhamnosus GG

Non-lethal high pressure treatment was evaluated regarding its potential of inducing heat resistance in Lactobacillus rhamnosus GG. Pressure pre-treated cells (at 100 MPa and 37 °C for 10 min) showed higher survivability than untreated ones when both were heat treated at 60 °C. Upon exposure to this lethal temperature for 5 min, cells survived better by 1.5 log-cycles in comparison to control group, when they were previously pressure treated at 100 MPa and 37 °C for 10 min. Apparently, only short-time heat resistance was positively affected by such pressure pre-conditioning step since at prolonged holding time at 60 °C (more than 7 min), no significant differences regarding the inactivation rate was observed. Various pressures (100–200 MPa), temperatures (37–50 °C), and holding times (5–10 min) were systematically investigated. The findings can serve as indications towards optimizing production of probiotic starter cultures using spray-drying.     

Microstructural changes of potato cells and starch granules heated in oil

A method that mimics the frying operation was implemented to observe microstructural changes in potato cells and starch. Isolated cells were obtained by successive treatments of potato tissue with acid and alkali solutions. Potato cells were heated in oil at a rate of 40 °C/min or directly in hot oil (180 °C) using a hot stage attached to a light microscope and studied by video-microscopy. Starch granules in cells underwent rapid gelatinization, deformation and compaction into one mass that occupied the whole volume of the cell. After dehydration cells showed a distinctive outer zone and a homogeneous core. Cell walls showed no signs of disruption but at temperatures >100 °C a reduction in cell area occurred. Immersion of cells in hot oil resulted in rapid swelling of starch. Relative projected area increased by 12% after 45 s of heating. Wet isolated starch granules began to expand and gelatinize at 65 °C and became disrupted at 120 °C. No changes were observed in dry starch granules subjected to a similar treatment.