VARIABLE CONTROL of A BATCH RETORT and PROCESS SIMULATION FOR OPTIMIZATION STUDIES

A microcomputer was interfaced with a batch retort for digital PI control of the retort temperature. Six retort temperature profiles (three isothermal, sinusoidal, step, and ramp) were examined for processing pea puree to nearly the same lethality values in 211 × 400 cans. Thiamine content and steam consumption were measured. Thiamine retentions in non-isothermal processes were greater than in the isothermal processes, but there was no significant difference in the steam consumption.
Lethality and thiamine retention were also simulated using a finite difference model for the same heating profiles, with good agreement with experimental results. the simulation suggests that differences in thiamine retention due to differing heating regimes are significant only for large lethality values and smaller cans. For F₀ values smaller than 10 min, the simulation suggests that the sinusoidal heating regime will provide better thiamine retention. Limitations of the model and suggestions for future improvement are also discussed.     

Correction Factors for Nondestructive Measurement of Residual Volume of Air in Retort Pouches

A destructive and a nondestructive method of estimating residual volume of air in retort pouches were compared. The difference between the volumes of air given by two techniques were found to be significant (P < 0.05) by paired t-test. An indirect calculation method was developed for estimation of internal pressure at neutral buoyancy and when used in conjunction with Archimedes’ Principle and Boyle's Law significantly improved the results. This method of calculating residual air in flexible pouches did not show any significant difference (P > 0.01) when compared to the standard destructive technique. The procedure outlined is easy to use.     

Water Activity Calculation by Direct Measurement of Vapor Pressure

An improved method for calculation of water activity from vapor pressure manometric data, which takes into account the volume expansion of nonadsorbing gases, the initial pressure in the desiccant flask and the pressure exerted by the desiccant, was developed. The water activity of pure water was measured over a range of nonadsorbing gas pressures and initial pressures in the desiccant flask. Standard deviations in a_w of pure water calculated using no corrections, corrections for volume expansion only, and full corrections were 0.221, 0.011, and 0.005, respectively. Correction only for volume expansion may under some conditions result in larger errors than when no corrections are made at all. Based on the improved equation, an experimental modification which reduces loss of water from samples during measurement was proposed.     

Rheological Behavior of Frozen and Thawed Low-Moisture, Part-Skim Mozzarella Cheese

Stress relaxation and dynamic profiles of low-moisture, part-skim (LMPS) Mozzarella cheese cylinders refrigerated 14 days (control), frozen and thawed, and stored frozen and refrigerated up to 90 days were compared. Samples were frozen at ?30°C and stored at ?20°C. Thawing and refrigerated storage were at 5°C. Stress relaxation tests were conducted at 20°C and dynamic spectrometry at 20°C and 60°C. The frozen and thawed Mozzarella cheese tested at 20°C became harder and more elastic with storage time, while refrigerated stored samples became softer and more elasticoviscous with time. Upon melting, both go-day-frozen and go-day-refrigerated cheeses were less elastic and less viscous than 14-day-refrigerated samples.     

Phase Equilibria and Distribution Coefficients of δ-Lactones in Supercritical Carbon Dioxide

A static method was used to measure the solubilities and distribution coefficients of δ-lactones (pure), inherent (natural levels) δ-lactones of anhydrous milk fat (AMF) and fortified (added) δ-lactones in AMF in supercritical carbon dioxide (SC-CO₂) at 40°C with pressures of 5–30 MPa. At 40°C distribution coefficients of δ-lactones increased with pressure and exhibited molecular weight-dependency. Using experimental data, a mathematical model based on thermodynamic equilibria and material balance was developed and used to predict the distribution of inherent δ-lactones of AMF in a continuous SC-CO₂ extraction system. At 24.1 MPa/40°C column conditions, AMF was separated into a raffinate and 5 fractions. Predicted δ-lactones distribution coefficients in the raffinate and extract were in good agreement with experimental data.     

RHEOLOGICAL BEHAVIOR OF PROCESSED MUSTARD. II. STORAGE EFFECTS

Samples of processed yellow mustard with three different particle size distributions (slightly-coarse, standard, and fine) were prepared according to a standard formulation and pilot-plant replication of the commercial methodology and stored for 3 months at 5°C, 25°C, and 45°C. After 3-month storage at 45°C, all samples showed visible liquid separation (syneresis) and aggregation of colloidal particles, as indicated by increase in d _pop and decrease in % colloids. The fine milled samples exhibited syneresis and an increase in d _pop after 3-month storage even at 25°C. The bimodal distribution did not show significant changes as a result of aggregation. In general, increase in d _pop increased apparent viscosity (η _ap ), flow behavior index (n) from the Herschel-Bulkley model, and exponent n" from the loss modulus (G") power law equation. As d _pop increased, Bingham yield stress _o), plastic apparent viscosity (η _p ), shear stress constant (A) from the Weltman stress decay model, yield stress (_o) and consistency index (m) from the Herschel-Bulkley model, exponent n' from the storage modulus (G') power law equation, and minimum values of loss tangent (tan δ) decreased. In heterodisperse semi-solid foods such as processed mustard, the particle size distribution influences the structure (i.e., packing) and the rheological behavior. Therefore, time and temperature dependent instabilities such as syneresis may be minimized by controlling particle size distribution during processing.     

Characterization of Thixotropic Behavior of Soft Cheeses

Time-dependent structural decay (γ) of four different spreadable cheeses was established using a Wells-Brookfield cone and plate viscometer. The γ values were determined at constant values of shear rate, (10 set⁻¹ to 200 sec⁻¹). The products studied were two samples of commercial Neufchatel cheese (one plain and one chocolate flavored) and two different saqples of cheese prepared using the Maubois, Mocquot and Vassal (MMV) method, which utilizes milk concentrated by ultrafiltration. All samples presented a structural decay characterizable by two different rate constants. Yield stress for Neufchatel cheese was observed to be one-third of the values obtained for MMV cheeses.     

MEASUREMENT OF FLOW ABILITY OF BUTTER BY CAPILLARY RHEOMETRY

Capillary rheometry was used to study and compare the rheological behavior of butters made from supercritically fractionated high melting triglyceride (HMT) and anhydrous milk fat (AMF) at different temperatures. Application of shear rates in the range of 17.37 to 346.93 _S⁻¹ showed a strong dependency of flow behavior on the capillary diameter, indicating slippage at the walls. Shear rates were corrected for wall-slippage using capillaries with different diameters but the same length, and for non-Newtonian behavior using the Rabinowitsch procedure. Shear stresses were corrected for entrance effect using capillaries of the same diameter but different lengths. Power law consistency indices (K), determined from loglog plots of corrected shear stresses versus shear rates, were found to have a negative correlation with temperature. Higher K values were observed for HMT butter depicting its more solid-like behavior due to greater concentration of high melting triglycerides. A slight increase in the K values was observed on storage probably due to rearrangement of fat crystals and formation of secondary bonds between them. Flow indices were found to be relatively insensitive to both time and temperature of storage. From the flow curves (apparent viscosity versus shear rates), master curves were generated using the time-temperature superposition principle.