The evaluation of water activity in aqueous solutions from freezing point depression

The water activity of various non-electrolyte and electrolyte solutions of interest for a _W inhibition of microorganisms, has been calculated from freezing point data and compared with a _W values measured at 25°C. It is shown that in the important range of a _W, 0.96–0.85, the error introduced by assuming that the a _W at the freezing point is the same as that at 'room' temperature, is relatively small. In the majority of cases the difference is not likely to be larger than about 0.01 a _W units.     

Equation for fitting desorption isotherms of durum wheat pasta

This work was undertaken to obtain water vapour desorption data for durum wheat pasta (spaghetti) between 40° and 90°C by an equilibrium method using sulphuric acid solutions. Sorption isotherms were also measured for semolina of the same composition. From these data the net heat of desorption, calculated by the Clausius-Clapeyron relationship, and BET monolayer values X _m were determined. In the range of water activity ( a _w) investigated, i.e. 0.10 < a _w < 0.90, the results were correlated to an Oswin type empirical relation X = f ( a _w, T ) with a mean precision of 7%. This equation should be of use in modelling pasta drying.     

Microbiological, physicochemical and organoleptic changes in dry-salted olives of Thassos variety stored under different modified atmospheres at 4 and 20 °C

Microbiological, physico-chemical and organoleptic changes were studied in dry-salted olives, cv. Thassos, stored under different atmospheres (100% carbon dioxide and nitrogen, 40% CO₂/30%O₂/30%N₂ and air) at 4 and 20 °C for 180 days. The initial microbial flora comprised of yeasts, no lactic acid bacteria, enterobacteria, pseudomonads or Staphylococcus aureus were detected, as the low water activity/high salt content does not favour their growth. At 4 °C, the population of yeasts declined steadily throughout storage but to a different extent depending on the gaseous atmospheres. At 20 °C, there was an initial decline in yeast counts in all samples followed by a steady increase until the end of the storage period. The CO₂ atmosphere was most effective at keeping the number of yeasts low at both storage temperatures. All gas atmospheres prevented fungal growth at both temperatures apart from the samples stored in air. The pH, a_w and salt content of the olives did not change significantly throughout the storage period. The prevailing yeast species was the salt tolerant Candida famata . The organoleptic characteristics did not differ significantly among differently treated olives. However, increased rancidity and reduced fruit colour was observed in the samples stored at 20 °C.     

A study of water activity lowering in meat during immersion-cooking in sodium chloride-glycerol solution. II. Kinetics of aW lowering and effect of some process variables

Beef slabs were dehydrated by immersion-cooking in a sodium chloride-glycerol solution of lower water activity ( a _W) in the range of temperatures 30 to 85°C. The present work studies the kinetics of a _W lowering in beef as a function of various processing variables (solution temperature, sample thickness) as well as the effect of various pretreatments applied to the beef.     

Chemical and sensory changes in haddock and herring stored under modified atmosphere

The effectiveness of storage in atmospheres with increased proportions of CO₂ to extend the shelf-life of haddock and herring was examined. Using a 40: 30: 30/CO₂: O₂: N₂ atmosphere with haddock no useful extension of shelf-life was achieved at +5°C and only limited extension at 0°C. A more useful extension of shelf-life at 0°C was obtained by storing haddock in a 60: 20: 2O/CO₂: O₂: N₂ atmosphere and with herring in a 60:40/CO₂::N₂: atmosphere. Total volatile bases (TVB) and hypoxanthine values (Hx) correlated with the cooked flavour. There were no significant differences in drip-loss between the modified atmosphere packaging (MAP) stored samples and the controls. Peroxide values (PV) in herring were lower in MAP stored samples. Whole haddock and whole herring were found to have a longer shelf-life when stored in MAP at 0°C than when stored as fillets.     

Stability of carotenoids in freeze dried papaya (Carica papaya)

Effect of water activity ( a _w) and storage temperature on the degradation of carotenoids in relation to keeping quality of freeze dried papaya is reported. Carotenoids were found to be most stable at 0.33 a _w and both below and above this level their rate of destruction was higher. Above 0.40 a _w browning limited the storage life. Freeze dried papaya has maximum stability between 0.22-0.33 a _w.     

Technical note: Sorption isotherms and monolayer moisture content of raw peas, and peas dehydrated after pressure cooking

The moisture sorption isotherms of raw and pre-cooked Century, MP 888 and MP 889 peas at 10, 25 and 40°C were determined by the standard salt solution technique. Pea cultivars, although different in composition, did not differ in equilibrium moisture content. At 40°C and in the water activity range 0.50 a _w 0.90, the sorption capacity of raw peas was higher than that of pre-cooked peas. All adsorption-desorption isotherms were of sigmoid shape and the quantity of sorbed water, at a given relative humidity, increased as the temperature was decreased. The monolayer moisture content of pre-cooked dehydrated peas stored at 40°C was 6.2%, with corresponding equilibrium relative humidity value of about 27%.     

Fate of Aflatoxin M1 during Manufacture and Storage of Feta Cheese

ABSTRACT:  The effect of feta cheese manufacture on aflatoxin M₁ (AFM₁) content was studied using an enzyme immunoassay technique. Feta cheese was made from milk spiked with 1 and 2 μg AFM₁ per kilogram milk. Pasteurization at 63 °C for 30 min caused <10% destruction of AFM₁. During cheese making, the remaining AFM₁ in milk was partitioned between curd and whey with two-thirds retained in the curd and one-third going into the whey. Cheeses were then stored for 2 mo in 8%, 10%, and 12% brine solutions at 6 and 18 °C. There was a 22% to 27% reduction of AFM₁ during the first 10 d of storage, with slightly more loss as salt concentration increased and when the cheese was stored at 18 °C. Further storage caused only slight decrease in AFM₁ and after 30 d of brining there was no difference in AFM₁ content of the cheese based upon salt concentration of the brine. At 18 °C, no further losses of AFM₁ occurred after 30 d, and at 6 °C, there was continued slight decrease in AFM₁ levels until 50 d. After 60 d of brining, there was a total loss of 25% and 29% of the AFM₁ originally present for cheese brined at 6 and 18 °C, respectively. Thus, the combination of pasteurization, conversion of milk into feta cheese, and at least 50 d storage of cheese in brine caused a total loss of about 50% of the AFM₁ originally present in the raw milk.     

Effect of temperature and initial moisture content on sorption isotherms of banana dried by tunnel drier

Sorption isotherms of intermediate moisture content (IMC) products are essential to predict shelf-life of packaged moisture-sensitive product by modelling moisture uptake during storage and distribution. The effect of temperature and initial moisture content (MC) of IMC banana on the relationship between MC and water activity were investigated. Raw bananas were dried in a tunnel dryer at 2% relative humidity (RH), 70 °C, and a 3.2 ± 0.2 m s^–1 air velocity. Drying procedure was carried out a number of times until various IMC levels were obtained (5%, 14%, 22% and 33% db for banana). Sorption isotherms of bananas were determined at 10, 20, 30 and 40 °C. The initial MC of IMC banana had no significant effect on the relationship between MC and water activity according to statistical analysis. All the sorption curves were found to be Type II. BET, GAB, modified GAB, Oswin, Halsey and modified Freundlich models were fitted to the data and it was found that the best results were obtained with a modified Freundlich equation. A secondary modified Freundlich model was built accounting for the effect of a _w and temperature on the sample MC.     

DEVELOPMENT OF STRESS AND SURVIVAL OF SALMONELLA TYPHIMURIUM ATCC 7136 IN A TAPIOCA SOYSTARCH PRODUCT CONTAINING POTASSIUM SORBATE

The effects of reduced water activity and 0.1% sorbate on Salmonella typhimurium ATCC 7136 in a soy-starch product were examined. The formulated product (final pH 6.7), was inoculated with cells to give a final concentration of approximately 2 × 10⁶ cells/g. Ten gram samples were placed in a series of desiccators over saturated salt solutions and stored at either 22°C or 40°C. Water activities ranged from 0.79 to 0.97. Samples were withdrawn at prescribed intervals and appropriate dilutions made. A dual plating procedure was used to monitor growth (tryptic soy agar) and development of injury (Levine's Eosin Methylene Blue Agar supplemented with 2% NaCl and 0.05% sodium desoxycholate). Cells stored at 22°C remained viable longer at an a_w of 0.79 than at a_w values of 0.96 to 0.97. As storage time increased, cells were stressed at a faster rate at higher a_w values. This effect was accelerated at 40°C and as storage time increased.     

SURVIVAL AND GROWTH OF SELECTED MICROORGANISMS IN KHOA DURING PREPARATION AND STORAGE

The survival of Escherichia coli and Staphylococcus aureus during the production of khoa, a heat concentrated Indian milk-based product, was investigated. Heat processing of milk containing from 3.6 to 6.5% fat at either 63 or 73C eliminated all E. coli. Under similar processing conditions , S. aureus was recovered, but only when heated in milk at 63C containing 6.5% fat. Potassium sorbate (3000 ppm) appeared more effective in inhibiting the growth of selected yeast and molds in khoa at 7C, compared to ascorbic acid (3000 ppm). Reducing the water activity (a _w) of khoa from 0.97 to 0.93 did not appear to enhance the preservative effect. The reduction of E. coli or S. aureus in khoa during prolonged storage at 6–7C, was less than one log cycle, regardless of a _w or preservative type. Survival of S. aureus in khoa appeared to be enhanced with a decrease in a _w. The potential for pathogens to survive in khoa during processing should be taken into consideration when formulating heating protocols.     

Water activity and salt content relationship in moist salted fish products

A linear relationship between water activity ( a _W) in moist salted fish products and NaCl molality was found: ( a _W) SF = 1.002–0.042 m ; where m is the NaCl molality considering it to be in true solution in the total water content of the product.
The linear approximation was analyzed with reference to the a _W values of pure NaCl solutions and to a thermodynamic approach for a _W prediction; it was found consistent and appropriate for a _W estimation in moist salted fish products, with errors within the accepted range for intermediate moisture foods (IMF).     

Storage behaviour of freeze dried watermelon juice powder

The effects of water activity ( a _w), sugar and citric acid addition on the keeping quality of freeze dried watermelon juice have been investigated. The stability of watermelon carotenoids increases with increase in a _w. Above 0.33 a _w caking limits the shelf life. Freeze dried watermelon has maximum stability between 0.22 and 0.25 a _w. The addition of sugar and citric acid has a beneficial effect on the stability of carotenoids and in retarding non-enzymic browning. The major pigment lycopene seems to be relatively more stable than β-carotene in watermelon juice powder during storage.     

Moisture sorption isotherms of potato slices

Water vapour sorption isotherms of Norchip, Russet Burbank and Norland potatoes were determined by the salt solution technique at 10, 25 and 40°C. The adsorption isotherms of potatoes were of sigmoid shape and were affected by drying method, temperature and sugars addition. The freeze dried products adsorbed more water vapour than the vacuum dried materials. Addition of glucose, sucrose or lactose to the potatoes caused the equilibrium moisture content to decrease in the low and intermediate water activity ranges. The sorption was temperature independent in the a _w range 0.11–0.35 and showed a hysteresis loop at intermediate moisture levels.     

Study of Caking in Powdered Foods Using Nuclear Magnetic Resonance Spectroscopy

Twenty-three powdered foods and ingredients were subjected to 2 treatments: temperature scan from -20 to 110 °C within 2 hours, and storage at 37 °C and 55 °C for 60 d. During temperature scans, the spin-spin relaxation time T₂ values of individual samples were determined using a nuclear magnetic resonance (NMR) technique, resulting in a temperature-T₂ curve for each sample. Four typical temperature-T₂ curve patterns were identified and were found to be closely related to the physical changes in the samples, including agglomeration, water vaporization, and caking observed during temperature scans. Based on the temperature-T₂ curve patterns, the samples were classified into 4 groups each having distinguished caking behavior. In the storage tests, the samples were removed from the incubators every half an hour during first 2 d of storage period and every day afterwards. The storage test also suggests a strong relationship between molecular mobility and caking. Monitoring changes in T₂ as a function of temperature provides information useful for predicting whether and when caking would occur.     

Fructo-oligosaccharide changes during the storage of dehydrated commercial garlic and onion samples

The composition of fructo-oligosaccharides (FOS), together with glucose, fructose and sucrose, has been determined in commercial dehydrated samples of onion ( Allium cepa L.) and garlic ( Allium sativum L.). No qualitative differences were detected among the FOS identified in both onion and garlic. Results show a lower FOS content in garlic than in onion in all samples analysed. In garlic samples, FOS with degree of polymerisation from 3 to 7 were identified and quantified for the first time. In addition, a study on the effect of the storage in these carbohydrates has been carried out under controlled conditions (50 °C, a _w 0.44). The storage seemed to promote important changes in the content of glucose and fructose, which could participate in the Maillard reaction and in the composition of sucrose and FOS, whose hydrolysis was noticeable during the storage time. Therefore, all these chemical changes should be considered to evaluate the quality of these foods.     

The prediction of water activity of aqueous solutions in connection with intermediate moisture foods III. aW Prediction in multicomponent strong electrolyte aqueous solutions

A study was made on the theoretical prediction of water activity ( a _W) in multicomponent aqueous solutions of strong electrolytes, in the range of a _W of most interest for intermediate moisture foods (IMF). A simplified model proposed by Ross (1975) is tested against a rigorous model equation developed on the basis of recent statistical theories of electrolytes. As a result of this study a new simple and very accurate model equation is proposed for predicting a _W in aqueous solutions of mixed strong electrolytes in the a _W range of most interest to IMF.     

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.     

Control of a range of food related microorganisms by a multi-parameter preservation system

Seventy-two process variables, based on a matrix of a _w nature of controlling solute, pH, and the addition of sodium citrate and sodium benzoate were examined with a view t o devising a food preservation system for ambient temperatures. Growth of a challenge'cocktail' of Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, Streptococcus faecalis, Lactobacillus casei and Clostridiurn perfringens, was assessed in a Brain Heart Infusion broth based model system. Each of the controlling factors had an effect on growth. The addition of sodium citrate (2% w /v) and sodium benzoate (2000 mg/I) enhanced the inhibitory effect. Reduction in a _w (1.00–0.94) and pH (7.0–5.5) in combination or as separate controlling factors increased any inhibition. Combinations of sodium chloride and glycerol were used as the controlling solute, and increasing the proportion of sodium chloride resulted in suppressed growth, irrespective of any change in a _w or pH. Combinations of sodium citrate and sodium benzoate with a pH less than 6.0 and an a, less than 0.95 inhibited any growth during a 42 day incubation period at 37°C, irrespective of the controlling solutes studied.