Changes in quality attributes throughout storage of strawberry juice processed by high-intensity pulsed electric fields or heat treatments

The effects of high-intensity pulsed electric field (HIPEF) processing (35 kV/cm for 1700 μs applying 4-μs pulses at 100 Hz in bipolar mode) on color, viscosity and PME and PG activities in strawberry juice were studied and compared to those of heat treatments (90 °C for 60 s or 30 s) through 63 days of storage. L^∗ and viscosity values of the HIPEF-processed juices were higher than those found in the thermally treated. In addition, HIPEF-treated juice exhibited lower 5-(hydroxymethyl)-2-furfural (HMF) concentration and browning index than heat-treated juices throughout storage. On the other hand, HIPEF-treated juice maintained low residual pectin methylesterase (PME) activity (13.1%) for 63 days, whereas in the case of the thermally treated, 22.2 and 48.8% was retained after 60 s and 30 s, respectively. Strawberry juice treated by HIPEF achieved lower residual polygalacturonase (PG) activity (73.3%) than those of heat-processed at 90 °C for 60 s (76.2%) or 30 s (96.8%). Thus, HIPEF could be a feasible alternative to thermal processing to minimize browning and viscosity loss in strawberry juice during storage.     

Effect of enterocin AS-48 in combination with high-intensity pulsed-electric field treatment against the spoilage bacterium Lactobacillus diolivorans in apple juice

Enterocin AS-48 was tested in apple juice against the cider-spoilage, exopolysaccharide-producing strain Lactobacillus diolivorans 29 in combination with high-intensity pulsed-electric field (HIPEF) treatment (35 kV/cm, 150 Hz, 4 μs and bipolar mode). A response surface methodology was applied to study the bactericidal effects of the combined treatment, with AS-48 concentration and HIPEF treatment time as process variables. At subinhibitory bacteriocin concentrations, microbial inactivation by the combined treatment increased as the bacteriocin concentration and the HIPEF treatment time increased (from 0.5 to 2.0 μg/ml and from 100 to 1000 μs, respectively). Highest inactivation (4.87 logs) was achieved by 1000 μs HIPEF treatment in combination with 2.0 μg/ml AS-48. While application of treatments separately did not protect juice from survivors during storage, survivors to the combined treatment were inactivated within the following 24 h of storage, and the treated samples remained free from detectable lactobacilli for at least 15 days at temperatures of 4 °C as well as 22 °C. The combined treatment could be useful for inactivation of exopolysaccharide-producing L. diolivorans in apple juice.     

Comparative study on shelf life of orange juice processed by high intensity pulsed electric fields or heat treatment

The effects of high intensity pulsed electric fields (HIPEF) processing (35 kV/cm for 1,000 μs; bipolar 4-μs pulses at 200 Hz) on the microbial shelf life and quality-related parameters of orange juice were investigated during storage at 4 and 22 °C and compared to traditional heat pasteurization (90 °C for 1 min) and an unprocessed juice. HIPEF treatment ensured the microbiological stability of orange juice stored for 56 days under refrigeration but spoilage by naturally occurring microorganisms was detected within 30 days of storage at 22 °C. Pectin methyl esterase (PME) of HIPEF-treated orange juice was inactivated by 81.6% whereas heat pasteurization achieved a 100% inactivation. Peroxidase (POD) was destroyed more efficiently with HIPEF processing (100%) than with the thermal treatment (96%). HIPEF-treated orange juice retained better color than heat-pasteurized juice throughout storage but no differences (p<0.05) were found between treatments in pH, acidity and °Brix. Vitamin C retention was outstandingly higher in orange juice processed by HIPEF fitting recommended daily intake standards throughout 56 days storage at 4 °C, whereas heat-processed juice exhibited a poor vitamin C retention beyond 14 days storage (25.2–42.8%). The antioxidant capacity of both treated and untreated orange juice decreased slightly during storage. Heat treatments resulted in lower free-radical scavenging values but no differences (p<0.05) were found between HIPEF-processed and unprocessed orange juice.     

Pilot-scale crossflow-microfiltration and pasteurization to remove spores of Bacillus anthracis (Sterne) from milk

High-temperature, short-time pasteurization of milk is ineffective against spore-forming bacteria such as Bacillus anthracis (BA), but is lethal to its vegetative cells. Crossflow microfiltration (MF) using ceramic membranes with a pore size of 1.4 μm has been shown to reject most microorganisms from skim milk; and, in combination with pasteurization, has been shown to extend its shelf life. The objectives of this study were to evaluate MF for its efficiency in removing spores of the attenuated Sterne strain of BA from milk; to evaluate the combined efficiency of MF using a 0.8-μm ceramic membrane, followed by pasteurization (72°C, 18.6 s); and to monitor any residual BA in the permeates when stored at temperatures of 4, 10, and 25°C for up to 28 d. In each trial, 95 L of raw skim milk was inoculated with about 6.5 log₁₀ BA spores/mL of milk. It was then microfiltered in total recycle mode at 50°C using ceramic membranes with pore sizes of either 0.8 μm or 1.4 μm, at crossflow velocity of 6.2 m/s and transmembrane pressure of 127.6 kPa, conditions selected to exploit the selectivity of the membrane. Microfiltration using the 0.8-μm membrane removed 5.91 ± 0.05 log₁₀ BA spores/mL of milk and the 1.4-μm membrane removed 4.50 ± 0.35 log₁₀ BA spores/mL of milk. The 0.8-μm membrane showed efficient removal of the native microflora and both membranes showed near complete transmission of the casein proteins. Spore germination was evident in the permeates obtained at 10, 30, and 120 min of MF time (0.8-μm membrane) but when stored at 4 or 10°C, spore levels were decreased to below detection levels (≤0.3 log₁₀ spores/mL) by d 7 or 3 of storage, respectively. Permeates stored at 25°C showed coagulation and were not evaluated further. Pasteurization of the permeate samples immediately after MF resulted in additional spore germination that was related to the length of MF time. Pasteurized permeates obtained at 10 min of MF and stored at 4 or 10°C showed no growth of BA by d 7 and 3, respectively. Pasteurization of permeates obtained at 30 and 120 min of MF resulted in spore germination of up to 2.42 log₁₀ BA spores/mL. Spore levels decreased over the length of the storage period at 4 or 10°C for the samples obtained at 30 min of MF but not for the samples obtained at 120 min of MF. This study confirms that MF using a 0.8-μm membrane before high-temperature, short-time pasteurization may improve the safety and quality of the fluid milk supply; however, the duration of MF should be limited to prevent spore germination following pasteurization.     

Changes on flavor compounds throughout cold storage of watermelon juice processed by high-intensity pulsed electric fields or heat

The application of HIPEF processing (35 kV/cm for 1727 μs using bipolar pulses of 4-μs at 188 Hz) on watermelon juice was evaluated as an alternative to conventional heat treatments (90 °C for 30 s or 90 s) in order to achieve better preservation of watermelon aroma compounds for 56 days of storage at 4 °C. HIPEF processing not only induced a rise (roughly 20%) in the concentrations of hexanal, (E)-2-nonenal, nonanal, 6-methyl-5-hepten-2-one and geranylacetone but also achieved less reductions on the retention of volatiles than the thermal treatment at 90 °C for 60 s. In contrast, the content of (Z)-6-nonenal, 1-nonanol and (Z)-3-nonen-1-ol in the untreated and processed juices remained unchanged after processing. Despite the decrease in overall flavor compounds observed during storage irrespective of the treatment applied, HIPEF-treated juices showed better flavor retention than heat-treated samples for at least 21 days of storage. Moreover, changes in aldehydes and ketones during storage of treated watermelon juices were well fitted by a model based on the Weibull distribution function. Therefore, the application of HIPEF may be appropriate to preserve the initial volatile profile of watermelon juices during storage.     

Maintenance of breast milk immunoglobulin A after high-pressure processing

Human milk is considered the optimal nutritional source for infants. Banked human milk is processed using low-temperature, long-time pasteurization, which assures microbial safety but involves heat denaturation of some desirable milk components such as IgA. High-pressure processing technology, the subject of the current research, has shown minimal destruction of food macromolecules. The objective of this study was to investigate the influence of pressure treatments on IgA content. Moreover, bacterial load was evaluated after pressure treatments. The effects of high-pressure processing on milk IgA content were compared with those of low-temperature, long-time pasteurization. Mature human milk samples were heat treated at 62.5°C for 30 min or pressure processed at 400, 500, or 600 MPa for 5 min at 12°C. An indirect ELISA was used to measure IgA in human milk whey obtained after centrifugation at 800 × g for 10 min at 4°C. All 3 high-pressure treatments were as effective as low-temperature, long-time pasteurization in reducing the bacterial population of the human milk samples studied. After human milk pressure processing at 400 MPa, 100% of IgA content was preserved in milk whey, whereas only 72% was retained in pasteurized milk whey. The higher pressure conditions of 500 and 600 MPa produced IgA retention of 87.9 and 69.3%, respectively. These results indicate that high-pressure processing at 400 MPa for 5 min at 12°C maintains the immunological protective capacity associated with IgA antibodies. This preliminary study suggests that high-pressure processing may be a promising alternative to pasteurization in human milk banking.     

Changes on phenolic and carotenoid composition of high intensity pulsed electric field and thermally treated fruit juice–soymilk beverages during refrigerated storage

The effects of high intensity pulsed electric fields (HIPEF) (35 kV/cm with 4 μs bipolar pulses at 200 Hz for 800 or 1400 μs) or thermal (90 °C, 60 s) treatments over phenolic and carotenoid compounds of a fruit juice–soymilk (FJ–SM) beverage stored at 4 °C were evaluated and compared, having the untreated beverage as a reference. Coumaric acid, narirutin and hesperidin were the most abundant phenolic compounds in the FJ–SM beverage, while the main carotenoids were lutein, zeaxanthin and β-carotene. Immediately after HIPEF or heat processing, hesperidin content of the beverage showed a huge rise, resulting in a significant increase on the total phenolic concentration. Regarding carotenoid concentration, HIPEF or thermal treatment lead to a significant decrease; lutein, zeaxanthin and β-cryptoxanthin being the most affected compounds. In contrast, the content of some individual phenolics and carotenoids increased with time, while others tended to decrease or remained with no significant changes with regards to their initial values. Total phenolic concentration seemed to be highly stable during storage; while, total carotenoid content gradually diminished, irrespectively of the treatment applied. Overall, the changes observed in HIPEF treated FJ–SM beverage were less than those in the heat processed one. Hence, HIPEF is a feasible technology to obtain FJ–SM beverages with extended shelf-life and a similar profile of antioxidant compounds to freshly made beverages.     

Yield, composition and rheological characteristics of cheddar cheese made with high pressure processed milk

High Pressure (HP) treatment of milk prior to cheese-making was shown to increase the yield of cheese due to increased protein and moisture retention in cheese. Cheeses were made with raw milk or milk treated with high temperature short-time (HTST) pasteurization, and HP treatments at two levels (483 and 676 MPa) at 10 °C, 483 MPa HP at 30 °C, and 483 MPa HP at 40 °C. Cheese yield, total solids, protein, fat and salt contents were evaluated, and fat and protein recovery indices were calculated. Cheeses from HP treatments of 676 MPa at 10 °C and 483 MPa at 30 °C exhibited wet yields of 11.40% and 11.54%, respectively. Protein recovery was 79.9% for HP treatment of 676 MPa at 10 °C. The use of slightly higher pressurization temperatures increased moisture retention in cheese. Visco-elasticity of cheeses was determined by dynamic oscillatory testing and a creep-recovery test. Rheological parameters such as loss (G″) and storage (G′) moduli were dependent on oscillation frequency. At high (173 rad/s) and low (2.75 rad/s) angular frequencies, cheeses made from milk treated at 483 MPa at 10 °C behaved more solid-like than other treatments. Creep tests indicated that cheeses from milk treated with 483 MPa HP at 10 °C showed the smallest instantaneous compliance (J_o), confirming the more solid-like behavior of cheese from the 483 MPa at 10 °C treatment compared to the behavior of cheeses from other treatments. Cheeses made with pasteurized milk were more deformable, exhibited less solid-like behavior than cheeses made with HP treated milk, as shown by the J_o value. With more research into bacteriological implications, HP treatment of raw milk can augment Cheddar cheese yield with better curd formation properties.     

Impact of high intensity pulsed electric field on antioxidant properties and quality parameters of a fruit juice-soymilk beverage in chilled storage

The effects of high-intensity pulsed electric fields (HIPEF) treatment (35 kV/cm, 4 μs bipolar pulses at 200 Hz for 800 or 1400 μs) on the microbial stability, quality parameters and antioxidant properties of a fruit juice-soymilk (FJ-SM) beverage along the storage time at 4 °C were compared to those obtained by thermal pasteurization (90 °C, 60 s). HIPEF processing for 800 μs ensured the microbial stability of the beverage during 31 days; however, longer microbial shelf-life (56 days) was achieved by increasing the treatment time to 1400 μs or by applying a thermal treatment. Peroxidase and lipoxygenase of HIPEF treated beverages were inactivated by 17.5-29% and 34-39%, respectively; whereas thermal treatment achieved 100% and 51%. During the storage, vitamin C content and antioxidant capacity depleted with time, and they were higher in FJ-SM beverages processed by HIPEF than in those thermally treated. Instead, total phenolic content of beverages did not present significant changes over the time, and it was higher in the 1400 μs-HIPEF treated beverages. In general, color, soluble solids, pH, and acidity values were not significantly affected by the processing treatments. Beverage viscosity increased over time, regardless of the treatment applied. Hence, application of HIPEF may be a good alternative treatment to thermal processing in order to ensure microbiological stability, high nutritional values and fresh-like characteristics of FJ-SM beverages.     

Inactivation of Cronobacter sakazakii by manothermosonication in buffer and milk

The inactivation of Cronobacter sakazakii by heat and ultrasound treatments under pressure at different temperatures [manosonication (MS) and manothermosonication (MTS)] was studied in citrate-phosphate pH 7.0 buffer and rehydrated powdered milk. The inactivation rate was an exponential function of the treatment time for MS/MTS treatments (35−68 °C; 200 kPa of pressure; 117 μm of amplitude of ultrasonic waves) in both media, and for thermal treatments alone when buffer was used as heating media. Survival curves of C. sakazakii during heating in milk had a concave downward profile. Up to 50 °C, the lethality of ultrasound under pressure treatments was independent of the treatment temperature in both media. At temperatures greater than 64 °C in buffer and 68 °C in milk, the inactivating effect of MTS was equivalent to that of the thermal treatments alone at the same temperature. Between 50 and 64 ºC for buffer and 50 and 68 °C for milk, the lethality of MTS was the result of a synergistic effect, where the total lethal effect was higher than the lethal effect of heat added to that of ultrasound under pressure at room temperature. The maximum synergism was found at 60 °C in buffer and at 56 °C in milk. A heat treatment of 12 min (60 °C) or 4 min of an ultrasound under pressure at room temperature treatment (35 °C; 200 kPa; 117 μm) would be necessary to guarantee the death of 99.99% of C. sakazakii cells suspended in milk. The same level of C. sakazakii inactivation can be achieved with 1.8 min of a MTS treatment (60 °C; 200 kPa; 117 μm). Damaged cells were detected after heat treatments and after ultrasound under pressure treatments at lethal but not at non-lethal temperatures.     

High temperature, short time pasteurization temperatures inversely affect bacterial numbers during refrigerated storage of pasteurized fluid milk

The grade A Pasteurized Milk Ordinance specifies minimum processing conditions of 72°C for at least 15 s for high temperature, short time (HTST) pasteurized milk products. Currently, many US milk-processing plants exceed these minimum requirements for fluid milk products. To test the effect of pasteurization temperatures on bacterial numbers in HTST pasteurized milk, 2% fat raw milk was heated to 60°C, homogenized, and treated for 25 s at 1 of 4 different temperatures (72.9, 77.2, 79.9, or 85.2°C) and then held at 6°C for 21 d. Aerobic plate counts were monitored in pasteurized milk samples at d 1, 7, 14, and 21 postprocessing. Bacterial numbers in milk processed at 72.9°C were lower than in milk processed at 85.2°C on each sampling day, indicating that HTST fluid milk-processing temperatures significantly affected bacterial numbers in fluid milk. To assess the microbial ecology of the different milk samples during refrigerated storage, a total of 490 psychrotolerant endospore-forming bacteria were identified using DNA sequence-based subtyping methods. Regardless of processing temperature, >85% of the isolates characterized at d 0, 1, and 7 postprocessing were of the genus Bacillus, whereas more than 92% of isolates characterized at d 14 and 21 postprocessing were of the genus Paenibacillus, indicating that the predominant genera present in HTST-processed milk shifted from Bacillus spp. to Paenibacillus spp. during refrigerated storage. In summary, 1) HTST processing temperatures affected bacterial numbers in refrigerated milk, with higher bacterial numbers in milk processed at higher temperatures; 2) no significant association was observed between genus isolated and pasteurization temperature, suggesting that the genera were not differentially affected by the different processing temperatures; and 3) although typically present at low numbers in raw milk, Paenibacillus spp. are capable of growing to numbers that can exceed Pasteurized Milk Ordinance limits in pasteurized, refrigerated milk.     

Fate of lysostaphin in milk from individual cows through pasteurization and cheesemaking

Transgenic cows secreting over 3 μg of lysostaphin/ mL of milk are protected against mastitis caused by Staphylococcus aureus, but it is unknown if active lysostaphin persists through dairy processing procedures or affects the production of fermented dairy foods. The objective of this study was to determine the fate of lysostaphin as milk was pasteurized and then processed into cheese. Raw milk from transgenic cows was heat treated at 63°C for 30 min, 72°C for 15 s (high temperature, short time), or 140°C for 2 s (UHT). Portions of the high temperature, short-time milk were manufactured into semi-hard cheeses. Aliquots taken at each processing step were assayed to determine the quantity (ELISA) and activity (ability to inhibit S. aureus growth) of lysostaphin. Results indicated that most of the lysostaphin was present in the aqueous portion of the milk and was not affected by pasteurization, although UHT treatment reduced enzyme concentration by 60%. The quantity and activity of the lysostaphin decreased during cheesemaking. Based on the amount of lysostaphin present in the starting cheesemilk, 10 to 15% of the lysostaphin was recovered in the whey, 21 to 55% in the cheese curd at d 1, and 21 to 36% in cheese stored at 4°C for 90 d. Enough of the lysostaphin secreted into milk by transgenic cows survived typical dairy processing conditions to impart potential value as a bioprotective agent against staphylococci in dairy foods.     

Thermal inactivation of foot-and-mouth disease virus in milk using high-temperature, short-time pasteurization

Previous studies of laboratory simulation of high temperature, short time pasteurization (HTST) to eliminate foot-and-mouth disease virus (FMDV) in milk have shown that the virus is not completely inactivated at the legal pasteurization minimum (71.7°C/15 s) but is inactivated in a flow apparatus at 148°C with holding times of 2 to 3 s. It was the intent of this study to determine whether HTST pasteurization conducted in a continuous-flow pasteurizer that simulates commercial operation would enhance FMDV inactivation in milk. Cows were inoculated in the mammary gland with the field strain of FMDV (01/UK). Infected raw whole milk and 2% milk were then pasteurized using an Arm-field pilot-scale, continuous-flow HTST pasteurizer equipped with a plate-and-frame heat exchanger and a holding tube. The milk samples, containing FMDV at levels of up to 10⁴ plaque-forming units/mL, were pasteurized at temperatures ranging from 72 to 95°C at holding times of either 18.6 or 36 s. Pasteurization decreased virus infectivity by 4 log₁₀ to undetectable levels in tissue culture. However, residual infectivity was still detectable for selected pasteurized milk samples, as shown by intramuscular and intradermal inoculation of milk into naïve steers. Although HTST pasteurization did not completely inactivate viral infectivity in whole and 2% milk, possibly because a fraction of the virus was protected by the milk fat and the casein proteins, it greatly reduced the risk of natural transmission of FMDV by milk.     

Shelf life of pasteurized microfiltered milk containing 2% fat

The goal of this research was to produce homogenized milk containing 2% fat with a refrigerated shelf life of 60 to 90 d using minimum high temperature, short time (HTST) pasteurization in combination with other nonthermal processes. Raw skim milk was microfiltered (MF) using a Tetra Alcross MFS-7 pilot plant (Tetra Pak International SA, Pully, Switzerland) equipped with Membralox ceramic membranes (1.4 μm and surface area of 2.31 m²; Pall Corp., East Hills, NY). The unpasteurized MF skim permeate and each of 3 different cream sources were blended together to achieve three 2% fat milks. Each milk was homogenized (first stage: 17 MPa, second stage: 3 MPa) and HTST pasteurized (73.8°C for 15 s). The pasteurized MF skim permeate and the 3 pasteurized homogenized 2% fat milks (made from different fat sources) were stored at 1.7 and 5.7°C and the standard plate count for each milk was determined weekly over 90 d. When the standard plate count was >20,000 cfu/mL, it was considered the end of shelf life for the purpose of this study. Across 4 replicates, a 4.13 log reduction in bacteria was achieved by MF, and a further 0.53 log reduction was achieved by the combination of MF with HTST pasteurization (73.8°C for 15 s), resulting in a 4.66 log reduction in bacteria for the combined process. No containers of MF skim milk that was pasteurized after MF exceeded 20,000 cfu/mL bacteria count during 90 d of storage at 5.7°C. The 3 different approaches used to reduce the initial bacteria and spore count of each cream source used to make the 2% fat milks did not produce any shelf-life advantage over using cold separated raw cream when starting with excellent quality raw whole milk (i.e., low bacteria count). The combination of MF with HTST pasteurization (73.8°C for 15 s), combined with filling and packaging that was protected from microbial contamination, achieved a refrigerated shelf life of 60 to 90 d at both 1.7 and 5.7°C for 2% fat milks.     

The effect of adding antimicrobial peptides to milk inoculated with Staphylococcus aureus and processed by high-intensity pulsed-electric field

The use of high-intensity pulsed-electric field (HIPEF) and antimicrobial substances of natural origin, such as enterocin AS-48 (AS-48), nisin, and lysozyme, are among the most important nonthermal preservation methods. Thus, the purpose of this study was to evaluate the combined effect on milk inoculated with Staphylococcus aureus of the addition of AS-48 with nisin or lysozyme, or both, together with the use of HIPEF. Synergy was observed in the reduction of Staph. aureus counts with the following combination methods: i) addition of AS-48 and nisin, ii) addition of AS-48 plus use of HIPEF, and iii) addition of AS-48 and nisin plus use of HIPEF. Specifically, when 28 arbitrary units/mL of AS-48 and 20 IU/mL of nisin were added to the milk, and it was treated with HIPEF for 800 μs, over 6 log reductions were observed in the microorganism. In general, Staph. aureus inactivation was dependent on HIPEF treatment time, antimicrobial doses, and medium pH. During storage of the treated milk, survivor population was related to peptide concentration and temperature. Final cell viability was influenced by the sequence in which the treatments were applied: the addition of AS-48 or AS-48 and nisin was more effective before than after HIPEF treatment. The results obtained indicate that the combination of HIPEF and antimicrobials could be of great interest to the dairy industry, although it is necessary to study further the way in which the combined treatments act.     

Carotenoid and phenolic profile of tomato juices processed by high intensity pulsed electric fields compared with conventional thermal treatments

The effect of high intensity pulsed electric fields (HIPEF) processing (35 kV/cm for 1500 μs of overall treatment time with bipolar pulses of 4-μs at 100 Hz) and heat pasteurisation (90 °C for 30 s or 60 s) on carotenoids and phenolic compounds as well as on some quality attributes (pH, soluble solids and colour parameters) of tomato juice was evaluated and compared, having the untreated juice as a reference. Processing enhanced some carotenoids (lycopene, β-carotene and phytofluene) and the red colour of juices, whereas no significant changes in phenolic compounds, pH and soluble solids were observed between treated and untreated juices. A slight decrease in overall health-related compounds was observed over time, with the exception of some carotenoids (β-carotene and phytoene) and caffeic acid. However, HIPEF-processed tomato juices maintained higher content of carotenoids (lycopene, neurosporene and γ-carotene) and quercetin through the storage time than thermally and untreated juices. Hence, the application of HIPEF may be appropriate to achieve not only safe but also nutritious and fresh like tomato juice.     

Flavour retention and related enzyme activities during storage of strawberry juices processed by high-intensity pulsed electric fields or heat

The effects of high-intensity pulsed electric fields (HIPEF) processing (35 kV/cm for 1700 μs using pulses of 4 μs at 100 Hz in bipolar mode) and thermal treatments (90 °C for 30 s or 60 s) on lipoxygenase (LOX) and β-glucosidase (β-GLUC) activities as well as on the production of volatile compounds were assessed in strawberry juice for 56 days of storage. HIPEF-treated juice kept higher residual LOX activity than heat-treated juices during the first 28 days of storage. Moreover, β-GLUC increased its initial activity just after HIPEF processing. The concentration of DMHF in HIPEF-processed strawberry juice was above those of untreated and heat-treated juices during the first 14 days of storage. On the other hand, concentrations of ethyl butanoate and 1-butanol obtained after HIPEF processing were better maintained than after thermal processing. However, thermally-treated samples showed an increase in the amount of 1-butanol beyond day 35, causing an unpleasant flavour to the product. Thus, flavour stability in HIPEF-processed strawberry juice was greater than in thermally-treated samples during storage.     

Combined pH and high hydrostatic pressure effects on Lactococcus starter cultures and Candida spoilage yeasts in a fermented milk test system during cold storage

The combined effects of high pressure processing (HPP) and pH on the glycolytic and proteolytic activities of Lactococcus lactis subsp. lactis, a commonly used cheese starter culture and the outgrowth of spoilage yeasts of Candida species were investigated in a fermented milk test system. To prepare the test system, L. lactis subsp. lactis C10 was grown in UHT skim milk to a final pH of 4.30 and then additional samples for treatment were prepared by dilution of fermented milk with UHT skim milk to pH levels of 5.20 and 6.50. These milk samples (pH 4.30, 5.20 and 6.50) with or without an added mixture of two yeast cultures, Candida zeylanoides and Candida lipolytica (10⁵ CFU mL⁻¹ of each species), were treated at 300 and 600 MPa (≤20 °C, 5 min) and stored at 4 °C for up to 8 weeks. Continuing acidification by starter cultures, as monitored during storage, was substantially reduced in the milk pressurised at pH 5.20 where the initial titratable acidity (TA) of 0.40% increased by only 0.05% (600 MPa) and 0.10% (300 MPa) at week 8, compared to an increase of 0.30% in untreated controls. No substantial differences were observed in pH or TA between pressure-treated and untreated milk samples at pH 4.30 or 6.50. The rate of proteolysis in milk samples at pH values of 5.20 and 6.50 during storage was significantly reduced by treatment at 600 MPa. Treatment at 600 MPa also reduced the viable counts of both Candida yeast species to below the detection limit (1 CFU mL⁻¹) at all pH levels for the entire storage period. However, samples treated at 300 MPa showed recovery of C. lipolytica from week 3 onwards, reaching 10⁶–10⁷ CFU mL⁻¹ by week 8. In contrast, C. zeylanoides did not show any recovery in any of the pressure-treated samples during storage.     

Immobilization of pepsin on chitosan beads

In this study, chitosan beads were prepared by using a cross-linking agent and the resulting beads were employed in immobilization process. Studies on free and immobilized pepsin systems for determination of optimum temperature, optimum pH, thermal stability, pH stability, operational stability, storage stability and kinetic parameters were carried out. The optimum temperature interval for free pepsin and immobilized pepsin were 30–40 and 40–50 °C, respectively. Free and immobilized pepsin showed higher activity at pH 2.0–4.0. Apparent K_m = 12.0 g L⁻¹ haemoglobin (1.56 mM tyrosine) and V_max = 5220 μmol (mg protein min)⁻¹ values were obtained for free pepsin, while apparent K_m = 20.0 g L⁻¹ haemoglobin (2.16 mM tyrosine) and V_max = 2780 μmol (mg protein min)⁻¹ values were obtained for immobilized pepsin. Thermal stability and storage stability of immobilized pepsin were higher than that of free pepsin. Milk clotting activity was used for evaluation of the applicability of pepsin immobilization to industrial process. Optimum milk clotting temperature was found as 40 °C for free pepsin and 50 °C for immobilized pepsin.