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.     

Microstructure and texture of khoa and gulabjamun made from cows' milk: Heat-induced changes during processing and frying

Scanning and transmission electron microscopy was carried out to study heat-induced structural changes that occur during the processing of khoa (a heat-desiccated Indian milk product) and gulabjamun (a product obtained by frying khoa, admixed with starch, in clarified butterfat and subsequently soaked in 60% sugar syrup). Constant boiling of milk during khoa manufacture led to the formation of casein-whey protein complexes, which coalesced gradually on the progress of boiling, forming a fuzzy-agglomerated mass and finally precipitated as heat-induced milk gels, joined together by thick protein bridges. Further heat desiccation of this gel resulted in the compaction of the protein agglomerates with reduction in void spaces and fat globules interspersed in between (ie khoa). Frying of khoa in clarified butterfat resulted in the enlargement of the voids, producing a loose matrix having starch particles interlinked loosely with the agglomerated protein bodies and the clumped fat globules cemented in it (ie gulabjamun). The laboratory and market gulabjamun had significant structural similarities in the fat and protein moieties but differed markedly in the textural values, probably due to the differences in the chemical composition and typical structural manifestations of the starchy materials in the market gulabjamun. Composition and texture can be explained by the microstructural attributes for both khoa and gulabjamun.     

Characterisation and comparison of khoa prepared from camel milk with that from cow and buffalo milk

Khoa, a heat‐concentrated milk product, is used as a base material for the manufacture of many popular sweets. The comparison was made between various chemical compositions and characteristics of the khoa prepared from the camel milk with that prepared from the cow and the buffalo milk samples. The khoa prepared from the camel milk had the higher moisture, ash, acidity, soluble nitrogen, free fatty acids and peroxide value, but lower in fat, protein and lactose contents than that prepared from the cow and buffalo milk samples. There was no significant (P > 0.05) difference in 5‐hydroxymethyl furfural between khoa samples prepared from the three milks.     

Effect of synthetic antioxidants on storage stability of Khoa – a semi-solid concentrated milk product

Effect of synthetic antioxidants on storage stability of freshly prepared khoa was evaluated. Free fatty acids (FFA), peroxide value (POV) and iodine values (IV) were used to assess the development of rancidity during 30 days of storage of khoa at 25 and 45 °C. Butylated hydroxy anisole (BHA) and butylated hydroxy toluene (BHT) were added to freshly prepared khoa to extend the storage life. After 30 days of storage at 45 °C, freshly prepared khoa containing 200 ppm of BHA and BHT showed lower values of FFA (0.066%, 0.058%) and POVs (23.0, 21.0 meq/kg) than the control samples (FFA 0.320%, POV 127 meq/kg). Iodine values of khoa sample containing 200 ppm of BHA and BHT were 67 and 69 after 30 days storage at 45 °C. However, iodine value of a khoa sample without antioxidant (control) after 30 days of storage at 45 °C was 30. Similarly, khoa samples treated with 100 ppm of BHA, along with 100 ppm of BHT, showed FFA value (19.0%), POV (0.049 meq/kg) and iodine value (72) after storage for 30 days at 45 °C. These results illustrate that synthetic antioxidants inhibited the development of rancidity during storage of khoa. Therefore, storage life of khoa can easily be extended for 30 days by the addition of BHA and BHT.     

Electrophoretic characterization of protein interactions suggesting limited feasibility of accelerated shelf-life testing of ultra-high temperature milk

Accelerated shelf-life testing is applied to a variety of products to estimate keeping quality over a short period of time. The industry has not been successful in applying this approach to ultra-high temperature (UHT) milk because of chemical and physical changes in the milk proteins that take place during processing and storage. We investigated these protein changes, applying accelerated shelf-life principles to UHT milk samples with different fat levels and using native- and sodium dodecyl sulfate-PAGE. Samples of UHT skim and whole milk were stored at 20, 30, 40, and 50°C for 28 d. Irrespective of fat content, UHT treatment had a similar effect on the electrophoretic patterns of milk proteins. At the start of testing, proteins were bonded mainly through disulfide and noncovalent interactions. However, storage at and above 30°C enhanced protein aggregation via covalent interactions. The extent of aggregation appeared to be influenced by fat content; whole milk contained more fat than skim milk, implying aggregation via melted or oxidized fat, or both. Based on reduction in loss in absolute quantity of individual proteins, covalent crosslinking in whole milk was facilitated mainly by products of lipid oxidation and increased access to caseins for crosslinking reactions. Maillard and dehydroalanine products were the main contributors involved in protein changes in skim milk. Protein crosslinking appeared to follow a different pathway at higher temperatures (≥40°C) than at lower temperatures, making it very difficult to extrapolate these changes to protein interactions at lower temperatures.     

Comparative study on the characteristics and oxidation stability of commercial milk powder during storage

The quality of milk powder can decrease during storage. In this study, the characteristics of 12 kinds of commercial milk powder from China were investigated. Changes in various indicators were tracked to comprehensively estimate the oxidation stability of different commercial milk powders. The components of the commercial milk powder were different. The percentages of milk fat, protein, and carbohydrates ranged from 9.8 to 28.5 g/100 g, 15.0 to 24.0 g/100 g, and 32.0 to 67.5 g/100 g, respectively. The water activities ranged from 0.2394 to 0.5286. The diameters of the milk fat globules in different commercial milk powder ranged from 13.99 to 41.09 nm. At the same time, the peroxide value of the control sample was low (≤0.14 mEq/kg). After 3 mo of storage, the peroxide values of some of the commercial milk powder increased significantly. The changes in the thiobarbituric acid values during storage did not follow a common trend. The contents of free fat in the different control samples were 0.21 to 1.67 g/100 g, and these values did not increase during storage. After 3 mo of storage, the hydroxymethyl furfural values and b color values of the different commercial milk powder reached their highest levels. The concentrations of typical oxidized flavor compounds in different commercial milk powder increased greatly with prolonged storage time. The level of hexanal was the highest, and the contents in all the samples ranged from 28.56 to 4,071.28 μg/kg after 6 mo of storage and from 5.91 to 6,281.37 μg/kg after 12 mo of storage. Free radicals were found in some of the stored milk powder, and these were shown as single peaks or multiple peaks. The ratios of the peak areas and masses reached 12.42 × 10⁶ to 14.26 × 10⁸. However, the presence of free radicals in the commercial milk powder was not consistent. The water activities and diameters of the fat globules in the commercial milk powder were highly correlated with their oxidation stabilities during storage.     

Amylose–Potassium Oleate Inclusion Complex in Plain Set‐Style Yogurt

Health and wellness aspirations of U.S. consumers continue to drive the demand for lower fat from inherently beneficial foods such as yogurt. Removing fat from yogurt negatively affects the gel strength, texture, syneresis, and storage of yogurt. Amylose–potassium oleate inclusion complexes (AIC) were used to replace skim milk solids to improve the quality of nonfat yogurt. The effect of AIC on fermentation of yogurt mix and strength of yogurt gel was studied and compared to full‐fat samples. Texture, storage modulus, and syneresis of yogurt were observed over 4 weeks of storage at 4 °C. Yogurt mixes having the skim milk solids partially replaced by AIC fermented at a similar rate as yogurt samples with no milk solids replaced and full‐fat milk. Initial viscosity was higher for yogurt mixes with AIC. The presence of 3% AIC strengthened the yogurt gel as indicated by texture and rheology measurements. Yogurt samples with 3% AIC maintained the gel strength during storage and resulted in low syneresis after storage for 4 wk.     

Effect of storage and separation of milk at udder quarter level on milk composition, proteolysis, and coagulation properties in relation to somatic cell count

Coagulation properties of milk are altered by elevated somatic cell count (SCC), partly due to increased proteolytic and lipolytic activity in the milk and, thereby, degradation of protein and fat during storage. Milk is commonly stored on the farm at cooling conditions for up to 2 d before transport to the dairy for processing. This study evaluated the effects of storage on milk with altered composition due to high SCC and the effects of exclusion of milk from individual udder quarters with high SCC on milk composition, proteolysis, and coagulation properties. Udder-quarter milk and cow-composite milk samples from 13 cows having at least 1 quarter with SCC above 100,000 cells/mL were collected on 1 occasion. In addition, commingled milk from only healthy quarters (<100,000 cells/mL) of each cow was collected, representing a cow sample where milk with elevated SCC was excluded. The milk samples were analyzed for total protein content; protein content in the whey fraction; casein, fat, and lactose contents; SCC; proteolysis; curd yield; coagulation time; and total bacterial count, on the day of sampling and after 2 and 5 d of storage at +4°C. In addition to SCC, duration of storage and total bacterial count had an effect on milk quality. The content of total protein, fat and protein contents in the whey fraction, and curd yield were found to have different storage characteristics depending on the level of SCC at udder-quarter level. The exclusion of milk from udder quarters with elevated SCC decreased the content of total protein and protein content in the whey fraction and increased the content of lactose at cow level. However, the effect of separating milk at udder-quarter level needs to be further studied at bulk tank level to evaluate the effect on overall total milk quality.     

Injury and death of frozen Listeria monocytogenes as affected by glycerol and milk components

A cell suspension of Listeria monocytogenes strain Scott A in phosphate buffer solution alone or with added glycerol, milk fat, lactose, or casein was frozen and stored at -18 degrees C. At suitable intervals, samples of cell suspensions were thawed at 35 degrees C and plated on suitable media to distinguish between surviving injured and noninjured cells of L. monocytogenes. Glycerol (2 or 4%) protected L. monocytogenes from death and injury during frozen storage for up to 6 mo; however, when 2% glycerol was present, 30 min of frozen storage had to elapse after completion of freezing before protection against death was evident. During short-term (2 wk or less) frozen storage, lactose, milk fat, and casein, each at 2%, provided better protection to L. monocytogenes than did 2% glycerol. During long-term frozen storage, milk components, each at 2%, protected L. monocytogenes against death and injury, but less than that provided by glycerol. Protection by lactose and milk fat against death during frozen storage was observed during 4 wk and against injury during 5 mo and 4 wk of frozen storage, respectively. Protection by casein against death and injury occurred during frozen storage for up to 6 mo. Salts that simulate milk ultrafiltrate provided almost no protection to L. monocytogenes during freezing and frozen storage. Increasing the concentration of milk fat from 2 to 4% resulted in almost no change in death of L. monocytogenes, but in a decrease in injury only during the first 24 h of frozen storage.(ABSTRACT TRUNCATED AT 250 WORDS)     

Development of the milk fat microstructure during the manufacture and ripening of Emmental cheese observed by confocal laser scanning microscopy

Changes in the physico-chemical properties and microstructure of milk fat globules were investigated during the manufacture and ripening of Emmental cheese. The measurement of fat globule size and apparent zeta-potential showed that they were slightly affected during cheese milk preparation, i.e. storage of cheese milk overnight at 4 °C and pasteurisation. After rennet-induced coagulation and heating of curd grains, coalescence caused the formation of large fat globules (i.e.>10 μm). The structure of fat in Emmental cheese was characterised in situ using confocal laser scanning microscopy (CLSM). The rennet-induced coagulation lead to the formation of a continuous network of casein strands in which fat globules of various sizes were entrapped. Heating of curd grains induced the formation of fat globule aggregates. Pressing of the curd grains resulted in the greatest disruption of milk fat globules, their coalescence, the formation of non-globular fat (free fat) and the release of the milk fat globule membrane (MFGM) material. This study showed that milk fat exists in three main forms in ripened Emmental cheese: (i) small fat globules enveloped by the MFGM; (ii) aggregates of partially disrupted fat globules and (iii) free fat, resulting from the disruption of the MFGM and allowing free triacylglycerols to fill voids in the protein matrix. The curd grain junctions formed in Emmental cheese were also characterised using CLSM: they are compact structures, rich in protein and devoid of fat globules.     

Effect of Milk Fat on the Stability of Vitamin A in Ultra-High Temperature Milk

The stability of vitamin A in ultra-high temperature milks with .15, 2.92, 6.16, and 9.7% fat during storage at 26°C was studied over 3 wk. Milks were fortified with synthetic retinyl palmitate to a final concentration of approximately 120 μg retinol equivalent/I 00 ml milk. The four milk samples were ultra-high temperature processed and packaged in 100 ml sterile milk dilution bottles and stored in the dark at 26°C for 3 wk. Vitamin A concentrations decreased rapidly during the first 2 wk of storage then stabilized. Degradation rates during the first 2 wk were linear and varied inversely with the fat content in the milk (the more fat, the slower the rate of degradation). Final vitamin A concentrations at the end of 3 wk of storage were higher in milk with high fat and closely corresponded to the native vitamin A concentrations present in milk prior to fortification. The results indicate a possible protective effect due to fat or a difference in stability between native and synthetic vitamin A.     

Fat content increases the lethality of ultra-high-pressure homogenization on Listeria monocytogenes in milk

Listeria monocytogenes CCUG 15526 was inoculated at a concentration of approximately 7.0 log₁₀ cfu/mL in milk samples with 0.3, 3.6, 10, and 15% fat contents. Milk samples with 0.3 and 3.6% fat content were also inoculated with a lower load of approximately 3.0 log₁₀ cfu/mL. Inoculated milk samples were subjected to a single cycle of ultra-high-pressure homogenization (UHPH) treatment at 200, 300, and 400 MPa. Microbiological analyses were performed 2 h after the UHPH treatments and after 5, 8, and 15 d of storage at 4°C. Maximum lethality values were observed in samples treated at 400 MPa with 15 and 10% fat (7.95 and 7.46 log₁₀ cfu/mL), respectively. However, in skimmed and 3.6% fat milk samples, complete inactivation was not achieved and, during the subsequent 15 d of storage at 4°C, L. monocytogenes was able to recover and replicate until achieving initial counts. In milk samples with 10 and 15% fat, L. monocytogenes recovered to the level of initial counts only in the milk samples treated at 200 MPa but not in the milk samples treated at 300 and 400 MPa. When the load of L. monocytogenes was approximately 3.0 log₁₀ cfu/mL in milk samples with 0.3 and 3.6% fat, complete inactivation was not achieved and L. monocytogenes was able to recover and grow during the subsequent cold storage. Fat content increased the maximum temperature reached during UHPH treatment; this could have contributed to the lethal effect achieved, but the amount of fat of the milk had a stronger effect than the temperature on obtaining a higher death rate of L. monocytogenes.     

Reduction of pasteurization temperature leads to lower bacterial outgrowth in pasteurized fluid milk during refrigerated storage: a case study

Bacterial numbers over refrigerated shelf-life were enumerated in high-temperature, short-time (HTST) commercially pasteurized fluid milk for 15 mo before and 15 mo after reducing pasteurization temperature from 79.4°C (175°F) [corrected] to 76.1°C (169°F). Total bacterial counts were measured in whole fat, 2% fat, and fat-free milk products on the day of processing as well as throughout refrigerated storage (6°C) at 7, 14, and 21 d postprocessing. Mean total bacterial counts were significantly lower immediately after processing as well as at 21 d postprocessing in samples pasteurized at 76.1°C versus samples pasteurized at 79.4°C. In addition to mean total bacterial counts, changes in bacterial numbers over time (i.e., bacterial growth) were analyzed and were lower during refrigerated storage of products pasteurized at the lower temperature. Lowering the pasteurization temperature for unflavored fluid milk processed in a commercial processing facility significantly reduced bacterial growth during refrigerated storage.     

Interactions of fat globule surface proteins during concentration of whole milk in a pilot-scale multiple-effect evaporator

The changes in milk fat globules and fat globule surface proteins during concentration of whole milk using a pilot-scale multiple-effect evaporator were examined. The effects of heat treatment of milk at 95 degrees C for 20 s, prior to evaporation, on fat globule size and the milk fat globule membrane (MFGM) proteins were also determined. In both non-preheated and preheated whole milk, the size of milk fat globules decreased while the amount of total surface proteins at the fat globules increased as the milk passed through each effect of the evaporator. In non-preheated samples, the amount of caseins at the surface of fat globules increased markedly during evaporation with a relatively small increase in whey proteins. In preheated samples, both caseins and whey proteins were observed at the surface of fat globules and the amounts of these proteins increased during subsequent steps of evaporation. The major original MFGM proteins, xanthine oxidase, butyrophilin, PAS 6 and PAS 7, did not change during evaporation, however, PAS 6 and PAS 7 decreased during preheating. These results indicate that the proteins from the skim milk were adsorbed onto the fat globule surface when the milk fat globules were disrupted during evaporation.     

Changes in free fatty acid composition during storage of whole milk powder

Changes in fat matter due to the microbial thermostable lipases are among the most significant deteriorations influencing the shelf life of whole milk powder. As a consequence, free fatty acids are produced, being the short-chain acids mainly responsible for the rancidity flavour. The aim of this work was to evaluate changes in different fatty acid concentrations during the storage of whole milk powder produced in two different seasons. Samples of whole milk powder produced in winter and summer and packed under inert atmosphere were used in the study. The samples were stored at 21°C and 40°C. Samples stored at 21°C were evaluated every 3 months, whereas samples stored at 40°C were evaluated monthly by free fatty acids profile and sensory analysis. Data were processed by principal component analysis and anova . Results showed that changes in the free fatty acids of milk powders were correlated with the season of manufacture. Temperature and storage time had little influence on that profile.     

Effect of Heating and Homogenization on the Stability of Coconut Milk Emulsions

: The effects of homogenization and heat treatment on the colloidal stability of coconut milk were studied. Fresh coconut milk (15% to 17% fat, 1.5% to 2% protein) was extracted and stored at 30 °C before homogenization at 40/4 MPa (stage I/stage II). Both homogenized and non‐homogenized samples were heated at 50 °C, 60 °C, 70 °C, 80 °C, and 90 °C for 1 h. Homogenization reduced the size of the primary emulsion droplets from 10.9 to 3.0 μm, but increased the degree of flocculation, presumably via a bridging mechanism. This flocculation was also responsible for increased viscosity of the homogenized samples. Heating increased the degree of flocculation in both non‐homogenized and homogenized samples. A slight amount of coalescence was also observed after heating above 80 °C. All samples creamed after 24 h of storage, but the heated samples formed a larger cream layer, presumably because the flocculated droplets packed together less efficiently. Optical microscopy was used to confirm the combination of flocculation and creaming responsible for changes in coconut milk quality. The information obtained from this study provides a better understanding of the emulsion science important in controlling coconut milk functionality.     

原生态与人工添加二十二碳六烯酸羊乳制品贮藏期脂肪酸稳定性比较

通过饲喂奶山羊富含二十二碳六烯酸（docosahexaenoic acid，DHA）的微藻粉，获得原生态DHA羊乳（DHA含量为30 mg/100 g原料乳），然后将其制作成超高温瞬时灭菌（ultra-high temperature instantaneous sterilization，UHT）乳及全脂乳粉，同时设立人工添加富含DHA微胶囊粉的UHT乳及全脂乳粉作为对照组，在常温（25 ℃）和高温（37 ℃）下进行为期28 d的贮藏实验，研究原生态与人工添加DHA羊乳制品贮藏期脂肪酸稳定性。结果表明，与人工添加组相比，贮藏期间原生态UHT乳及全脂乳粉的DHA含量下降速率明显减缓，在UHT乳中，人工添加组乳制品DHA含量降低率在37 ℃下最高达（40.92±3.52）%（贮藏第28天），此时原生态组DHA降低率为（36.70±4.84）%。贮藏期间，原生态与人工添加DHA的UHT乳及全脂乳粉中多不饱和脂肪酸相对含量总体均下降，且与人工添加DHA的乳制品相比，原生态组中多不饱和脂肪酸相对含量更高，更易氧化生成碳链更短的脂肪酸。此外，随着贮藏期的延长，原生态DHA乳制品组中的油脂氧化指标过氧化值和酸价上升速率明显低于人工添加DHA乳制品组。综上，本实验可为制备富含DHA的天然奶制品提供理论参考。     

Supercritical CO2 Extraction for Total Fat Analysis of Food Products

Supercritical CO₂ extraction (SFE) is a promising alternative to solvent extraction for the measurement of total fat in food products. Total fat was extracted by SFE from samples of food products of different compositions and fat contents. Results using SFE were statistically compared with those using standard methods. This feasibility study showed very promising results. A suitable method was developed for analysis of full milk powder and dietetic milk powder and recoveries were 97.4 and 98.4% respectively. The content of the major fatty acids of fat extracted with SFE was similar to that of Röse-Gottlieb solvent extraction.     

Flow behavior studies of kefir type systems

Kefir type samples were prepared using two different commercial starter cultures and bovine full fat milk. The milk samples were fermented at either 30 or 35 °C. Separate series of samples to which 1% or 2% or 3% caseinates were added, prepared the same way. All samples were rheologically tested in stirred as well as in set form. Shear viscosity measurements showed that the apparent viscosities exhibited by the kefir samples were starter culture type, fermentation temperature and caseinate content dependent. Similar results were obtained from lubricated squeeze flow viscometry measurements. The experimental data was discussed in the light of possible mechanisms involved in the structure net work formation of the samples during their preparation.     

Development and application of confocal scanning laser microscopy methods for studying the distribution of fat and protein in selected dairy products.

Confocal scanning laser microscopy (CSLM) methods were developed to identify fat and protein in cheeses milk chocolate and milk powders. Various fluorescent probes were assessed for their ability to label fat or protein in selected food products in situ. Dual labelling of fat and protein was made possible by using mixtures of probes. Selected probes and probe mixtures were then used to study (a) structure development of Mozzarella cheese during manufacture and ripening, and (b)) the distribution of fat and protein in milk chocolate made with milk powders containing varying levels of free fat. Microstructural changes in the protein and fat phases of Mozzarella cheese were observed at each major step in processing. Aggregation of renneted micelles occurred during curd formation; this was followed by amalgamation of the para-casein into linear fibres during plasticization. Following storage, the protein phase of the Mozzarella became more continuous; entrapping and isolating fat globules. Chocolate made with a high free-fat spray-dried powder blend showed a homogeneous fat distribution, similar to that of chocolate made with roller-dried milk. Chocolate made with whole milk powder containing 10 g free fat/100 fat showed a non-homogeneous fat distribution with some fat occluded within milk protein particles. These differences in fat distribution were related to Casson yield value and Casson viscosity of the chocolates.