Post-processing of fermented milk to stirred products: Reviewing the effects on gel structure

BackgroundThe structure of stirred fermented milk products such as yoghurt, Greek-style yoghurt and fresh cheese is an essential criterion for consumer acceptance. It is determined by various factors ranging from the milk base composition to starter culture to mechanical forces occurring during filling. The effect of upstream processing parameters and fermentation conditions on the structure of stirred fermented milk gels have been extensively studied and reviewed by different authors.Scope and approachThis review examines the effect of mechanical stresses in different unit operations downstream of fermentation, or post-processing, on the milk coagulum, often referred to as a microgel suspension. The process steps can be indispensable, e.g. pumping, cooling and filling, and concentration for concentrated milk products, or optional, e.g. mechanical or thermal treatment. They are evaluated with regards to their impact on the structure of microgel suspensions. In-situ laboratory scale experiments in rheometers provide an insight into structural changes of milk gels due to shearing.Key findings and conclusionsThe structural properties, including rheological parameters, of the microgel suspension depend on the cumulative effect of the type, magnitude and duration of the mechanical stresses occurring in the different post-processing unit operations. The latter can be optimised to achieve the desired gel properties and save raw material, mainly protein and stabilisers. It can also be adjusted for low viscosity at high protein contents. Thus, the efficiency of the process can be improved by considering the design of the process downstream of fermentation.     

Tailoring cream by modifying the composition of the fat and interfacial proteins to modulate stirred milk gel texture

The formulation-structure-texture relationship in stirred emulsion-filled food gels has rarely been analysed, let alone in realistic conditions. By studying thermal (calorimetry), structural (laser diffraction, confocal microscopy and mathematical morphology analysis) and textural (rheology and tribology) properties, this work advanced the understanding of this relationship in stirred gels made entirely from milk ingredients. Indeed, tailoring the fat composition (AMF, olein or stearin fractions) and interfacial proteins (native or heat-aggregated WPI) in cream resulted in different properties. Crystallisation of the fat droplets and probably their interactions (aggregation or partial coalescence), pore size, microgel size and the coarseness of the protein network in stirred milk gels were all modified by the cream formulation. The changes in properties led to different textures and lubrication behaviours of the stirred milk gels. The highlighted relationships between formulation, structure and texture are recapitulated in a concluding diagram.     

Fermented milk obtained with kefir grains as an ingredient in breadmaking

The objectives of this research were to study the effect of the addition of lyophilised kefir milk to premixes for household production of bread and evaluate the quality attributes of them. Four lyophilised samples were obtained from the followings: skim milk, acidified skim milk, fermented skim milk and neutralised fermented skim milk. Breads were prepared with commercial wheat flour, lyophilised milk samples and yeast through a straight dough process. Quality was assessed through loaf volume, crumb porosity and moisture, crumb texture and crust colour. Changes in texture and starch recrystallisation by X‐ray diffractometry were determined after 1 and 3 days of storage at room temperature. Breads with acidified milks showed the highest specific volumes and crumbs with the best texture properties. Crystallinity in bread with fermented milks was higher than for skim milk sample. This would indicate that there would be a certain effect of the type of milk processing on the promotion of starch retrogradation.     

Major advances in concentrated and dry milk products, cheese, and milk fat-based spreads

Advances in dairy foods and dairy foods processing since 1981 have influenced consumers and processors of dairy products. Consumer benefits include dairy products with enhanced nutrition and product functionality for specific applications. Processors convert raw milk to finished product with improved efficiencies and have developed processing technologies to improve traditional products and to introduce new products for expanding the dairy foods market. Membrane processing evolved from a laboratory technique to a major industrial process for milk and whey processing. Ultra-filtration and reverse osmosis have been used extensively in fractionation of milk and whey components. Advances in cheese manufacturing methods have included mechanization of the making process. Membrane processing has allowed uniform composition of the cheese milk and starter cultures have become more predictable. Cheese vats have become larger and enclosed as well as computer controlled. Researchers have learned to control many of the functional properties of cheese by understanding the role of fat and calcium distribution, as bound or unbound, in the cheese matrix. Processed cheese (cheese, foods, spreads, and products) maintain their importance in the industry as many product types can be produced to meet market needs and provide stable products for an extended shelf life. Cheese delivers concentrated nutrients of milk and bio-active peptides to consumers. The technologies for the production of concentrated and dried milk and whey products have not changed greatly in the last 25 yr. The size and efficiencies of the equipment have increased. Use of reverse osmosis in place of vacuum condensing has been proposed. Modifying the fatty acid composition of milkfat to alter the nutritional and functional properties of dairy spread has been a focus of research in the last 2 decades. Conjugated linoleic acid, which can be increased in milkfat by alteration of the cow's diet, has been reported to have anticancer, anti-atherogenic, antidiabetic, and antiobesity effects for human health. Separating milk fat into fractions has been accomplished to provide specific fractions to improve butter spreadability, modulate chocolate meltability, and provide texture for low-fat cheeses.     

Characterization of probiotic bacteria involved in fermented milk processing enriched with folic acid

Yogurt products fermented with probiotic bacteria are a consumer trend and a challenge for functional food development. So far, limited research has focused on the behavior of the various probiotic strains used in milk fermentation. In the present study, we characterized folic acid production and the sensory and textural characteristics of yogurt products fermented with probiotic bacteria. Yogurt fermented with Lactobacillus plantarum had improved nutrient content and sensory and textural characteristics, but the presence of L. plantarum significantly impaired the growth and survival of Lactobacillus delbrueckii ssp. bulgaricus during refrigerated storage. Overall, L. plantarum was a good candidate for probiotic yogurt fermentation; further studies are needed to understand the major metabolite path of lactic acid bacteria in complex fermentation.     

Potential of functional strains, isolated from traditional Maasai milk, as starters for the production of fermented milks

The purpose of this research was the evaluation of technological features and of the ability of functional LAB strains with desirable sensory characteristics, to produce fermented milk. Eight strains of Lactobacillus plantarum, Lactobacillus acidophilus, Lactobacillus paracasei and Lactococcus lactis, isolated from Maasai traditional fermented milk in Kenya and previously tested for their probiotic properties, were selected for this investigation. Technological features such as growth kinetics in fresh heat-treated whole milk medium and survival in the final product during storage at 4 degrees C, were studied. The strains Lb. acidophilus BFE 6,059, Lb. paracasei BFE 5,264 and Lc. lactis BFE 6,049 showed the best potential and were thus selected for use as starter cultures in further trials with the objective to improve their technological performance and to optimise the sensory features of fermented milk obtained. The effects of fat (F), non-fat milk solids (S) and fermentation temperature (T), modulated according to a Central Composite Design, on fermentation rates and viability losses during refrigerated storage of the chosen starters, and on product texture parameters, were studied. From the data analysis, it was possible to select optimum conditions for enhancing positive sensory traits of final products and for improving the survival of these potentially probiotic cultures.     

Constituents of nutritional relevance in fermented milk products commercialised in Italy

Fermented milk products represent an increasing share of the dairy products consumed in Italy. The nutritional value of these products is related to the milk utilised and to the eventual presence of other ingredients (milk powder, sugar, fruit puree and fruit extracts), whereas the microrganisms used can affect texture and organoleptic characteristics. In this paper constituents of nutritional relevance such as protein, fat, total carbohydrate, amino acids, minerals, vitamin A, vitamin E and cholesterol, have been evaluated in yoghurts, fermented milks (plain and with essences) and Quark cheeses (plain and with fruits). This study confirms the high nutritional quality of fermented milks and stresses the role of non-milk ingredients in modifying and, sometimes, improving the dietary contribution of these products.     

EFFECT OF HIGH HYDROSTATIC PRESSURE PROCESSING ON RHEOLOGICAL AND TEXTURAL PROPERTIES OF PROBIOTIC LOW-FAT YOGURT FERMENTED BY DIFFERENT STARTER CULTURES

The effect of milk processing on rheological and textural properties of probiotic low‐fat yogurt (fermented by two different starter cultures) was studied. Skim milk fortified with skim milk powder was subjected to three treatments: (1) thermal treatment at 85C for 30 min; (2) high hydrostatic pressure (HHP) at 676 MPa for 5 min; and (3) combined treatments of HHP (676 MPa for 5 min) and heat (85C for 30 min). The processed milk was fermented using two different starter cultures containing Streptococcus thermophilus, Lactobacillus delbrueckii ssp. bulgaricus, Lactobacillus acidophilus and Bifidobacterium longum at inoculation rates of 0.1 and 0.2%. Rheological parameters were determined and a texture profile analysis was carried out. Yogurts presented different rheological behaviors according to the treatment used, which could be attributed to structural phenomena. The combined HHP and heat treatment of milks resulted in yogurt gels with higher consistency index values than gels obtained from thermally treated milk. The type of starter culture and inoculation rate, providing different fermentation pathways, also affected the consistency index and textural properties significantly. The combined HHP and heat treatment of milks before fermentation, and an inoculation rate of 0.1% (for both cultures), led to desirable rheological and textural properties in yogurt, which presented a creamy and thick consistency that does not require the addition of stabilizers.     

Innovative food processing technologies on the transglutaminase functionality in protein-based food products: Trends,opportunities and drawbacks

BackgroundConsumption of protein-based food products has a key role in the improvement of human health. The crosslinking agent microbial transglutaminase (mTGase) is an effective and promising tool to modify animal proteins used in the food industry. Improvement in the gelation process, physicochemical and textural quality, and consumer's demand of protein-based food products could be attained by combining mTGase and some non-conventional food processing technologies.Scope and approachNew perspectives and key areas for future research in the development of high-quality food proteins and protein-based products as a function of interaction effect of mTGase and some new processing techniques (e.g. high pressure processing (HPP), ultrasound, microwave (MW) and ultraviolet (UV) irradiation) are reviewed. The effect of conventional thermal and emerging processing methods on the mTGase crosslinking activity and protein gel functionality are also compared.Key findings and conclusionsThe crosslinking density and functional properties of protein gels can be strongly promoted by the synergistic action of mTGase and innovative processing methods. Compared to the conventional heating, HPP with further increase of mTGase affinity to proteins can result in products with better physicochemical quality and more complex and firmer gel structure. The yield, water holding capacity, surface hydrophobicity, strength, and viscoelastic characteristics of mTGase-catalyzed protein gels can be significantly increased by ultrasonication treatments. mTGase-crosslinked hydrogels subjected to high-intensity ultrasonic pretreatment have potential to be used as delivery vehicles for a wide spectrum of bioactive compounds. The application of MW and UV light can substantially improve the surface, textural and structural features of gels generated by mTGase-technology.     

Kinetic modelling of textural changes in ready-to-eat breakfast cereals during soaking in semi-skimmed milk

Summary Ready‐to‐eat breakfast cereals immersed in milk undergo undesirable changes in texture because of sudden moisture uptake. The textural changes are ascribable to a plasticizing effect of water, which modifies the mechanical strength of products by softening the starch/protein matrix. In this work, some textural parameters of different cereal flakes were derived from the force–displacement curves monitored during 300 s of immersion in milk. Hardness loss, deformability increment and changes in the force–displacement curve profile were calculated and plotted against soaking time. The application of a sugar coating process to a model cereal flake increased the initial product hardness and improved the preservation of the textural parameters during immersion. The Peleg model closely fitted the experimental data, with regression coefficients from 0.967 to 0.999.     

Structural and textural characteristics of reduced-fat cheese-like products made from W1/O/W2 emulsions and skim milk

The chemical composition, yield, structural arrangement, instrumental textural characteristics, and preference sensory evaluation of reduced-fat cheese-like products manufactured from skim milk and different water-in-oil-in-water (W₁/O/W₂) emulsions were determined. A full-fat white fresh cheese (WFC) was prepared from milk containing 27 g of milk-fat L⁻¹, and five reduced-fat white fresh cheese-like products (EC) were made from skim milk added with 25 g of multiple emulsions L⁻¹ containing canola oil and stabilized/emulsified by amidated low-methoxyl pectin (LMP), carboxymethylcellulose (CMC), gum Arabic (GA), and blends of GA-CMC or GA-LMP. The chemical composition, yield, structural arrangement and texture of the cheese-like products were affected by the biopolymers used as emulsifying/stabilizing agents of the multiple emulsions. CMC produced an EC with similar textural behaviour than the WFC cheese. GA contributed to a higher yield and fat content in the EC cheese in comparison with CMC and LMP cheese. GA and LMP contributed to increased values of hardness and chewiness of the EC cheese. The cheese made with multiple emulsions incorporating GA and LMP emulated best the textural characteristics of the WFC cheese. All of the EC cheese showed marked differences in microstructure.     

发酵乳的最新进展

综述了发酵乳在科学、枝术和市场方面最新进展。介绍了在分子水平和菌株水平的乳酸菌和益生菌的菌种鉴定、乳酸菌和新益生菌株的安全性、乳酸菌胞外多糖、减少发酵乳生产过程中黏度降低的因素，以及乳酸菌和益生菌的功能保健作用。根据近年来消费者对酸奶的温和味道、香气和质地更高的要求，叙述了酸奶和其他发酵乳新产品开发的进展。     

Laccase-catalysed cross-linking of a yoghurt-like model system made from skimmed milk with added food-grade mediators

Lactic acid fermented foods, such as yoghurt, suffer from structural losses in post-processing steps due to their shear sensitivity. The acidic pH optimum of fungal laccases offers potential to compensate for these losses and enhance the textural characteristics by cross-linking of milk proteins. Physical properties of skimmed milk yoghurt, as measured by dynamic oscillation rheology, changed upon addition of laccase alone or in combination with food-grade mediators. Among seven mediators examined, vanillin, vanillic acid, and caffeic acid were found to be most efficient. Variation of enzyme activity, mediator type and concentration showed best viscoelastic properties for the combination of 3 U laccase per gram yoghurt and 5 mmol L⁻¹ caffeic acid. Evaluation of the loss factor, tan δ, showed increased elastic properties. SDS-PAGE showed a changed pattern of protein bands after the treatments. Overall, the combination of laccase with common food components provides an alternative to improve yoghurt texture.     

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.     

Carbohydrate-based fat replacers in the modification of the rheological, textural and sensory quality of yoghurt: comparative study of the utilisation of barley beta-glucan, guar gum and inulin

Barley beta‐glucan, partially hydrolysed guar gum and inulin were used in the processing of low‐fat yoghurts. The possible beneficial effects of carbohydrate fat replacers on the rheological, textural and sensory quality of low‐fat yoghurt‐based products were determined. Comparisons were made between the sample yoghurts made from a low‐fat milk base, and full‐fat and low‐fat yoghurt controls. The inclusion of the carbohydrate components reduced product syneresis and improved the texture and rheological properties of the low‐fat‐based products so that their quality characteristics were similar to yoghurt made with full‐fat milk. Both the type and also the amount of carbohydrate component altered product characteristics. Beta‐glucan addition at low level (0.5%) was effective in improving serum retention of the yoghurt and its viscoelastic nature (G′, G′ and tan δ). In contrast, higher levels (above 2%) of inulin and guar gum were needed to exert significant improvements in the textural characteristics of yoghurt. Sensory analysis conducted on the samples illustrated that the inclusion of carbohydrate‐based fat replacers could be successfully utilised to mimic full‐fat products.     

蒙古族传统乳制品奶皮子和奶豆腐的工艺研究及营养价值分析

奶皮子和奶豆腐是蒙古族的两种传统特色乳制品。研究了奶皮子和奶豆腐的加工工艺,并对两种制品的营养成分进行分析,为进一步研制开发民族乳制品提供一定的理论依据。     

Effect of milk supplementation and culture composition on acidification, textural properties and microbiological stability of fermented milks containing probiotic bacteria

In the present work, the combined effect of milk supplementation and culture composition on acidification, textural properties, and microbiological stability of fermented milks containing probiotic bacteria, was studied. Three powders (whey, casein hydrolysate, and milk proteins) were tested as supplementation. Two strains of probiotic bacteria, Lactobacillus acidophilus (LA5) and Lactobacillus rhamnosus (LC35), were used in pure culture, and in mixed culture with Streptococcus thermophilus (ST7). Acidifying activity was enhanced with mixed cultures, compared to pure cultures resulting in a shorter time to reach pH 4.5. Acidifying activity was greatly improved with casein hydrolysate, with a reduction of the fermentation time by about 55% by comparison with the other supplementations. The stability of probiotic bacteria was weakly affected by milk supplementation and culture composition. However, pure cultures were more stable than mixed cultures. The texture of the fermented products was not dependent on culture composition, but strongly dependent on milk supplementation. Sweet whey supplementation gave products with lower firmness and viscoelasticity than products supplemented with casein hydrolysate or milk proteins (decrease by 70%). It was observed that all products containing probiotic counts over 2.2×10⁷ CFU mL⁻¹ are suitable for the development of a lactic beverage containing probiotics.     

开菲尔榛子发酵乳的工艺研究

利用传统酸牛乳酒(Kefir)的发酵剂—开菲尔粒(Kefir Grains)制作的发酵剂,对榛子乳与牛乳的混合原料进行发酵,采用L16(45)正交试验设计筛选制备开菲尔榛子乳的最佳发酵条件。结果表明:当榛子乳与鲜牛乳混合比例8∶2,接种量为3%,发酵温度25℃,加糖量10%,发酵时间14 h时,所得的产品的酸度为83°T,乙醇体积分数为0.26%,风味柔和,口感独特。     

Major advances in fresh milk and milk products: fluid milk products and frozen desserts

Major technological advances in the fluid milk processing industry in the last 25 yr include significant improvements in all the unit operations of separation, standardization, pasteurization, homogenization, and packaging. Many advancements have been directed toward production capacity, automation, and hygienic operation. Extended shelf-life milks are produced by high heat treatment, sometimes coupled with microfiltration or centrifugation. Other nonthermal methods have also been investigated. Flavored milk beverages have increased in popularity, as have milk beverages packaged in single-service, closeable plastic containers. Likewise, the frozen dairy processing industry has seen the development of large-capacity, automated processing equipment for a wide range of products designed to gain market share. Significant advancements in product quality have been made, many of these arising from improved knowledge of the functional properties of ingredients and their impact on structure and texture. Incidents of foodborne disease associated with dairy products continue to occur, necessitating even greater diligence in the control of pathogen transmission. Analytical techniques for the rapid detection of specific types of microorganisms have been developed and greatly improved during this time. Despite tremendous technological advancements for processors and a greater diversity of products for consumers, per capita consumption of fluid milk has declined and consumption of frozen dairy desserts has been steady during this 25-yr period.