In vitro evaluation of the effect of yogurt acid whey fractions on iron bioavailability.

A side effect of the raised consumption of Greek yogurt is the generation of massive amounts of yogurt acid whey (YAW). The dairy industry has tried several methods for handling these quantities, which constitute an environmental problem. Although the protein content of YAW is relatively low, given the huge amounts of produced YAW, the final protein amount in the produced YAW should not be underestimated. Taking into consideration the increased interest for bioactive peptides and the increased demand for dietary proteins, combined with protein and peptides content of YAW, efforts should be made toward reintroducing the latter in the food supply chain. In this context and in view of the prevalent dietary iron deficiency problem, the objective of the present study was the investigation of YAW fractions' effect on Fe bioavailability. With this purpose, an in vitro digest approach, following the INFOGEST protocol, was coupled with the Caco2 cell model. To evaluate whether YAW digest fractions exert positive, negative or neutral effect on Fe bioavailability, they were compared with the ones derived from milk, a well-studied food in this context. Milk and YAW showed the same effectiveness on both Fe bioavailability and the expression of relative genes (DCYTB, DMT1, FPN1, and HEPH). Focusing further on YAW fractions, by comparison with their blank digest control counterparts, it resulted that YAW 3- to 10-kDa digests fraction had a superior effect over the 0- to 3-kDa fraction on Fe-uptake, which was accompanied by a similar effect on the expression of Fe metabolism-related genes (DCYTB, FPN1, and HEPH). Finally, although the 3- to 10-kDa fraction of bovine YAW digests resulted in a nonsignificant increased Fe uptake, compared with the ovine and caprine YAW, the expression of DCYTB and FPN1 genes underlined this difference by showing a similar pattern with statistically significant higher expression of bovine compared with ovine and bovine compared with both ovine and caprine, respectively. The present study deals with the novel concept that YAW may contain factors affecting Fe bioavailability. The results show that it does not exert any negative effect and support the extensive investigation for specific peptides with positive effect as well as that YAW proteins should be further assessed on the prospect that they can be used in human nutrition.

Production of nisin from Lactococcus lactis in acid-whey with nutrient supplementation.

The production of Nisin, an FDA-approved food preservative, was attempted by Lactococcus lactis subsp. lactis ATCC® 11454 using the underutilized milk industry effluent, acid-whey, as a substrate. Nisin production was further improved by studying the effect of supplementation of nutrients and non-nutritional parameters. The addition of yeast extract (6% w/v) as nitrogen source and sucrose (4% w/v) as carbon source were found to be suitable nutrients for the maximum nisin production. The changes in the medium pH due to lactic acid accumulation during batch fermentation and its influence on the production of nisin were analyzed in the optimized whey medium (OWM). The production characteristics in OWM were further compared with the nisin production in MRS media. The influence of nisin as an inducer for its own production was also studied and found that the addition of nisin at 0.22 mg/ml promote the nisin production. The analysis of consumption of various metal ions present in the OWM during the nisin production was also analyzed, and found that the copper ions are the most consumed ion. The highest nisin yield of 2.6 × 105 AU/mL was obtained with OWM.

Sonication as a pre-processing step to alter acid whey generation during Greek yoghurt manufacture.

We investigated sonication as a pre-processing step to reduce acid whey generation during Greek yoghurt manufacture. The generation of a large amount of acid whey during the production of Greek yoghurt is an ongoing problem in the dairy industry and many studies are currently focusing on reducing it. We focused on the use of ultrasonication as a novel approach to minimize the casein fraction in the acid whey stream and simultaneously improve the gel properties. Ultrasound applied before the fermentation modified the structural properties and bonding behaviours of milk proteins, and enhanced the retention of casein in the yoghurt gel after the fermentation and straining steps. Therefore, the use of low-frequency ultrasonication as a pre-processing step may have the potential to provide significant economic benefits to the Greek yoghurt manufacturing process. Moreover, it improved the nutritional and physicochemical properties compared to regular Greek yoghurts.

Metabolomics of acid whey derived from Greek yogurt.

Acid whey, a byproduct of Greek yogurt production, has little commercial value due to its low protein content and is also environmentally harmful when disposed of as waste. However, as a product of microbial fermentation, acid whey could be a rich source of beneficial metabolites associated with fermented foods. This study increases understanding of acid whey composition by providing a complete metabolomic profile of acid whey. Commercial and laboratory-made Greek yogurts, prepared with 3 different bacterial culture combinations, were evaluated. Samples of uncultured milk and cultured whey from each batch were analyzed. Ultra-high-performance liquid chromatography-tandem mass spectrometry metabolomics were used to separate and identify 477 metabolites. Compared with uncultured controls, acid whey from fermented yogurt showed decreases in some metabolites and increases in others, presumably due to the effects of microbial metabolism. Additional metabolites appeared in yogurt whey but not in the uncultured control. Therefore, the effect of microbial fermentation is complex, leading to increases or decreases in potentially bioactive bovine metabolites while generating new microbial compounds that may be beneficial. Metabolite production was significantly affected by combinations of culturing organisms and production location. Differences between laboratory-made and commercial samples could be caused by different starting ingredients, environmental factors, or both.

Invited review: Acid whey trends and health benefits.

In recent years, acid whey production has increased due to a growing demand for Greek yogurt and acid-coagulated cheeses. Acid whey is a dairy by-product for which the industry has long struggled to find a sustainable application. Bulk amounts of acid whey associated with the dairy industry have led to increasing research on ways to valorize it. Industry players are finding ways to use acid whey on-site with ultrafiltration techniques and biodigesters, to reduce transportation costs and provide energy for the facility. Academia has sought to further investigate practical uses and benefits of this by-product. Although modern research has shown many other possible applications for acid whey, no comprehensive review yet exists about its composition, utilization, and health benefits. In this review, the industrial trends, the applications and uses, and the potential health benefits associated with the consumption of acid whey are discussed. The proximal composition of acid whey is discussed in depth. In addition, the potential applications of acid whey, such as its use as a starting material in the production of fermented beverages, as growth medium for cultivation of lactic acid bacteria in replacement of commercial media, and as a substrate for the isolation of lactose and minerals, are reviewed. Finally, the potential health benefits of the major protein constituents of acid whey, bioactive phospholipids, and organic acids such as lactic acid are described. Acid whey has promising applications related to potential health benefits, ranging from antibacterial effects to cognitive development for babies to human gut health.

Engineering Yarrowia lipolytica for the utilization of acid whey.

Acid whey, a byproduct in cheese and yogurt production, demands high costs in disposal at large quantities. Nonetheless, it contains abundant sugars and nutrients that can potentially be utilized by microorganisms. Here we report a novel platform technology that converts acid whey into value-added products using Yarrowia lipolytica. Since wild type strains do not assimilate lactose, a major carbon source in whey, a secreted ß-galactosidase was introduced. Additionally, to accelerate galactose metabolism, we overexpressed the relevant native four genes of the Leloir pathway. The engineered strain could achieve rapid total conversion of all carbon sources in acid whey, producing 6.61 g/L of fatty acids (FAs) with a yield of 0.146 g-FAs/g-substrates. Further engineering to introduce an omega-3 desaturase enabled the synthesis of α-linolenic acid from acid whey, producing 10.5 mg/gDCW within a short fermentation time. Finally, PEX10 knockout in our platform strain was shown to minimize hyphal formation in concentrated acid whey cultures, greatly improving fatty acid content. These results demonstrate the feasibility of using acid whey as a previously untapped resource for biotechnology.

Bioactive Compounds in Fermented Sausages Prepared from Beef and Fallow Deer Meat with Acid Whey Addition.

The present study examined the effect of the type of meat (beef and fallow deer) and the addition of freeze-dried acid whey on nutritional values and the content of bioactive compounds (peptides, L-carnitine, glutathione, and conjugated linoleic acid (CLA)) in uncured fermented sausages. The antioxidant properties of isolated peptides (ABTS, DPPH radical scavenging activity, and ferric-reducing antioxidant power) were also evaluated. The results showed that fallow deer sausages had higher peptide content than beef products. The addition of acid whey caused a decrease in the content of peptides, especially in fallow deer sausages. The glutathione content in beef sausages (22.91-25.28 mg 100 g-1 of sausage) was quite higher than that of fallow deer sausages (10.04-11.59 mg 100 g-1 of sausage). The obtained results showed a significantly higher content of CLA in beef sausages than in products from fallow deer meat. In conclusion, products prepared from fallow deer meat have generally higher nutritional value because of the content of peptides, their antioxidant properties, and the content of L-carnitine, while beef products have higher levels of CLA and glutathione.

Influence of freeze-dried acid whey addition on biogenic amines formation in a beef and deer dry fermented sausages without added nitrite.

OBJECTIVE: Aim of this study was to evaluate the influence of freeze-dried acid whey addition and the use a game meat (fallow deer) on a microbial content and the biogenic amines formation in dry fermented sausages. METHODS: The experiment involved dry fermented sausages made in two variants from beef and from fallow deer. Each variant was divided into five groups: control (with a curing mixture), reference (with a sea salt), sample with a liquid acid whey and two samples with the addition of reconstituted freeze-dried acid whey in different concentrations. Changes in lactic acid bacteria (LAB), Enterobacteriaceae content and biogenic amines content were determined. RESULTS: The microbial content changes suggest that addition of acid whey slightly affected lactic acid bacteria content in comparison with the control and reference sample, but the addition of freeze-dried acid whey resulted in a reduction of Enterobacteriaceae content in the sausages from fallow deer or a similar level in the beef sausages compared with the control and reference sample. Both changes in lactic acid bacteria and Enterobacteriaceae content were more evident in case of sausages made from fallow deer. Addition of acid whey (liquid and a higher amount of freeze-dried) and use of fallow deer meat to produce the sausages resulted in a significant reduction of total biogenic amines content. CONCLUSION: The addition of acid whey (liquid and higher amount of freeze-dried) resulted in a significant reduction of total biogenic amines content in dry fermented sausages made from fallow deer meat.

Comparison of the effect of freeze-dried acid whey on physicochemical properties of organic fermented sausages made from beef and fallow deer meat.

The objective of this study was to compare the effect of freeze-dried acid whey on physicochemical properties and microbial changes of organic fermented sausages made from beef and fallow deer meat. Five formulations of sausages from each species were made. The results show that processing time and species of meat were the high significant factor on tested parameters. Variants and interactions between main factors influenced at different levels of significance on some tested attributes. At the end of processing fallow deer sausages were characterised by the lower pH (4.79 ± 0.01-4.90 ± 0.02 for fallow deer and 5.04 ± 0.00-5.25 ± 0.03 for beef sausages) and the content of 2-thiobarbituric acid reactive substances (1.54 ± 0.09-2.81 ± 0.23 and 1.64 ± 0.15-5.06 ± 0.25 respectively) than sausages made from beef meat. In conclusion, the addition of freeze-dried acid whey in varying amounts did not significantly affect the physicochemical characteristics of sausages from both fallow deer and beef. However, further research is needed to compare the effect of acid whey on the nutritional values of raw fermented sausages from fallow deer and beef.

Use of acid whey protein concentrate as an ingredient in nonfat cup set-style yogurt.

Acid whey resulting from the production of soft cheeses is a disposal problem for the dairy industry. Few uses have been found for acid whey because of its high ash content, low pH, and high organic acid content. The objective of this study was to explore the potential of recovery of whey protein from cottage cheese acid whey for use in yogurt. Cottage cheese acid whey and Cheddar cheese whey were produced from standard cottage cheese and Cheddar cheese-making procedures, respectively. The whey was separated and pasteurized by high temperature, short time pasteurization and stored at 4°C. Food-grade ammonium hydroxide was used to neutralize the acid whey to a pH of 6.4. The whey was heated to 50°C and concentrated using ultrafiltration and diafiltration with 11 polyethersulfone cartridge membrane filters (10,000-kDa cutoff) to 25% total solids and 80% protein. Skim milk was concentrated to 6% total protein. Nonfat, unflavored set-style yogurts (6.0 ± 0.1% protein, 15 ± 1.0% solids) were made from skim milk with added acid whey protein concentrate, skim milk with added sweet whey protein concentrate, or skim milk concentrate. Yogurt mixes were standardized to lactose and fat of 6.50% and 0.10%, respectively. Yogurt was fermented at 43°C to pH 4.6 and stored at 4°C. The experiment was replicated in triplicate. Titratable acidity, pH, whey separation, color, and gel strength were measured weekly in yogurts through 8 wk. Trained panel profiling was conducted on 0, 14, 28, and 56 d. Fat-free yogurts produced with added neutralized fresh liquid acid whey protein concentrate had flavor attributes similar those with added fresh liquid sweet whey protein but had lower gel strength attributes, which translated to differences in trained panel texture attributes and lower consumer liking scores for fat-free yogurt made with added acid whey protein ingredient. Difference in pH was the main contributor to texture differences, as higher pH in acid whey protein yogurts changed gel structure formation and water-holding capacity of the yogurt gel. In a second part of the study, the yogurt mix was reformulated to address texture differences. The reformulated yogurt mix at 2% milkfat and using a lower level of sweet and acid whey ingredient performed at parity with control yogurts in consumer sensory trials. Fresh liquid acid whey protein concentrates from cottage cheese manufacture can be used as a liquid protein ingredient source for manufacture of yogurt in the same factory.

Effects of replacing buttermilk with yogurt acid whey in ranch dressing.

Greek-style yogurt has expanded from 5 to 50% of the US yogurt market in the past decade, accompanied by a corresponding increase in production of its by-product: yogurt acid whey (YAW). Yogurt acid whey qualities (e.g., low pH, mineral content, astringency, and saltiness) present challenges for processing, disposal, and ingredient use. A shelf-stable ranch dressing was formulated by replacing buttermilk in the control with YAW and concentrated YAW (6.3 to 25.2 °Brix). Added salt, gums, and acids were adjusted. The effects of buttermilk substitution on stability were studied on pasteurized samples (8 mo at room temperature). A consumer sensory study (n = 96) was conducted utilizing hedonic and just-about-right scales. Purchase intent and demographic data were also collected. A focus group (n = 7) evaluated the sensorial attributes of the samples after 6 mo. The experiment was performed in triplicate and all instrumental analyses (pH, soluble solids as °Brix, water activity, refraction index, and color) were conducted in triplicate for statistical analysis. Increasing the gum content in YAW samples resulted in equivalent texture liking compared with the control. Matching the control's NaCl concentration resulted in undesirable higher saltiness. The pH of the 18.9 °Brix YAW ranch sample without lactic acid added was under 4.6, with no effect on flavor liking. Increasing concentrations of YAW decreased L* and water activity, and increased the refractive index and hue. The YAW samples presented minimum changes over 8 mo of storage and had better water retention than the control. We conclude that 15 to 17 °Brix YAW is the optimal replacement for buttermilk in a dressing. The formulation of dressings may be accomplished successfully, sustainably, and cost effectively, with minor processing adjustment, by substituting buttermilk with YAW.

Short communication: Composition of coproduct streams from dairy processing: Acid whey and milk permeate.

This article provides composition information for 3 abundantly available but little characterized dairy coproduct streams: acid whey from Greek yogurt (GAW), acid whey from cottage cheese (CAW), and milk permeate (MP). Three replicate samples obtained on different dates from several dairy processors were analyzed. The main component in all streams was lactose, with up to 3.5, 2.1, and 11.9% in GAW, CAW, and MP, respectively. Crude protein content ranged from 1.71 to 3.71 mg/g in GAW, 1.65 to 5.05 mg/g in CAW, and 3.2 to 4.35 mg/g in MP, and pH ranged from 4.21 to 4.48, 4.35 to 4.51, and 5.4 to 6.37, respectively. Chemical oxygen demand varied from 52,400 to 62,400 mg/L for GAW, 31,900 to 40,000 mg/L for CAW, and 127000 to 142,000 mg/L for MP; biochemical oxygen demand ranged from 45,800 to 50,500 mg/L (GAW), 32,700 to 40,000 mg/L (CAW), and 110,000 to 182,000 mg/L (MP). The GAW had the lowest pH (4.21-4.48) and highest mineral content of all streams. These data will assist processors and researchers in developing value-added uses of these dairy coproducts.

Leveraging endogenous barley enzymes to turn lactose-containing dairy by-products into fermentable adjuncts for Saccharomyces cerevisiae-based ethanol fermentations.

Acid whey, a by-product of strained yogurt production, represents a disposal challenge for the dairy industry. Utilization schemes are currently limited; however, acid whey contains valuable components that could be used to create value-added products. One potential scheme would be the fermentation of acid whey into an alcoholic beverage. Sour beers are gaining popularity and acid whey, which is sour to begin with, could provide a new product opportunity. However, the main sugar of acid whey, lactose, cannot be fermented by the traditional brewer's yeast, Saccharomyces cerevisiae. It has been reported that barley contains enzymes capable of hydrolyzing lactose to glucose and galactose, which are fermentable by S. cerevisiae. We investigated whether a barley-based mash resulted in detectable hydrolysis of lactose into sugars fermentable by S. cerevisiae. We demonstrated the ability to hydrolyze lactose in acid whey using a barley-based mash, resulting in the average release of 3.70 g/L of glucose. Additionally, the subsequent liquid was fermented by S. cerevisiae to an average ethanol concentration of 3.23% alcohol by volume. This work demonstrates the ability to hydrolyze the lactose in acid whey using barley and the opportunity to use acid whey as a fermentable sugar source in beer production.

Probiotic fermented beverages based on acid whey.

Acid whey is a byproduct of cheesemaking that is difficult to use because of its low pH and less-favorable processing properties compared with rennet whey. The aim of this study was to evaluate the qualities of fermented beverages made using acid whey. In manufacturing the beverages, we used probiotic cultures Lactobacillus acidophilus LA-5 or Bifidobacterium animalis ssp. lactis BB-12. The production process included combining pasteurized acid whey with UHT milk, unsweetened condensed milk, or skim milk powder. We introduced milk to enrich casein content and obtain a product with characteristics similar to that of fermented milk drinks. The products were stored under refrigerated conditions (5 ± 1°C) for 21 d. During storage, we assessed the beverages' physicochemical properties and organoleptic characteristics. The properties of the beverages depended on their composition, microbial culture, and storage time. Beverages containing L. acidophilus had higher acidity, which increased during storage; the acidity of samples containing B. animalis was more stable. Beverages made with skim milk powder (La1 and Bb1) had higher acetaldehyde content, but this parameter decreased in all samples during storage. The hardness of the samples did not change during storage and was highest in beverage La3, made from whey, condensed milk, and L. acidophilus. Beverage La2, made from whey, milk, condensed milk, and L. acidophilus, had the best sensory properties. The whey beverages we developed provided a good medium for the probiotic bacteria; bacteria count throughout the storage period exceeded 8 log cfu/mL, distinctly higher than the minimum therapeutic dose.

Positive Impact of Pulsed Electric Field on Lactic Acid Removal, Demineralization and Membrane Scaling during Acid Whey Electrodialysis.

The drying of acid whey is hindered by its high mineral and organic acid contents, and their removal is performed industrially through expensive and environmentally impacting serial processes. Previous works demonstrated the ability to remove these elements by electrodialysis alone but with a major concern-membrane scaling. In this study, two conditions of pulsed electric field (PEF) were tested and compared to conventional DC current condition to evaluate the potential of PEF to mitigate membrane scaling and to affect lactic acid and salt removals. The application of a PEF 25 s/25 s pulse/pause combination at an initial under-limiting current density allowed for decreasing the amount of scaling, the final system electrical resistance by 32%, and the relative energy consumption up to 33%. The use of pulsed current also enabled better lactic acid removal than the DC condition by 10% and 16% for PEF 50 s/10 s and 25 s/25 s, respectively. These results would be due to two mechanisms: (1) the mitigation of concentration polarization phenomenon and (2) the rinsing of the membranes during the pause periods. To the best of our knowledge, this was the first time that PEF current conditions were used on acid whey to both demineralize and deacidify it.

Short communication: Multi-component interactions causing solidification during industrial-scale manufacture of pre-crystallized acid whey powders.

Acid whey (AW) is the liquid co-product arising from acid-induced precipitation of casein from skim milk. Further processing of AW is often challenging due to its high mineral content, which can promote aggregation of whey proteins, which contributes to high viscosity of the liquid concentrate during subsequent lactose crystallization and drying steps. This study focuses on mineral precipitation, protein aggregation, and lactose crystallization in liquid AW concentrates (∼55% total solids), and on the microstructure of the final powders from 2 independent industrial-scale trials. These AW concentrates were observed to solidify either during processing or during storage (24 h) of pre-crystallized concentrate. The more rapid solidification in the former was associated with a greater extent of lactose crystallization and a higher ash-to-protein ratio in that concentrate. Confocal laser scanning microscopy analysis indicated the presence of a loose network of protein aggregates (≤10 µm) and lactose crystals (100-300 µm) distributed throughout the solidified AW concentrate. Mineral-based precipitate was also evident, using scanning electron microscopy, at the surface of AW powder particles, indicating the formation of insoluble calcium phosphate during processing. These results provide new information on the composition- and process-dependent physicochemical changes that are useful in designing and optimizing processes for AW.

How electrodialysis configuration influences acid whey deacidification and membrane scaling.

With the rising popularity of Greek-style yogurts in the past few years, the production of acid whey has drastically increased. If sweet whey is usually further processed, the acid whey valorization comes with challenges because its drying is jeopardized by its high mineral and organic acid contents. For this reason, prior demineralization and deacidification are usually performed at industrial scale using a combination of ion exchange resins and electrodialysis. This whole process represents large amounts of resources and energy consumption as well as an important production of effluents. The optimization of the electrodialysis technique, currently the focus of a few studies, could result in the replacement of the serial processes and would provide a cost-effective and eco-efficient alternative. In this work, the demineralization and deacidification of acid whey were compared via 2 electrodialysis configurations: one conventional and one using bipolar membranes. Both configurations allowed to reach interesting demineralization (67%) and deacidification (44%) rates. However, even though the appearance of fouling or scaling has never been reported, scalings of different natures were observed on membranes using both configurations. Amorphous calcium phosphate was identified on the anion exchange membranes for both configurations while calcite and brucite were identified on cation exchange ones using the bipolar membrane configuration. These scaling formations were linked to the migration of divalent ions and water splitting phenomenon caused by a high demineralization rate or by an already formed significant scaling.

Addition of pectin and whey protein concentrate minimises the generation of acid whey in Greek-style yogurt.

The objective of the study reported in this Research Communication was to investigate the effects of pectin and whey protein concentrate (WPC) on the generation of acid whey during Greek-style yogurt (GSY) processing. Yogurt samples were prepared using pectin (0·05%, w/v) and whey protein concentrate (WPC-80) (1%, w/v) as possible ingredients that reduce the acid whey production. Control yogurt sample was prepared without addition of these ingredients. The results showed that yogurt made with pectin plus WPC had significantly higher water holding capacity (~56%) than the control (33%). Similarly, yogurt supplemented with pectin plus WPC exhibited 15% less susceptibility to syneresis compared to the control (P < 0·05). Viability of L. bulgaricus and S. thermophilus in all yogurts remained ≥7·0 and ≥8·0 log CFU/g respectively. Native PAGE analysis showed an interaction between pectin and WPC. Pectin hinders the formation of large oligomeric aggregates of whey protein which correlates with an increase in WHC and a decrease in syneresis. Our results demonstrated that a combination of pectin and WPC have the potential to limit the quantity of acid whey generation in GSY manufacturing. Thus, these ingredients have positive implications for dairy industry in the production of GSY.

The influence of acid whey on the antioxidant peptides generated to reduce oxidation and improve colour stability in uncured roast beef.

BACKGROUND: The effect of marinating beef in acid whey on the antioxidant peptides generated, and their influence on lipid oxidation, colour stability, sensory analysis and protein degradation products in uncured roasted beef stored 6 weeks in vacuum conditions (T = 4 °C) were determined. Measurements of pHe, water activity, oxidation-reduction potential, colour, TBARS, the cutting force, texture and amino acid profile, the total content of peptides, and antioxidant activity of isolated peptides were conducted immediately after production and during 42 days of storage. RESULTS: The non-nitrite control batch (C) was characterized by a lower a* value (6.33-6.70) during the whole storage period compared to the non-nitrite sample with acid whey (W). It also appears that meat with the worst colour stability has the poorest oxidative stability (C = 1.57 mg MDA kg-1 , W = 0.76 mg MDA kg-1 ). Activity against reactive forms of oxygen to fraction A and fraction B (P < 0.05) showed an increasing trend with time and it was the largest in sample C and W. CONCLUSION: The results indicated that bioactive peptides could be generated in uncured roasted beef. The <3.5 kDa peptides have strong antioxidant activity, as a result of which they function as inhibitors of lipid oxidation and colour discoloration during prolonged storage. © 2018 Society of Chemical Industry.

Synthesis of Galactooligosaccharides in Milk and Whey: A Review.

Galactooligosaccharides (GOS) are synthesized by the enzyme ß-galactosidase during the hydrolysis of lactose. In this so-called transgalactosylation reaction the galactosyl moiety is transferred to another sugar molecule instead of water resulting in oligosaccharides of different chain lengths and glycosidic linkages. Because their structures are similar to oligosaccharides present in human breast milk, they act as prebiotics, which has been shown for infants and adults to be alike. While so far most of the research to maximize GOS yield has been carried out using buffered lactose solution as a starting material, more and more work is now conducted with dairy by-products such as whey and whey permeate, or even milk, for direct GOS synthesis in order to develop new GOS-enriched dairy products. This review aims to summarize the results obtained with various dairy liquids, and it rates their suitabilities to act as raw material for GOS production. Most of the studies using whey or milk have been carried out with enzymes from Aspergillus oryzae, Kluyveromyces lactis, Bacillus circulans, Streptococcus thermophilus, and several Lactobacillus species. As the initial lactose concentration (ILC) is known to be a crucial factor for high GOS yield, most of the research has been done with concentrated or supplemented milk and whey. However, a clear dependency on ILC could only be observed for the A. oryzae lactase, indicating a strong influence of milk components like minerals and proteins on the transfer activities of most enzymes.