Batch lactase addition to pasteurised whole milk is carried out in jacketed silos with low-shear bottom agitation before UHT sterilisation. The enzyme preparation from Kluyveromyces lactis is dosed as a liquid or powder; typical addition ranges between 0.5–2.0 NLU/L when target residual lactose is below 0.1 g/100 mL. Hydrolysis proceeds at 4–8 °C for 12–24 h, which avoids mesophilic spore germination while neutral lactase remains active at pH 6.5–6.7. Initial lactose concentration in raw milk varies seasonally from 4.6–5.2 % (w/v); the holding time is adjusted against this value. In-process lactose concentration is monitored by ISO 22662:2007 HPLC-RI on samples drawn from the silo mid-depth. Production-line failure modes include pH drift caused by lactic acid bacteria in raw milk and premature enzyme inactivation when sodium hypochlorite residues from CIP enter the silo. Before enzyme injection, the silo is chilled to 4 °C and rinsing is continued until conductivity drops below 50 μS/cm. The enzyme preparation is added through a sterile dosing skid with positive displacement pump; batch-to-batch variation is corrected by FCC lactase unit assay. Hydrolysis is terminated by UHT final heating at 137–142 °C for 3–5 s in a tubular or plate steriliser, which denatures residual lactase. Terminal product is lactose-free whole milk, semi-skimmed milk or flavoured milk packed in aseptic cartons. Lactase from Kluyveromyces lactis is affirmed as GRAS under 21 CFR 184.1553. Final lactose content is verified after 7 days at 25 °C accelerated storage to detect post-pack hydrolysis from any surviving enzyme activity.

Whey Permeate Lactose Hydrolysis and the Yeast Fermentation Bottleneck

Sweet whey permeate generated after ultrafiltration of rennet cheese whey contains lactose at 4.0–4.5 % (w/v). Acid whey from Greek-style yogurt or quark lines presents pH 4.0–4.6; neutral lactase from Kluyveromyces lactis is inactive in this matrix. Acid lactase from Aspergillus oryzae is therefore applied without pH correction, with activity optimum at pH 3.5–4.5 and 45–55 °C. The substrate is held in a batch reactor with heating jacket and side-mounted mixer. Dose-response is adjusted to reduce lactose below 1.0 g/L for subsequent yeast fermentation of monosaccharides to ethanol or single-cell protein. Processing conflicts arise with pH correction: potassium hydroxide addition to raise acid whey pH above 6.0 destabilises calcium phosphate complexes and increases ash load. Published data for exact neutral lactase survival in acid whey are limited; process design should avoid neutral enzyme in acid streams unless ultrafiltration and diafiltration remove organic acids first. Hydrolysate is cooled to 30–32 °C before Saccharomyces cerevisiae inoculation in factory fermenters with working volumes up to 200 m³. Enzyme residues are inactivated during pasteurisation at 72 °C for 15 s. If hydrolysed whey concentrate is routed to spray drying, inlet air temperatures above 180 °C increase Maillard browning because reducing sugars are present. Final products are reduced-lactose whey syrup, bioethanol, or yeast biomass used in animal nutrition.

How Does Transgalactosylation Shift Product Distribution in Concentrated Lactose Syrup?

When lactose concentration is raised above 300 g/L, hydrolytic activity of β-galactosidase competes with transgalactosylation transfer to carbohydrate acceptors. Batch GOS synthesis is run in a stirred reactor at 50–60 °C, pH 4.5–6.0, with initial lactose dissolved in deionised water to 400–600 g/L. Dissolution requires heating to 70–80 °C before cooling to reaction temperature. The enzyme dose is expressed as total lactase activity per gram of lactose; commercial preparations from Aspergillus oryzae or Bifidobacterium bifidum are used. The reaction mixture develops a turbid appearance as galacto-oligosaccharides accumulate. Product distribution after 20–30 h includes unconverted lactose, glucose, galactose, and GOS species with degree of polymerisation from 2 to 6. Common literature values report GOS yield typically not exceeding 30–35 % of total carbohydrate under batch operation. Kinetic limitations are significant: glucose and galactose act as competitive inhibitors, and the ratio of hydrolysis to transfer is governed by water activity. Agitation speed above 300 rpm in a pilot-scale vessel improves mass transfer but increases air entrainment and may reduce enzyme stability. The reaction is stopped by heat treatment at 90 °C for 10 min. Terminal syrup is refined through activated carbon and ion-exchange demineralisation to produce a clear GOS concentrate of 45–55 % solids. GOS syrup is incorporated into infant formula and adult nutritional products under EU Regulation (EU) 2016/127 where applicable. No single ISO method covers all GOS species; GOS content is typically determined by HPAEC-PAD, while residual lactose is quantified by ISO 22662:2007.

Lactose crystallisation defects in frozen dairy desserts are managed by adding neutral lactase at 0.5–1.5 NLU/L to the ice cream mix before high-temperature short-time pasteurisation at 85 °C for 20–30 s, with hydrolysis completed during subsequent ageing at 4 °C for 4–12 h; the terminal product is a soft-serve or hard-pack ice cream with reduced sandiness after storage at −18 °C. The enzyme preparation must conform to Food Chemicals Codex lactase activity specifications.

Immobilised β-Galactosidase in Continuous Membrane Reactor Configurations

Continuous lactose hydrolysis with enzyme immobilised on ion exchange resin is applied to whey permeate and retentate streams in dairy processing lines. The carrier is a styrene-divinylbenzene anion exchange resin with particle size 300–700 μm; enzyme is immobilised through glutaraldehyde crosslinking or covalent coupling. A packed bed reactor with aspect ratio above 3:1 and inlet distributor plate is operated at 30–40 °C for neutral lactase. Process control variables include pressure drop across the bed, substrate residence time, and enzyme leakage. In long-term runs, pressure drop above 1.5 bar may indicate resin fines accumulation or microbial biofilm growth. Backwashing with 0.1 M sodium acetate buffer at pH 5.0 restores flow. Continuous stirred tank reactors with membrane recirculation avoid bed compression but require ultrafiltration membranes with molecular weight cut-off 10–30 kDa to retain enzyme. Hydrolysis degree is controlled by feed flow rate and lactose load; residual lactose below 1.0 g/L in whey permeate is achievable at hydraulic retention times between 4–8 h. Terminal products are reduced-lactose whey concentrates and lactose-free milk ingredients for sports nutrition. Resin capacity is expressed as grams lactose hydrolysed per kilogram resin per hour; published data for exact capacity under industrial whey permeate are limited due to varying ash content. Microbial contamination in continuous reactors is managed by periodic sanitation with peracetic acid at 100–200 ppm after enzyme separation. Operational boundaries include divalent cation fouling on resin and enzyme thermal denaturation above 55 °C; cleaning with sodium hydroxide above 0.5 M may strip immobilised protein. Immobilised catalyst activity is verified with FCC lactase assay after desorption from resin.

When Lactase Is Deployed as an Analytical Reagent for Lactose QC

In quality control laboratories, β-galactosidase is used as a cleaving reagent in enzymatic lactose determination based on ISO 5765-1:2002 for dried milk and combined with glucose oxidase-peroxidase-linked chromogenic detection. Analytical-grade β-galactosidase from Escherichia coli or Kluyveromyces lactis is used; the preparation is free of glucose oxidase and hexokinase interference. The manual or automated assay proceeds at 37 °C and pH 6.6 in pipetting platforms with photometric read at 505 nm or 340 nm depending on the chromogen. Each millilitre of reagent contains sufficient β-galactosidase to hydrolyse the lactose present in 0.10 mL of diluted sample within 15 min. The method is linear up to 0.6 g/L lactose in the cuvette; higher concentrations require dilution with deionised water. Matrix interferences from galactose, glucose, and ascorbic acid are corrected with blank samples. The terminal output is a certificate of analysis for lactose content in milk powders, infant formula, and whey products. The enzyme reagent is stored lyophilised at 2–8 °C; reconstituted working solution remains stable for 24 h. Correlation between enzymatic and HPLC methods follows ISO 22662:2007 with a bias below 2 % for routine dairy matrices. Quality control laboratories follow ISO 5725-2 for repeatability and reproducibility verification.