An evaluation of modern production methods fo...

13/09/2021  •  22 views
An evaluation of modern production methods for lactose-free dairy products. BGLA

For lactose intolerant people, it is nowadays not necessary to completely avoid the indulgence and nutritional value of dairy products [4]. There are very good solutions that rely on the hydrolysis of lactose into glucose and galactose with the enzyme lactase. These monosaccharides are readily adsorbed in the small intestine and prevent the occurrence of lactose intolerance symptoms. Two different type of lactases are currently commercially available: neutral lactases and acid lactases [5]. Neutral lactases are mainly used in the production of lactose-free dairy products on an industrial scale, although in some countries, this enzyme is also offered to consumers for treating milk at home. Acid lactase is available to consumers as a nutritional supplement to be taken together with regular dairy products and works by splitting lactose in the stomach.

The enzyme used for the production of lactose-free dairy products has traditionally been the neutral β-galactosidase derived from the dairy yeast Kluyveromyces lactis (and its close relatives Saccharomyces lactis, K. marxianus or K. fragilis) [5]

Other commercial β-galactosidases from, for example, Bacillus circulans or Aspergillus oryzae are less suitable for producing lactose-free products in most dairy matrices because of the different pH or temperature optimums [5] and are mainly sold for either galacto-oligosaccharide (GOS) production or as a nutritional enzyme.
In general, the majority of potable milk in Northern European countries, North America and Australia is pasteurized and stored under cooled conditions for a couple of weeks. In Southern European countries, South America and Asia most milk is UHT sterilized and stored at ambient temperatures up to 9 months.

Currently, two processes (batch and aseptic) are in use to produce lactose-free milk [9,10], and both these processes use soluble lactase enzyme.

However, the immobilization of lactase was not used in industrial practice for lactose-free milk production until today due to problems with the microbial stability of the final product. The recycling of the immobilized enzyme in an industrial setting is therefore limited, making the procedure less cost-effective and more prone to quality defects than hydrolysis with a soluble enzyme.

In the batch process, a neutral lactase sample is added to a tank of raw or thermized milk and, commonly, incubated for approx. 24 h under slow stirring to prevent creaming. Since the milk at this stage is not sterile yet, this process has to be performed at cooled conditions (normally 4–8 °C) to prevent microbial growth. After this incubation, the milk is pasteurized, homogenized and packaged (Figure 2). Also, some producers of UHT milk use the batch process, although the aseptic process has become much more popular for this market segment in recent years. Since the enzyme is inactivated during the pasteurization/sterilization of the milk, no residual enzyme activity remains in the final product when the milk is produced via the batch process, which is an advantage of the regulation and labelling in some countries.

A number of aspects are important to consider when one uses the batch process for making lactose-free milk:

    The dosage of the enzyme should be sufficient to reach the required boundary for lactose-free during the limited time and low temperature of the incubation. Therefore, the enzyme dosage is relatively high. Enzymes available for this process are selected for their relatively high activity at neutral pH and low temperature. The process control is high since the lactose conversion may be measured in the tank and since the enzyme dosage or incubation time may be adapted during the process.

    The batch incubation requires the occupation of a tank in the factory and the holdup of the milk for a day. The process is, therefore, discontinuous, which may pose a problem for some factories, especially when the productivity is high. A lactase with a higher specific activity under these conditions (like Maxilact® Smart) may help shorten the production time and, hence, may increase the throughput of the factory. Since the pasteurization of the milk is postponed for a day, the milk quality should be impeccable to prevent microbial spoilage.

    The milk product produced with the batch process is relatively insensitive for possible side activities in the enzyme preparation. This is due to the limited time of storage of the milk at refrigerated conditions and to the pasteurization/sterilization after the enzyme incubation, inactivating most enzymatic activities. Although in the past some lactase preparations showed proteolytic side activities [12], these problems seem to be over and very few complaints occur for lactose-free milk produced with the batch process.

    Since lactose hydrolysis leads to a doubling of the sweetness of milk, processes were developed to remove part of the lactose using chromatography or (ultra and nano) filtration techniques combined with the hydrolysis of the remaining lactose so an exact sweetness is regenerated [13,14]. The resulting lactose-free milk produced with this process is of excellent quality, and the taste is almost identical to regular milk. This feature is especially appreciated by the pasteurized milk drinkers in Northern Europe and North America and is, therefore, frequently used in conjunction with the batch process.

In the aseptic process, the milk is first sterilized using the UHT procedure, after which a sterile lactase preparation is injected into the milk just before packaging [15]. The lactose conversion in the milk will take place in the milk package (Figure 2). Since UHT milk is often kept in quarantine for approx. 3 days at ambient temperature, there is sufficient time for complete hydrolysis before the milk is shipped to the retailer. Since there is no quarantine period for pasteurized milk, the aseptic process is not used for this type of lactose-free milk. There are, in essence, two different procedures for obtaining a sterile lactase. In the first procedure, the lactase enzyme is presterilized by the manufacturer of the enzyme, and special sterile dosing equipment is required for the sterile injection (e.g., the Tetrapak (Lund, Sweden) Flexdos® system). In the second procedure, the unsterile enzyme is filter-sterilized just before addition to the sterile milk at the dairy factory (via, for example, the Tetrapak Aldose® system).

A number of aspects are important to consider when one uses the aseptic process for making lactose-free milk:

• The dosage of the enzyme can be much lower compared to the batch process, since both the incubation time and temperature are higher. Process control is, however, absent since the enzyme is only active in the final milk package. E.g., the storage temperature in un-thermostated warehouses may deviate from summer to winter, and the dairy producer should take these aspects into account when dosing the enzyme.

• The aseptic process requires special equipment and consumable costs, and especially for the in-factory filtration, it requires highly skilled operators to prevent microbial contamination of the milk during lactase injection. However, the process can be operated full-continuous when organized properly, and that is a major advantage for factories that require a high throughput.

• The aseptic process for making lactose-free UHT milk could only be fully developed after major improvements in the quality of the lactase enzymes. Besides the removal of proteolytic side activity, it was also found that arylsulfatase side activity in the lactose preparation may lead to severe medicinal off-flavors during storage due to p-cresol formation from sulphonated-cresol that is naturally present in the milk [16]. A producer of lactose-free UHT milk should consider using only the highest quality lactases for this process to prevent problems during shelf life. Arylsulfatase-free lactases (like all Maxilact® products) are currently commercially available.

• Lactose hydrolysis in milk leads to an increased presence of monosaccharides, and therefore, the Maillard reaction is more efficient. Limited proteolysis by proteases present in the milk or originating from the lactase preparation may enhance the reaction. This results in the increased formation of off-flavors, in the browning of lactose-free milk when compared to regular milk and in a reduced nutritional value when stored at increased temperatures [10,17,18]. The increased Maillard reaction is probably the most important determinant of the reduced shelf life of lactose-free UHT milk compared to regular UHT milk. Although it has been suggested in the past that lactose-free UHT milk production using the batch process may lead to even more browning compared to milk produced via the aseptic process [19], recent data show that the storage conditions (temperature) and choice of the lactase are much more relevant for determining shelf life [10]. Excellent shelf life was found for lactose-free UHT milk produced with the batch process, and milk browning during storage is, therefore, largely independent of the production process that is used

The presence of a substantial amount of lactose in most fermented milk products,
like 30–40 gram/kg in yoghurt [3], would suggest that lactose-intolerant people will have a problem
with these dairy products. However, this seems to be much less the case than expected from the
amount of lactose that is consumed. There have been two different theories put forward to explain this
phenomenon (reviewed by References [20,21]).
• It has been suggested that the lactic acid bacteria present in yoghurt will survive the stomach,
and the lactase enzyme present in these bacteria aids the digestion of (or part of) the lactose in
the small intestine. The monosaccharides are both consumed by the bacteria and taken up in the
small intestine, so lactose intolerance symptoms are reduced. Some dairy companies claim to
produce a yoghurt containing special cultures that have this effect. This hypothesis will only hold
if the yoghurt bacteria and their intracellular lactase enzyme will survive the stomach. Indeed,
the pasteurization of yoghurt seems to worsen lactose intolerance symptoms.
• A second explanation that was put forward suggests that the lactose in yoghurt is better digested
due to the decreased transit time of a viscous yoghurt meal compared to liquid milk. Due to
this, any residual lactase in the small intestine will have more time to digest lactose and, thereby,
reduces intolerance symptoms. It has indeed been found that having a meal together with a glass
of milk will reduce symptoms, suggesting that transit time may play a role in lactose digestion.

Regardless of which of these effects play the major role in tolerating fermented milk products by lactose
intolerant people, the most reliable remedy seems to be the complete enzymatic digestion of lactose in
yoghurt. This can be done by incubating the milk with lactase before pasteurization (familiar to the
batch process for milk) or adding the lactase together with the culture after the pasteurization of the
milk (Figure 3). Most yoghurt producers opt for the latter, co-hydrolysis, approach since predigestion
seems to inhibit the activity of some yoghurt cultures (see, for example, Reference [22]), probably due
to the switch from lactose to glucose as a main carbon source or to the increased osmotic pressure in
lactose-hydrolyzed milk.
When the (neutral) lactase is added to the milk at the same time as the yoghurt culture, only a
limited time is left for lactose digestion. Most neutral lactases are completely inactivated at a
pH < 5.5 [5], which is reached after 2.5–3 hours of incubation in a regular yoghurt making process.
Hence, the lactase dosage must be relatively high to obtain a lactose-free status. Acid stable lactases
are presently on the market, and these may reduce the total enzyme dosage.

When the (neutral) lactase is added to the milk at the same time as the yoghurt culture, only a
limited time is left for lactose digestion. Most neutral lactases are completely inactivated at a pH < 5.5
[5], which is reached after 2.5–3 hours of incubation in a regular yoghurt making process. Hence, the
lactase dosage must be relatively high to obtain a lactose-free status. Acid stable lactases are presently
on the market, and these may reduce the total enzyme dosage. However, since these enzymes are not
inactivated by the low pH in the final product, the addition has to be labeled on the yoghurt package
in some countries.
A major advantage of using lactases in the production of yoghurt is the increase in sweetness
due to the splitting of lactose [23,24]. Hence, the total added sugar can be reduced by 1.5–2 g/100 g
without changing the flavor profile. The enzyme addition can be reduced for this application since
the final product does not have to be lactose-free, but a slightly higher residual lactose is allowed
without noticeable difference in sweetness. Since K. lactis lactase is inactivated in the yoghurt process
due to the pH drop, the labelling of the enzyme on the yoghurt package is often not required. Hence,
many yoghurt manufacturers use the enzyme as a label-friendly solution to reduce the sugar
Another advantage of the digestion of lactose in yoghurt is that the post-acidification during
shelf life can be reduced when specific yoghurt cultures are used [25]. Apparently, some yoghurt
bacteria are less active in the absence of lactose or have difficulties in switching from one carbon
source (lactose) to another (glucose), leading to a better sensory stability of the product.
Invertase activity was present as a side activity in many commercial lactases in the past.
Invertase digests sucrose into glucose and fructose and, thereby, influences the sweetness perception
in a fermented milk product that contains added sucrose. This problem was recognized and has led
to the development of special lactases without invertase activity, such as Maxilact® LGi.
Other relevant side activities