Encyclopedia
Subjects:
Biotechnology & Applied Microbiology
1. Introduction
Recent findings have confirmed the therapeutic properties of human milk components and have left no doubt that it constitutes an indispensable part of newborns’ nutritional treatment, especially premature babies with very low (VLBW) and extremely low (ELBW) birth weight. Initiating lactation in preterms’ mothers and maintaining mother’s milk supply for NICU infants remains challenging. Donor milk has become the standard way of feeding newborns who cannot receive the milk of their biological mothers [1]. Donor milk administration to prematurely born children needing longer hospital stays requires necessary collecting procedures, freezing, storage and pasteurization. This may considerably lower its nutritional and therapeutic values. So far, the best characterized and commonly used method of human milk preservation is holder pasteurization. However, the knowledge about the significant negative impacts of this method on the active components in human milk results in the development of novel techniques that could better preserve nutritional and non-nutritional factors.
The aim of this narrative review was to analyze the best characterized and current methods of human milk processing in the context of minimizing qualitative and quantitative losses of its bioactive elements and ensuring safety in clinical situations.
2. Materials and Methods
The literature review included electronic searches of MEDLINE (January 2000–January 2019), EMBASE (January 2000–January 2019) and conference proceedings. The electronic search used the following text words and MeSH terms: donor milk, human milk, breast milk, banked milk, (human milk OR donor milk) AND pasteurization [–] (human milk OR breast milk) AND, preservation (human milk OR breast milk) AND holder pasteurization (human milk OR breast milk) AND, high-pressure (human milk OR breast milk), UV–treatment (human milk OR breast milk), microwave (human milk OR breast milk). Reference lists of the previous reviews and relevant studies were examined. Research that had been reported only as abstracts were eligible for inclusion if sufficient information was available from the report.
3. Results and Discussion
Microbiological safety is the primary criterion that has to be fulfilled as far as food for newborns is concerned. Therefore, firstly we focus on the effectiveness of the proposed human milk processing methods in human milk pathogen elimination (Table 1).
Table 1. Studies of Holder Pasteurization, High-Temperature Short-Time, High Pressure Processing and Microwave Irradiation HoP, HTST, HPP, MI on microbiological and viral components of human milk.
| Tested Component | HoP | HTST | HPP | MI | References |
|---|---|---|---|---|---|
| Bacteriostatic effect on E.coli and L. innocua | 48% reduction 28% reduction |
64% reduction 39% reduction |
not studied | not studied | [5,6] |
| Inactivation of selected microorganisms (L. monocytogenes S. agalactiae, E. coli, S. aureus) | inactivation | not studied | inactivation | not studied | [7] |
| Inactivation of selected microorganisms (Enterobacteriaceae) | inactivation | not studied | inactivation | not studied | [8] |
| Inactivation of selected microorganisms (S. aureus ATCC 6538, Enterobacteriaceae) | not studied | not studied | inactivation | not studied | [9] |
| Antibacterial efficacy (Coagulase-negative staphylococci, Gram- negative bacteria, Enterococcus species) | reduced bacterial counts | reduced bacterial counts | not studied | not studied | [10] |
| Microbiological quality (vegetative forms of microorganisms present in raw milk samples) | destroyed commensal and contaminant vegetative microorganisms except Bacillus sp. | destroyed all vegetative forms of microorganisms except Bacillus sp. and E. faecalis | not studied | not studied | [11] |
| Inactivation of selected microorganisms (S.aureus) | inactivation | not studied | inactivation | not studied | [12] |
| Inactivation of selected microorganisms (S.aureus, B. cerues) | partial inactivation | not studied | inactivation | not studied | [13] |
| Inactivation of selected microorganisms (E.coli, P. aeruginosa, S. aureus, S. epidermidis) | inactivation | not studied | not studied | inactivation | [14] |
| Ebola Virus | inactivation | not studied | not studied | not studied | [15] |
| Marburg Virus | inactivation | not studied | not studied | not studied | [15] |
| Zika virus | inactivation | not studied | not studied | not studied | [16] |
| CMV | Inactivation destroy viral infectivity |
destroy viral infectivity | not studied | inactivation | [17,18,19] |
| HTLV HIV |
inactivation | inactivation | not studied | not studied | [20] |
| HPV high-risk (types 16 and 18), low-risk (type 6) | inactivation | not studied | not studied | not studied | [21] |
HoP–Holder Pasteurization, HTST–High-Temperature Short-Time, HPP–High Pressure Processing, MI–Microwave Irradiation CMV–cytomegalovirus, HTLV–human T lymphotropic virus, HIV–human immunodeficiency virus.
Microbiota associated with human milk mostly reflects the mother’s health status but contributes to both maternal and infant homeostasis [2]. Small amounts of bacteria from the nipple and the areola, such as Escherichia coli, Serratia marcescens, and Pseudomonas aeruginosa are often present in human milk. Moreover, due to milk flow back from the newborn’s mouth cavity into the mammary ducts, Streptococcus sp. and Staphylococcus sp. are present in mother’s milk in physiological conditions [3]. On the other hand, bacteria connected with mastitis, namely Staphylococcus aureus, Streptococcus agalacitiae and Corynebacterium, are potentially dangerous for newborns. Human milk may also become contaminated with other bacteria such as Listeria monocytogenes, Enterobacter cloacae and Klebsiella pneumoniae as a result of improper expressing, collection and storage [4]. Antibodies against numerous pathogenic bacteria, fungi and viruses are present in human milk and passed to the breastfed infant to prevent mother-to-child disease transmission.
Due to this fact, even though the milk of an infected mother does contain pathogens, bioactive components of the natural food effectively prevent the entry of bacteria and viruses into the digestive tract of a child. Free fatty acids and monoglycerides (products of triglycerides lipolysis) found in human milk exhibit anti-fungal activity [22]. The bacteriostatic properties of human milk depend on its specific antibodies but also result from toxin neutralization by cellular components of the immune system, as well as bacterial translocation blocking by intestinal mucosa. Therefore, the binding of Streptococcus pneumoniae and Escherichia coli to their proper receptors is inhibited by human milk oligosaccharides (HMO) and in the case of Campylobacter jejuni, by fucosyl oligosaccharides. Kappa–casein is a specific ligand for Helicobacter pylori. Neutralization of Escherichia coli, Clostridium difficile, Salmonella enterica and Shigella enterotoxins is the reason why breastfeeding successfully protects against bacterial diarrhea [23]. Also, the binding of rotaviruses by stable sedimentation of 46 kDa glycoprotein (lactadherin), resulting in the alleviation of infectious symptoms, has been confirmed [24]. Therefore, with proper hygiene during the process of expressing and storage conditions, bacterial infections of infants caused by the milk of their mothers are extremely rare. Donor milk that is intended for a child other than a biological child undergoes more restrictive handling [25]. Human milk may also be a source of cytomegalovirus (CMV), hepatitis B virus (HBV) and human T lymphotropic retrovirus (HTLV I and II), as well as HIV 1, 2 infection. Even though transmission through human milk is rare, the pasteurization parameters of milk intended for children should ensure the deactivation of pathogenic viruses HIV, HTLV, CMV, rubella virus and herpes virus (HCV) (hepatitis virus is relatively heat-resistant) [26].
3.1. Methods of Human Milk Processing
Low temperature long time (LTLT) pasteurization, also known as the holder method (HoP), is considered to be the standard for human milk pasteurization. Milk is incubated for 30 min at 62.5 °C in a water bath or other device that ensures effective heating [27,28]. Modern milk pasteurizers guarantee precise measurement of the temperature inside the bottle, automatic control of the process and the possibility of efficient and safe milk cooling to low temperature (usually 4 °C) [29]. Other methods of high temperature pasteurization at 72–75 °C, the so-called HTST (high temperature short time (HTST) or FHP flash-heat pasteurization (FHP), are used to effectively eliminate microorganisms [30,31]. Flash pasteurization applies especially in countries with a high risk of HIV infection. It was adopted as a simple, universally accessible method and does not require any sophisticated equipment. Today a simple, portable, low resource piece of equipment for FHP is available on the market. Escuder-Vieco and coworkers [11] recently demonstrated an HTST equipment for the continuous processing of human milk that could be adopted in milk banks. International research has shown that the high temperature short time method was effective in eradicating HIV, Escherichia coli, Staphylococcus aureus and Streptococcus A and B, while protecting the nutritional composition of human milk including important vitamins [32,33,34]. Also, investigations of methods combining thermal processing with homogenization techniques, for example ultrasound, are being carried out at present. Other physical factors such as UV, microwaves, electric pulses, and high pressures as alternatives to the traditional thermal process of inactivating microorganisms in human milk have been tested [35,36,37,38,39,40,41].
One of the most promising non-thermal method is high pressure processing (HPP), which is becoming a more and more popular method of food preservation on an industrial scale. It seems to be more efficient in achieving microbial purity of human milk, while still preserving its maximal therapeutic value [7,42,43].
3.2. Temperature and High Pressure Influence on Microbial Purity of Human Milk
The main goal of human milk pasteurization is to remove pathogens that might possibly be the cause of an infection when donor milk is given to an infant as an alternative to formula feeding (in accordance with WHO, UNICEF and AAP guidelines) [1,44]. The efficiency of pathogen elimination such as Mycobacterium tuberculosis by heating milk to 62.5 °C was first checked in cow’s milk [28]. Later research confirmed the possibility of removing vegetative forms of numerous types of bacteria present in human milk as well as the risk that endosporic forms and toxins will survive, like in the case of Bacillus cereus, still remain [45]. This is why in many European countries, the microbial purity of human milk is controlled twice, before and after pasteurization, eliminating milk with a level of Staphyloccocus aureus above 104 CFU (colony forming units) before pasteurization and/or an overall bacteria count above 105 CFU. In the absence of a unified acceptance criteria for milk before pasteurization with respect to microbial purity, in order for milk to be given to someone other than the biological child of the donor, vegetative forms of bacteria must be eliminated (Table 1).
In the case of Ebola virus, Marburg and Zika viruses, inactivation happens only by using thermal processing (HoP). So far, there is no data on whether HTST or HPP are equally effective as HoP in eliminating these viruses [15,16].
HTST pasteurization (72 °C, 5–16 s) seems to be more effective than the holder method in eliminating bacteria and viruses with lipid envelopes (HIV, HTLV), as well as model viruses for HCV and hepatitis B virus that cannot be otherwise deactivated [20,46]. Even high temperatures do not completely eliminate viruses without lipid envelopes like Parvovirus B19, but apart from experimental conditions, no risk of infection transferred through human milk has been reported [31]. HTST was reported to efficiently destroy CMV infectivity in a process at 72 °C for 5 s [17]. Microwave radiation at high-power settings has proved to inactivate CMV in human milk as well [19] (Table 1).
Recently, Escuder-Vieco reported that HTST processing at 72 °C for at least 10 s efficiently destroyed all vegetative forms of microorganisms present in raw milk [11]. However, sporulated Bacillus sp. survived this thermal process. In earlier studies Klotz and coworkers did not find a difference in the reduction of naturally present microorganisms in raw milk after HTST in comparison to HoP treatment [10]. High pressure processing (HPP) is becoming an increasingly popular method for food preservation on an industrial scale. The selective effects of high pressures allows the conditions (pressure, temperature, presence of water) to be chosen, in order to selectively destroy pathogenic cells and preserve more of the valuable human milk components. HPP is a successful method of eliminating Gram-positive and Gram-negative bacteria, though vegetative cells are more effectively destroyed than endosporic forms. The destruction of Listeria monocytogenes, Eschericha coli, Staphylococcus aureus, Staphylococcus agalactiae and Salmonella spp. within the pressure range of 300–400 MPa has been proved safe for a lot of milk proteins with active hormones and enzymes. Viruses such as HIV and CMV are also deactivated [7,31,43]. HPP was shown to be even more effective in comparison to holder pasteurization in the elimination of inoculated microbiological flora (total viable count microorganisms, S. aureus) of raw milk [12]. A recent study showed that high pressure processing can be an effective method of eliminating bacteria which produce spores, like Bacillus cereus [13]. Foods pasteurized by means of the HPP method are freer of microbes than products of thermal processing. Therefore, HPP may possibly be used in the future in milk banks, providing nutrition for newborns with special dietary needs.
4. Conclusions
Mother’s own fresh expressed milk is an extremely valuable source of components with both nutrient and bioactive activities, such as bacteriostatic factors, oligosaccharides, vitamins and growth factors. The combined effect of the nutritional and bioactive components decides on the short-term and long-term health benefits of a human milk-based diet in the preterm newborn population. Some of the latest research has shown a partial activity of human milk after holder pasteurization for important biological implications such as anti-infective properties, immune components, microbiota and growth factors [79,81,82].
Improvement of the techniques allowing the preservation of the bioactivity of donor milk is profitable in the context of the health effects related to child feeding choice. Proven and safe techniques adopted from the food industry have created opportunities to minimize losses as a consequence of human milk processing. The new methods applied to human milk should be at least as effective as the old ones to ensure microbiological safety.
Data evaluating the effectiveness of interventions with thermal pasteurized donor milk are not clear [83,84,85,86,87], and there are no clinical trials concerning new techniques. Therefore, studies with human participants are needed to be carried out with the most promising new techniques of human milk processing in comparison to holder pasteurization.
It is essential to bear in mind if there is a real possibility of introducing new equipment into the market and its usage in routine milk bank services [88].
