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Abstract

Tea consumption is practiced as a tradition and has shown potential to improve human health. Maximal uptake of tea antioxidants and milk proteins without a negative impact on tea flavor is highly desired by consumers. There is conflicting evidence for the effect of milk addition to tea on antioxidant activity. Differences in the type of tea, the composition, type and amount of milk, preparation method of tea-milk infusions, the assays used to measure antioxidant activity, and sampling size likely account for the different findings. Interactions between tea polyphenols and milk proteins, especially between catechins and caseins, could account for a decrease in antioxidant activity, although other mechanisms are also possible given the similar effect between soy and bovine milk. The role of milk fat globules and the milk fat globule membrane surface is also important when considering interactions and loss of polyphenolic antioxidant activity, which has not been addressed in the literature.
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Critical Reviews in Food Science and Nutrition
ISSN: 1040-8398 (Print) 1549-7852 (Online) Journal homepage: http://www.tandfonline.com/loi/bfsn20
Addition of Milk to Tea Infusions: Helpful or
Harmful?; Evidence from In vitro and In vivo
Studies on Antioxidant Properties
Ali Rashidinejad, E. John Birch, Dongxiao Sun-Waterhouse & David W. Everett
To cite this article: Ali Rashidinejad, E. John Birch, Dongxiao Sun-Waterhouse & David W.
Everett (2015): Addition of Milk to Tea Infusions: Helpful or Harmful?; Evidence from In vitro
and In vivo Studies on Antioxidant Properties, Critical Reviews in Food Science and Nutrition,
DOI: 10.1080/10408398.2015.1099515
To link to this article: http://dx.doi.org/10.1080/10408398.2015.1099515
Accepted author version posted online: 30
Oct 2015.
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Addition of Milk to Tea Infusions: Helpful or Harmful? Evidence From In Vitro and In
Vivo Studies on Antioxidant Properties
ALI RASHIDINEJAD1,2, E. JOHN BIRCH1, DONGXIAO SUN-WATERHOUSE3 and
DAVID W. EVERETT1,2,*
1 Department of Food Science, University of Otago, PO Box 56, Dunedin 9054, New Zealand
2 Riddet Institute, Private Bag 11 222, Palmerston North 4442, New Zealand
3 School of Chemical Sciences, The University of Auckland, Private Bag 92019, Auckland, New
Zealand
Corresponding author: Ali Rashidinejad
E-mail: ali.rashidinejad@otago.ac.nz
Tel. +64 3 479 7545
Fax +64 3 479 7567
* Present address: Department of Animal Science, California Polytechnic State University, San
Luis Obispo, CA 93407-0255 United States.
HIGHLIGHTS
Milk addition to tea can decrease or completely inhibit tea antioxidant properties
Milk caseins interact with polyphenolic catechins from tea
Skim milk has a more negative effect on tea health benefits than whole milk
Proteins from soy and milk similarly affect the bioavailability of tea antioxidants
ABSTRACT
Tea consumption is practiced as a tradition and has shown potential to improve human
health. Maximal uptake of tea antioxidants and milk proteins without a negative impact on tea
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2
flavor is highly desired by consumers. There is conflicting evidence for the effect of milk
addition to tea on antioxidant activity. Differences in the type of tea, the composition, type and
amount of milk, preparation method of tea-milk infusions, the assays used to measure
antioxidant activity, and sampling size likely account for the different findings. Interactions
between tea polyphenols and milk proteins, especially between catechins and caseins, could
account for a decrease in antioxidant activity, although other mechanisms are also possible given
the similar effect between soy and bovine milk. The role of milk fat globules and the milk fat
globule membrane surface is also important when considering interactions and loss of
polyphenolic antioxidant activity, which has not been addressed in the literature.
Keywords Polyphenols, milk proteins, antioxidant activity, bioavailability, milk fat globules,
chemical interactions.
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INTRODUCTION
Tea is the second most popularly consumed beverage worldwide after water. The top ten tea-
producing countries are China, India, Kenya, Sri Lanka, Turkey, Vietnam, Iran, Indonesia,
Argentina, and Japan (FAO and FOODS 2010). Different varieties of tea, including black, green,
oolong, white, and yellow tea are all derived from Camellia sinensis, but climatic differences and
processing methods account for differences in the composition and degree of antioxidative
behavior (Hayat et al. 2015). For example, black and green tea, the two most common tea
varieties, mainly differ in the presence or absence of a fermentation step, respectively.
Polyphenol oxidase in tea leaves can aerobically oxidize tea phenolics after the enzyme comes
into contact with the substrate upon the disruption of the intact leaf cell structure (Graham 1992).
Black tea is fully fermented, thus, possesses stronger flavor compared to less oxidized teas.
Green tea accounts for only 20% out of 2.5 million metric tons (on a dried basis) of
manufactured tea leaves, with less than 2% for oolong tea. The chemical composition of tea
determines the various putative health benefits such as anticarcinogenic effects (Butt and Sultan
2009; Butt et al. 2015; Dufresne and Farnworth 2000; Gupta et al. 2002; Sun et al. 2006),
antioxidative potential (Benzie and Szeto 1999), and mitigation of cardiovascular diseases (Sano
et al. 2004; Kuriyama et al. 2006; Wang et al. 2011), strokes (Keli et al. 1996), atherosclerosis
(Vinson et al. 2004; Tijburg et al. 1997), and other heart diseases (Hayat et al. 2015; Vinson
2000; Hertog et al. 1997; Arts et al. 2001).
The major constituents responsible for these health benefits are polyphenols, that account for
30% of the dry leaf weight and constitute the largest group of chemical components in green tea,
followed by carbohydrates and proteins. The polyphenols consist of catechins, theaflavins,
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tannins, and flavonoids (Leenen et al. 2000). Catechins, as the predominant polyphenolic
substances in tea, can be partially converted to other components, such as theaflavins and
thearubigins by oxidation in black tea (Hollman et al. 1997). The major catechins in green tea are
epigallocatechin-3-gallate (EGCG), epicatechin (EC), epicatechin gallate (ECG),
epigallocatechin (EGC), gallocatechin (GC), and catechin (C), with EGCG being the most
abundant and potent compound (Huang et al. 2011; Goto et al. 1996). These antioxidants are
well-known effective electron donors and scavengers of physiologically relevant reactive oxygen
species in vitro, including superoxide anions, peroxyl radicals, and singlet oxygen, and have
been shown to account for some of the health benefits of tea consumption, especially antioxidant
activity of human plasma (Yashin et al. 2011; Michalak 2006). Tea is a good source of methyl
xanthines, which are primarily found in the form of caffeine, but the caffeine content of tea is
only around one-third of coffee (known as the best source of caffeine amongst all foodstuffs).
Small quantities of methyl xanthines, theobromine, and theophylline are also present in tea.
Theanine, a specific and unusual non-protein amino acid derived from L-glutamic acid
(glutamate), is thought to be largely responsible for the unique flavor of tea. This amino acid
constitutes approximately one-half of the total amino acid composition of tea. Some other
products, such as carotenoids, important as precursors in black tea, are also well-known in tea
although they are present at low levels. β-Carotene, violaxanthin, lutein, and neoxanthin have
been also identified (Engelhardt 2010).
Milk is recognized as a natural and nutrient-rich beverage that provides good nutrition for
new-borne mammals, including humans. Although many kinds of milk are consumed by
humans, including milk from sheep, goat, buffalo, camel, and horse, 85% of all milk produced
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by the dairy industry is from bovine cows (Gerosa and Skoet 2012). The standard bovine milk in
the market is a opaque white liquid containing water (87%), proteins (3.0 - 3.5%, with ~80%
associated into casein micelles), fat (3.4 - 4.0%), lactose (4.8- 4.9%), minerals (mainly calcium
phosphate, 0.2%), and vitamins (~0.03%) (Walstra et al. 2010; Fennema 1996). The casein
micelles contain most of the calcium phosphate mineral. Fat is found in an emulsified form
coated by the native milk fat globule membrane.
Although tea is consumed without any additional ingredients in many Asian countries,
addition of milk to tea is a ubiquitous practice. Many studies have been carried out on the effects
of milk components, especially proteins, on the antioxidant properties of tea catechins. These
include: 1) a totally inhibitory (negative) or masking effect (Arts et al. 2002; Dubeau et al. 2010;
Ryan and Petit 2010; Sharma et al. 2008; Ryan and Sutherland 2011), 2) a neutral effect (non-
masking, with neither inhibition nor enhancement) (Kyle et al. 2007; Leenen et al. 2000), 3) a
dual effect (both positive and negative for different attributes) (Dubeau et al. 2010), and 4) a
positive effect (enhancement) of milk on catechin absorption of tea (Xie et al. 2013). The effect
of milk addition on tea polyphenolic activity depends upon the ratio of milk to tea, milk
composition, tea type, temperature of brewing, and infusion method. There are no in vivo or in
vitro reports published on the effect of milk addition on the activity, availability, and
accessibility of tea caffeine. This review focuses on the effect of adding different types of milk to
tea, and the resultant phenolic content, antioxidant activity, bioaccessibility, and bioavailability
of tea polyphenols.
TEA PRODUCTION AND COMPOSITION
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Understanding the manufacture methods and leaf composition of various types of tea, and
most importantly, the fundamental chemical changes that occur during the production of
commercial tea products, is essential to interpret the outcome of adding milk to tea. Two major
species of tea, Camellia sinensis var. sinensis and Camellia sinensis var. assamica, are steeped
for consumption. The sinensis variety has small leaves of 5-12 cm whereas the leaves of the
assamica variety can be as long as 20 cm (Graham 1992). The exact classification of tea is
mostly based upon the degree of fermentation, i.e. white, green, oolong, black, pu-erh, and
flavored teas (Figure 1). The properties, appearance, flavor, and the methods of preparing
infusions of the main tea categories are presented in Table 1. Tea blends, through the combined
use of these main categories of tea, also exist on the market. It is worth noting that many herbal
infusions are also called tea although they are entirely unrelated to the tea plant (Camellia
sinensis). The composition of the two main types of tea (black and green), which vary with
fermentation processing, climatic conditions, handling practices, season, leaf age, and tea
variety, is shown in Table 2.
The development of more effective extraction, separation, and characterization methods
should enable the appreciation of the full spectrum of bioactive phytochemicals, including
catechin digallates, methylated catechins, and chalcan-flavans in fresh tea leaves. Tea contains
flavor compounds that are also found in food spices (Dufresne and Farnworth 2001).
Approximately 60 volatile components, including esters, acids, carbonyls, alcohols, and cyclic
compounds, have also been identified in fresh tea leaves that contribute to the specific flavor of
tea (Graham 1992). Dimeric proanthocyanidins, phenolic compounds found in fresh tea leaves,
contribute to tea flavor and astringency, and are derived products from tea catechins with a pyran
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ring and an A ring linked via C-C bonds (Quideau et al. 2011). Some important phenolic acids,
such as gallic acid, are also present in a free form in tea leaves where they enter into oxidation
reactions during fermentation. Quinic acid and caffeoylquinic acid are also found in the fresh
leaf (Engelhardt 2010). The protein fraction of tea mainly includes the enzymes that are normally
associated with the metabolism of plant cells. These enzymes are also responsible for catechin
synthesis. Polyphenol oxidase facilitates aerobic oxidation of tea catechins, especially in black
tea. The presence of other enzymes, such as glycosidase (which catalyzes the hydrolysis of
several aroma precursors), lipoxidase (which generates volatile aldehydes), and the enzymes
responsible for synthesis of methyl xanthine, have also been reported (Komes et al. 2010).
MILK COMPOSITION
The composition of bovine milk is required to understand the mechanisms of interaction
between proteins and tea polyphenols. Bovine milk contains casein proteins s1-, αs2-, β-, and κ-
caseins) and soluble serum proteins (~18%, collectively called whey proteins, including α-
lactalbumin, β-lactoglobulin, serum albumin, immunoglobulins, and enzymes). Caseins form
spherical aggregates (sized ~0.1 µm), commonly called casein micelles, consisting of protein
molecules bound together via nanoparticles of calcium phosphate (Horne 2006; Dalgleish and
Corredig 2012). The outermost layer of the casein aggregate consists of negatively charged κ-
casein that maintains colloidal stability under physiological conditions (Goff 2010).
Milk fat naturally occurs in the form of milk fat globules with a triacylglycerol content of up
to 96--98%, surrounded by a milk fat globule membrane (MFGM) containing phospholipids and
proteins. Milk fat composition and the size of fat globules varies according to breed and species
of cows, and factors related to season, lactation stage, feed composition, feeding time, and
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nutritional status (Butler et al. 2008). Milk fat contains fat-soluble carotenoids and vitamins such
as vitamins A, D, E, and K, along with more than 400 short-, medium and long-chain saturated
and unsaturated fatty acids, including linoleic and linolenic acids and some medium-chain fatty
acids with reported health benefits. Homogenization can reduce the diameter of milk fat globules
from 2-4 to ~ 0.4 µm (MacGibbon and Taylor 2006).
Some of the compounds in milk may also be counted as sources of extractable phenolic
compounds and endogenous antioxidants. These include vitamin A, hydroxycinnamic acids, and
flavonoids (Rashidinejad et al., 2013; Hilario et al., 2010) which are mostly transferred from the
diet (O'Connell & Fox, 2001).
THE ADDITION OF MILK TO TEA INFUSIONS
It is debatable if complexation between tea catechins and milk proteins can decrease or
increase the antioxidant capacity of these bioactive compounds, and subsequently decrease the
nutritional value of milk proteins. The nutritional impact of the addition of milk to tea is a
controversial debate that has been ongoing for many years. Some of the contradictory findings of
milk addition to tea are reviewed in the following sections.
The method of making the perfect cup of tea (considering water temperature and the type of
tea), as well as the suitability and method of adding milk to tea, has been debated for many years
and contradictory findings have been reported. Although milk is usually only added to black tea,
milk can also be added to green, oolong, or even herbal infusions. It is generally accepted that
adding milk to tea is either a dietary habit or an approach to reduce the astringency of plain tea
and/or decrease the temperature of hot tea for immediate consumption (Hertog et al. 1997). It is
thought that people more likely drink tea due to their interest in the benefits of tea rather than
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that of milk. Moreover, adding milk to tea before pouring tea in times past could temper the tea
and prevent porcelain from breaking (Dubrin 2010). A major barrier to adding milk to tea is a
concern about the potential reduction of the health benefits of tea (Graham 1992; Gupta et al.
2002; Katiyar and Mukhtar 1996; Liu et al. 2005; Kuriyama et al. 2006; Nath et al. 2012; Sun et
al. 2006; Payton 2012; Song et al. 2005; Xie et al. 2013).
THE EFFECT OF MILK ADDITION ON THE HEALTH BENEFICIAL PROPERTIES OF
TEA
Results from previous studies on the controversial topic of adding milk to tea is summarized
in Table 3. As stated before, the effects of adding milk to tea can be classified into negative,
neutral, dual, and positive effect categories.
Negative (masking) effects of milk addition to tea: possible interactions between tea and milk
Reports on the negative effects of adding milk to either green tea or black tea outnumber reports
on the neutral and positive effects. Interestingly, Hertog et al. (1997) reported that consumption
of flavonols after adding milk to tea was not inversely associated with an increased ischemic
heart disease (IHD) risk for Welsh men aged 45-49, due to suppression of plasma antioxidant-
raising capacity. In contrast, some epidemiologic studies report that consumption of polyphenols
(flavonols, such as catechins) derived from tea are inversely associated with the occurrence of
coronary problems in elderly Dutch men (Hertog et al. 1993) and stroke incidences in middle-
aged Dutch men (Keli et al. 1996).
A high proportion of publications report that negative effects of milk addition to tea could be
related to interactions between tea polyphenols (catechins) and milk proteins (Sabouri et al.,
2015; Korir et al. 2014; Moser et al. 2014; Livney 2010; Xiao et al. 2011; Kanakis et al. 2011;
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Hemar et al. 2011; Arts et al. 2002; Bourassa et al. 2010; Langley-Evans 2000; Tewari et al.
2000; Serafini et al. 1996). Recently, Moser et al. (2014), Haratifar and Corredig (2013) and Ye
et al. (2013) demonstrated that not only casein micelles, but also whey proteins in milk can bind
to tea catechins. These authors also pointed out that casein micelles exhibit greater affinity to
highly polymerized tea polyphenols, whereas whey proteins showed a greater degree of binding
to smaller polyphenolic molecules. The high affinity of casein micelles to phenolic compounds is
well-recognized in many studies, brought about by the high level of proline amino acids in
casein, resulting in binding to polyphenols via hydrogen bonds between the peptide carbonyl and
the phenolic hydroxyl along with other interactions between phenolic rings and hydrophobic
amino acid residues (Hofmann et al. 2006; Pascal et al. 2008). The cyclic structure of proline,
despite decreasing the formation of hydrogen bonds inside the peptide backbone, can facilitate a
more open and flexible protein conformation enabling interactions between large-sized and
proline-rich proteins with polyphenols (Williamson 1994). In comparison, proteins that are
tightly coiled and globular-shaped likely have much lower affinities for polyphenols. Proline is
also unique in its capability to form cis peptide bonds that are very rigid when forming
complexes (Luck et al. 1994). A good example of this phenomenon is the salivary proteins that
are rich in proline. Phenolic compounds, such as those from tea, can also crosslink with proteins
in food (Tantoush et al., 2012). Jöbstl et al. (2006) reported that β-casein molecules from bovine
milk can wrap around green tea catechins, such as EGCG, leading to a stiffer and more tightly
packed structure. These researchers (Jöbstl et al. 2006) concluded that this kind of binding can
reduce the bioavailability of tea catechins while stabilizing the structure of caseins in milk.
Generally, non-covalent binding between phenolic compounds and proteins correlates with the
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physicochemical parameters of proteins, such as sequence of the amino acids, the isoelectric
point, and the secondary structure (Nagy et al., 2012). For example, some polyphenols, such as
phloretin and EGCG, have a stronger affinity to proteins containing a high number of positive
and negative charges, whereas others, such as procyanidin, can bind strongly to proteins
containing a relatively higher number of proline residues (Nagy et al., 2012).
When a food containing a high polyphenol content is consumed, astringency is mostly perceived
because of the binding of salivary proteins to polyphenols (Hofmann et al. 2006). Milk caseins,
as do salivary proteins, contain a high content of proline residues in the amino acid sequences
thus milk caseins have a relatively open structure. For instance, β-casein (the second most
abundant milk protein) contains 35 prolines out of 209 amino acid residues. In β-casein, proline
residues are evenly distributed throughout the peptide sequences with phosphorylated serine
amino acids at five locations close to the N-terminus (by which β-casein peptide exerts an
amphiphilic property to form casein micelles) (Eskin and Shahidi 2012). Thus, it is
understandable how milk addition could induce polyphenol--protein interactions similar to those
described herein. Although bovine milk contains a considerable amount of protein, it also
contains a similar amount of fat in the form of globules (MacGibbon and Taylor 2006) stabilized
by the milk fat globule membrane (Keenan and Mather 2006). Therefore, when adding high-fat
milk to tea infusions, interactions between tea polyphenols and milk fat globules should be
considered.
Differences among the tea phenolic compounds, such as chemical structure and molecular
size may impact upon the affinity for casein micelles (Ye et al. 2013). Ryan and Petit (2010)
compared the antioxidant capacity of five brands of tea with and without the addition of different
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amounts of whole, semi-skimmed, and skimmed milk. These authors concluded that the three
types of milk significantly (P≤0.05) decreased the total antioxidant capacity of all brands of tea
measured by the ferric reducing antioxidant power (FRAP) assay, although the effect of whole
milk was less than for semi-skimmed or skimmed milk addition. Whole milk, compared with
skimmed milk, contains several fat-soluble antioxidants, including carotenoids, tocopherols and
retinols, that can protect milk against lipid peroxidation and peroxyl/superoxide radical
generation, thereby maintaining milk quality (Lindmark-Månsson and Åkesson 2000). Thus, a
decreased fat content of milk lowers the amount of fat soluble antioxidants in milk and
consequently lowers the total antioxidant property of the milk (Ryan and Petit 2010).
Some studies report no negative effect on the plasma antioxidant activity of black tea after
adding whole milk (Reddy et al. 2005; Hollman et al. 2001; Leenen et al. 2000), whereas others
report a negative effect (Hertog et al. 1997; Tewari et al. 2000; Kartsova and Alekseeva 2008).
In comparison, most of the studies on the effect of low-fat milk on tea show a decrease (although
to a different extent) in both the antioxidant activity of tea-infused milk and the bioavailability of
tea antioxidants in vitro or in vivo (Moser et al. 2014; Egert et al. 2013; Xie et al. 2013; Dubeau
et al. 2010; Ryan and Sutherland 2011; Ryan and Petit 2010; Lorenz et al. 2009; Lorenz et al.
2007). It should be noted that all types of milk contain enzymatic antioxidants, such as
superoxide dismutase, catalase, glutathione, and peroxidase, along with non-enzymatic
antioxidants such as lactoferrin and vitamin C (Lindmark-Månsson and Åkesson 2000).
Milk caseins, especially β-casein, can actively interact with most tea catechins, particularly
EGCG, forming an association (complex) with a very compact core (Haratifar and Corredig
2013; Guinee et al. 2004; Jöbstl et al. 2006). Precipitation of both β-casein and bovine serum
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albumin is possible at different concentrations of EGCG (Jöbstl et al. 2006; Pascal et al. 2008).
Heratifar and Corredig (2013) reported that EGCG in tea forms complexes with caseins,
especially -casein, and decreasing EGCG concentration causes rennet gel formation time to
increase. Based on these observations, it was proposed that EGCG associates with the four
proline residues near the hydrolysis point of -casein (in the 98--111 sequence of His-Pro-His-
Pro-His-Leu-Ser-Phe-Met-Ala-Ile-Pro-Pro-Lys). Finding an alternative to bovine milk to
minimize the impact of milk on the health benefits of tea has generated further investigations.
Soy-based emulsion beverages were proposed as a substitute since soy does not contain any
casein (Ryan and Sutherland 2011). All the five brands of soy beverage used in this study had
much higher antioxidant activities than semi-skimmed milk in the absence of tea. Three studies
have recently showed that soy beverage can suppress vascular effects (Lorenz et al. 2009),
bioavailability (Egert et al. 2013), and total antioxidant activity of tea (Ryan & Sutherland,
2011).
Many in vivo studies have been carried out with different results (Egert et al. 2013; Henning
et al. 2004; van het Hof et al. 1998; Roura et al. 2007). Egert et al. (2013) reported on the
inhibitory effects of skimmed bovine milk, caseinate, and soy beverage on the bioavailability of
galloylated catechins from green tea based on the plasma concentrations of tea catechins in 24
non-smoking women (aged 23-32 years old) with normal body weight. A negative effect of soy
beverage on the antioxidant activity of tea polyphenols in vivo (Lorenz et al. 2009) as well as in
vitro (Ryan & Sutherland, 2011) have been reported. Lorenz et al. (2009) found that 10% plain
soy beverage can suppress the vascular antioxidant property of tea, possibly through interactions
between proteins, such as β-conglycinin and glycinin, with tea antioxidants. Thus, soy proteins
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might exert effects similar to bovine milk caseins on the bioavailability and beneficial effects of
tea antioxidants (Rawel et al. 2002; Egert et al. 2013; Ryan and Sutherland 2011). As Tewari et
al. (2000) explained, the negative effect of milk on antioxidant activity of tea phenolic
compounds is associated with the interference of increasing gastric pH (caused by milk) with the
absorption of simple phenolics. Gastric acids are normally weak acids and easily absorbed in
their non-ionized form, but are increasingly ionized after milk addition, causing a reduced
passage rate through the gastric mucosa.
Neutral (non-masking) effects of milk addition to tea
Reports have shown weak to non-existent effects of milk addition to tea (Hollman et al.
2001; Leenen et al. 2000; van het Hof et al. 1998; Kyle et al. 2007; Reddy et al. 2005), but less in
number than reports of negative effects. As mentioned before, milk is an inherently good source
of antioxidants, thus addition of milk should increase the total antioxidant potential of a tea-milk
beverage in the absence of any synergistic of confounding effects. A non-masking effect occurs
when the inhibitory effects of added milk on the antioxidant potential in tea infusions are very
weak and insignificant. For example, the addition of milk only weakly affected (P<0.07, too
weak to be measured as a negative effect) kaempferol activity (one of the tea flavonols) after tea
intake (Kyle et al. 2007). Langley-Evans (2000) reported that tea flavonoids appear to be taken
up across the gastrointestinal tract membrane, and van het Hof et al. (1998) showed that
simultaneous consumption of black tea and milk did not impair catechin bioavailability based on
the presence of tea catechins in plasma that decrease the concentration of circulating low density
lipoproteins.
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It is worth noting that the absorption of other flavonoids, such as theaflavins and thearubigins
in tea, has not yet been completely studied and more investigations are needed to confirm if there
is a neutral effect of milk addition on flavonoid activity. Variations in the methods used for
measuring plasma antioxidant potential of polyphenols may lead to inconsistent results. For
example, Maxwell and Thorpe (1996) reported no effect of tea consumption on plasma
antioxidant status in the absence of added milk or derived milk products, even though catechins
from tea were found to be absorbed rapidly into blood after ingestion (Langley-Evans 2000). A
good demonstration of the antioxidant properties of tea flavonoids needs to be clearly shown.
Thus, a lack of consistency in the findings may reflect the variety in methodologies used to
measure plasma antioxidant potential of catechins, and this might be one of the reasons why
researchers such as Maxwell and Thorpe (1996) reported no effect of tea consumption without
any additives on plasma antioxidant status. Similarly, Benzie and Szeto (1999) claimed a small
and transient effect of green tea on the plasma antioxidant activity (by the FRAP assay) after
ingestion. But this transient effect may be spurious as green tea flavonoids, especially catechins,
have a short half-life (4.8 h) in human plasma (van het Hof et al. 1998).
Dual effects of milk addition to tea
Among all of the studies reviewed, only one study describes the dual effect of milk addition
on the antioxidant capacity of green and black teas (Dubeau et al. 2010). These authors examined
the in vitro antioxidant activity of tea infusions, and infusions containing 5% semi-skimmed
milk, using three complementary methods, ABTS+ (2,2'-azino-bis(3-ethylbenzothiazoline-6-
sulfonic acid)) free radical scavenging, lipid oxidation, and voltammetry assays. Dubeau et al.
(2010) reported that milk decreased the antioxidant activities, as measured by the ABTS+ and
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voltammetry assays, in all of the tea infusions, but increased the chain--breaking antioxidant
property as determined by the lipid peroxidation assay. This study also showed that Darjeeling
tea (usually available in the form of black tea) possessed the largest polyphenolic content and
antioxidant capacity amongst the tested green and black teas, a result in disagreement with the
findings of other studies where green tea has been shown to have a greater antioxidant content
than black tea (Ye et al. 2013; Leenen et al. 2000; van het Hof et al. 1998; Anesini et al. 2008).
The antioxidant property of milk, due to intrinsic antioxidants (Lindmark-Månsson and Åkesson
2000), could have been masked by the negative effect of milk on antioxidant capacity of teas.
Although Dubeau et al. (2010) found that the antioxidant capacity of tea-milk infusions by
voltammetry and ABTS+ assays was significantly lower than for tea alone, these authors
reported that milk addition positively affected (P = 0.004) the inhibition of lipid peroxidation by
tea samples, suggesting a dual effect between tea polyphenols and milk components in the
emulsion, such as linoleic acid. There are limitations in the study of Dubeau et al. (2010) given
that only one milk concentration was examined (5%) and no in vivo bioavailability study was
carried out. A part from that, the data obtained from only one antioxidant activity assay (ABTS+)
may not be enough to draw conclusions about the negative or positive effects of milk on
antioxidant activity of tea polyphenols. Previously, it was reported that oxygen radical
absorbance capacity was the most suitable assay for measuring antioxidant activity of dairy
products due to the suitability of the method in both hydrophilic and lipophilic systems
(Rashidinejad et al. 2013).
Positive effects of milk addition to tea
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Some studies report on the ability of milk proteins to facilitate intestinal transport of catechins
originating from green tea by enhancing the absorption of these catechins. Xie et al. (2013)
observed that the content of catechins increased significantly in digested green tea containing
10% skimmed milk compared to the corresponding undigested tea-milk samples, based on a
Caco-2 cell in vitro model (a human colon epithelial cancer cell line). These authors also found
that adding 10% milk significantly improved the recovery of all catechins in Caco-2 cells after 2
h incubation, whereas milk addition at 25% resulted in a slight decline in the recovery due to
binding of green tea catechins to milk proteins (in agreement with the discussion in Section 5.1).
Xie et al. (2013) pointed out that all types of green tea catechins, with EGCG as the predominant
compound, are still present at substantial concentrations in the digestive fluids, except for some
degradation of catechins during digestion. Enzyme treatment can considerably increase the
recovery of catechins i.e. ~59% of EGCG, possibly due to disruption of the interactions between
milk proteins and tea catechins after hydrolysis of proteins into peptides (Xie et al. 2013). The
positive effect of milk addition to black tea was confirmed by Weisburger et al. (1997) in trials
using rats with either mammary gland cancer or colon cancer. These researchers found that black
tea alone decreased the mammary gland cancer, and black tea with milk exerted a greater
protection against colon and breast tumors. Full-fat milk (4.5% fat) led to an even greater
additive effect on the antioxidant property. In summary, these two studies show the positive
effects of milk addition, although the mechanisms and the quantitative analysis of absolute
values (rather than relative results) are currently not known.
Differences in the type of tea tested, type, composition and concentration of milk
products, method of preparation for tea and milk-tea infusions, the assay used to measure in vivo
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or in vitro antioxidant activity, and sampling size for analysis can account for variations amongst
the findings reported by different authors. Moreover, bioaccessibility as a prerequisite for
bioavailability of any substance, such as polyphenols in the gut, is an important index. The
effectiveness of releasing bioactives from a food matrix, in addition to gastrointestinal stability,
determines bioavailability (Xie et al. 2013). Chemical alteration of catechins by enzymatic
glucuronidation, methylation and sulfation is possible during absorption through intestinal
epithelial cells (Lambert et al. 2007), which also affects the bioavailability assessment.
CONCLUSIONS
Although it is a logical step to translate various and controversial scientific findings into a
public health message, it will be a challenge, at least not straightforward, to do so for the impact
on health from milk addition to tea. Based on the results of previously published studies with
consideration of the variations among individuals, tea type, milk type, and methods of assessing
antioxidant properties, the evidence appears to point toward a negative (masking) effect for
adding milk, especially skimmed milk, to tea beverages because of the putative association
between tea polyphenols (such as catechins) and milk proteins (specifically caseins). This effect
may vary depending upon the concentration of milk proteins and tea phenolics. Despite of the
various traditional habits for drinking tea consumers are increasingly aware of the health benefits
of both tea polyphenols and milk proteins, but there is insufficient evidence to support any
recommendation for the appropriate ratio of milk to tea. Furthermore, it appears that the role of
milk fat has been neglected, despite milk containing fat globules which may interact with tea
catechins as well. Another important aspect is the purpose of consuming tea by different people.
Some may drink tea for pleasure whereas others may do so for the health benefits of either
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caffeine or antioxidants. Although the negative effects of the addition of milk on the activity of
tea antioxidant have been broadly reported, there is lack of information about the effect of milk
addition on the activity or bioavailability of caffeine in tea infusions. Further studies are required
on the effect of milk addition to tea on the bioaccessibility and bioavailability of tea polyphenols
and milk proteins. Even though the habits for drinking tea and the use of milk addition in
different societies, or even between individuals, might be different, tea antioxidants can play an
important role in the prevention of many chronic diseases and disorders in individuals who drink
it regularly.
ACKNOWLEDGEMENTS
The lead author (A.R.) was supported by a doctoral scholarship from the University of Otago,
Dunedin, New Zealand.
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Table 1: The characteristics, appearance, flavor, and methods of infusion preparation for the
main tea categories based on the degree of fermentation and processing.
Tea type
Properties
Appearance
of the tea
plant
Production
method
Appearance
and flavor of
the beverage
Applicable
methods for
preparing the
beverage
White
Almost the
rarest tea
category, and
mainly
produced in
China.
Silvery
white hairs
found on the
undeveloped
parts of the
plant
(whitish
buds).
Naturally sun or
steam dried.
Relatively
pale yellow
with slightly
sweet flavor
and a sort of
mellow nutty
or creamy
quality.
Brewing in hot
water.
Green
Nearly 20% of
all of the tea
produced
globally.
Green or
dark green-
leaves.
Withering,
followed by
steaming or
frying (usually
pan-frying).
Either artisanal
or modern ways
Greenish
yellow with
having an
astringent
grassy taste of
the fresh tea
leaves.
Brewing or
steeping in hot
water.
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of firing might
be used.
Oolong
Semi-fermented
traditional
Chinese tea
leaves.
Red-brown
leaves.
Short period of
fermentation.
Pale yellow
with a
specific
unique floral
and fruity
taste.
Brewing or
steeping in hot
water.
Black
Most common
tea type. People
in China and
neighboring
countries may
call it red tea.”
Black
leaves.
Firstly withered
(by blowing air)
after harvesting
and then fully
fermented for
several hours
under controlled
temperature and
humidity and
finally treated
by heating or
drying
processes.
Reddish
brown color
contains a
wide range of
flavors with a
hearty and
more
assertive
quality than
other types of
tea.
Brewing or
steeping in hot
water.
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Pu-erh
A thin layer of
mould on the
tea leaves- used
for medicinal
purposes
Dark red-
Leaves
Fully
anaerobically
fermented with
microbes
through the
processing
phase. It is
usually
fermented twice
instead of once
(double
oxidation
process along
with a period of
maturation)
Dark red with
a distinctive
soil-like
flavour
recognized as
off-putting by
some people
Brewing in hot
water
Flavoured
Not regarded as
a different type
of tea but refers
to the tea leaves
have been
flavoured with
the addition of
Mainly
black-
Leaves
Mainly fully
fermented
Depends on
the added
flavour
Brewing or
steeping in hot
water
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various flavours
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Table 2: The major constituents of green and black tea leaves; modified from Dufresne and
Farnworth (2001).
Occurrence (% dry weight)
Green tea
Black tea
Structure
Catechins
30-42
10-12
epigallocatechin
gallate
11
B (−)2,3-cis R1 = OH R2 = A
epicatechin gallate
2
B (−)2,3-cis R1 = H R2 = A
gallocatechin gallate
2
B (+)2,3-trans R1 = OH R2 = A
epicatechin
10
B (−)2,3-cis R1 = R2 = H
epigallocatechin
ND*
B (−)2,3-cis R1 = OH R2 = H
gallocatechin
ND
B (−)2,3-trans R1 = OH R2 = H
catechin
ND
B (−)2,3-trans R1 = R2 = H
Teaflavin
3-6
theaflavin-3-gallate
ND
C R1 = OH R2 = OH
theaflavin--gallate
ND
C R1 = A R2 = OH
theaflavin-3,3˴-
ND
C R1 = OH R2 = A
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gallate
Thearubigens
2-3
C R1 = A R2 = A
Theogallin
ND
12-18
proanthocyanidin
ND
Flavonols
5-10
6-8
quercetin
ND
D R1 = OH R2 = H R3 = OH
kaempferol
ND
D R1 = R2 = H R3 = OH
rutin
ND
D R1 = OH R2 = H R3 = O-
rutinose
Methylxanthines
7-9
8-11
caffien
3-5
E R1 = R2 = CH3
theobromine
0.1
E R1 = H R2 = CH3
theophylline
0.02
E R1 = CH3 R2 = H
Amino acids
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theanine
4-6
F
*Not determined
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Table 3: Summary of the previous studies carried out on the effect of milk addition to tea.
Author
Tea
type
tested
Milk type
added
Milk
ratio
(v/v %)
added
The effect reported
Korir et al.
(2014)
Black
and
green
tea
Fresh cows
milk (full-fat)
2
Milk addition had a significant effect on
decreasing the antioxidant activity (DPPH) of
the tea infusions in Swiss albino mice in a
concentration-dependent manner.
Ye et al.
(2013)
Black
and
green
tea
Full-fat milk
(3.3% fat)
40
There are strong interactions between milk
proteins and polyphenols when added to both
black and green tea. These bindings could alter
the secondary structures of proteins in milk.
Egert et al.
(2013)
Green
tea
Skimmed
milk (0.1%
fat)
20
The bioavailability of total catechins in tea was
significantly reduced by addition of skimmed
milk.
Xie et al.
(2013)
Green
tea
Skimmed
milk (fat
content not
reported)
10 and
25
Gallated catechins including ECG and EGCG
were bound to milk proteins more than non-
gallated catechins (EC and EGC), but in
general, milk addition at both concentrations
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increased bioavailability of catechins via
enhancing their transepithelial absorption and
their uptake from green tea.
Ryan and
Sutherland
(2011)
Black
tea
Semi-
skimmed milk
(1.6% fat).
Different
brands of soy
milk (1.2-
2.2% fat
content)
10
The addition of either bovine or soy milk
decreased the total antioxidant activity (FRAP)
while the effect of milk was greater than soy
milk.
Dubeau et
al. (2010)
Green
and
black
tea
Semi-
skimmed milk
(2% fat)
5
Milk decreased the antioxidant activity
(measured by the ABTS+ methoda) of both
types of tea. In contrast, it improved the chain--
breaking antioxidant capacity measured by a
lipid peroxidation method.
Ryan and
Petit
(2010)
Black
tea
Whole milk
(3.5 -3.7%
fat), semi-
skimmed milk
(1.7 -2% fat),
5, 7.5,
10
The addition of all three concentrations of all
three types of milk decreased the total
antioxidant activity (FRAP) of tea but the
masking effect of skimmed milk was greater
than other two.
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and skimmed
milk (<0.5%
fat)
Lorenz et
al. (2009)
Black
tea
Skimmed
milk and soy
milk
10
Both skimmed and soy milk hampered
beneficial tea-induced vascular effects
Kartsova
and
Alekseeva
(2008)
Black
and
green
tea
Milk (fat
content not
reported)
5-10
The milk addition can decrease the
concentration of free polyphenols in tea due to
their binding with milk caseins.
Sharma et
al. (2008)
Black
tea
Milk (fat
content not
reported)
40
Milk reduced the total phenolic content and
radical scavenging activity (DPPH) of tea but
stabilized the antioxidant activity (β-carotene-
linoleic acid model).
Lorenz et
al. (2007)
Black
tea
Skimmed
milk (fat
content not
reported)
10
The vascular protective effects of tea were
completely blunted by the addition of milk.
Kyle et al.
(2007)
Black
tea
Skimmed
milk (fat
content not
33.3
Total phenolic content, antioxidant capacity
(FRAP), and catechin contents of tea were not
affected by milk addition.
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reported)
Reddy et
al. (2005)
Black
tea
Whole milk
(3.5% fat)
20
The plasma antioxidant capacity of black tea
may not be adversely affected by the addition
of milk, although the absorption/bioavailability
of tea catechins might be affected.
Hollman et
al. (2001)
Black
tea
Milk (fat
content not
reported)
10
Addition of milk did not change the plasma
concentration of antioxidants (ORAC) in tea.
Richelle et
al. (2001)
Black
tea
Milk (fat
content not
reported
10
Antioxidant activity of tea infusions (LDL
oxidation assay) was not impacted by milk
addition.
Leenen et
al. (2000)
Black
and
green
tea
Full-fat milk
(fat content
not reported)
20
Addition of milk did not remove the plasma
antioxidant activity (FRAP) of neither teas.
Tewari et
al. (2000)
Black
tea
Milk (fat
content not
reported)
Not
reported
(taking
300 mL
of tea
with
Tea without milk showed better in vivo
antioxidant potentials than tea with milk.
Addition of milk can form a complex of
polyphenols- milk protein which results in
increasing ionization of polyphenols and thus
lowering their absorption through gastric
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a 2,2'-azino-bis
milk)
mucosa.
Langley-
Evans
(2000)
Black
tea
Semi-
skimmed milk
(fat content
not reported)
10
The effect of tea on plasma antioxidant
potential (FRAP) appeared to be negated
totally by the addition of milk.
van het
Hof et al.
(1998)
Black
tea
Semi-
skimmed milk
(fat content
not reported)
16.7
The bioavailability of the tea catechins was not
impaired by milk addition.
Weisburger
et al.
(1997)
Black
tea
Full-fat milk
(4.5% fat)
32
Milk potentiated the inhibiting effects of tea on
mammary and colon tumour induction in rats.
Hertog et
al. (1997)
Black
tea
Milk (fat
content not
reported)
Not
reported
Adding milk removed the in vivo plasma
antioxidant-raising capacity of tea.
Serafini et
al. (1996)
Black
and
green
tea
Whole milk
(fat content
not reported)
25
Simultaneous consumption of tea and milk
resulted in a totally inhibition of in vivo activity
of tea polyphenols by milk.
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Figure 1: Processes for manufacturing different types of tea.
Tea plant (Camellia
sinensis)
Whitish buds and very
young leaves
White
tea
Drying
Tea
leaves
Green tea
Wither
ing
Rolling
Drying
Semi-
ferment
ation
Oolong tea
Drying
Wither
ing
Rolling
Fermen
tation
aeration
Black tea
Drying
Steaming or
frying
Rolling
Microbi
al
ferment
Pu-erh tea
Drying
Wither
ing
Rolling
Fermen
tation
aeration
Flavoured
tea
Drying
Adding
flavours
Steaming or
frying
Rolling
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Abstract Green tea is the most widely consumed beverage besides water and has attained significant attention owing to health benefits against array of maladies e.g. obesity, diabetes mellitus, cardiovascular disorders, and cancer insurgence. The major bioactive molecules are epigallocatechin-3-gallate (EGCG), epicatechin, epicatechin-3-gallate, epigallocatechin, etc. The anti-carcinogenic and anti-mutagenic activities of green tea were highlighted some years ago. Several cohort studies and controlled randomized trials suggested the inverse association of green tea consumption and cancer prevalence. Cell culture and animal studies depicted the mechanisms of green tea to control cancer insurgence i.e. induction of apoptosis to control cell growth arrest, altered expression of cell cycle regulatory proteins, activation of killer caspases, and suppression of nuclear factor kappa-B activation. It acts as carcinoma blocker by modulating the signal transduction pathways involved in cell proliferation, transformation, inflammation, and metastasis. However, results generated from some research interventions conducted in different groups like smokers & non-smokers, etc. contradicted with aforementioned anticancer perspectives. In this review paper, anticancer perspectives of green tea and its components have been described. Recent findings and literature have been surfed and arguments are presented to clarify the ambiguities regarding anticancer perspectives of green tea and its component especially against colon, skin, lung, prostate, and breast cancer. The heading of discussion and future trends is limelight of the manuscript. The compiled manuscript provides new avenues for researchers to be explored in relation to green tea and its bioactive components.
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The effect of (+)‐catechin on total phenolic content (TPC) and antioxidant properties in low‐fat hard cheese were examined over a 90‐day ripening period at 8 °C. Antioxidant activity (AA) in cheese was measured by ferric reducing antioxidant power, oxygen radical absorbance capacity (ORAC) and 2,2‐diphenyl‐1‐picrylhydrazyl assays and compared with TPC. Catechin retention coefficients in cheese curds were in the range of 0.63–0.75 and decreased the pH of cheese without affecting protein, fat or moisture content. Both TPC and AA increased during the 90‐day ripening period. Oxygen radical absorbance capacity was the most suitable technique for evaluating AA in cheese due to the high correlation with TPC and suitability in both lipophilic and hydrophilic systems.
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As a consequence of industrial development, the environment is increasingly polluted with heavy metals. Plants possess homeostatic mechanisms that allow them to keep correct concentrations of essential metal ions in cellular compartments and to minimize the damaging effects of an excess of nonessential ones. One of their adverse effects on plants is the generation of harmful active oxygen species, leading to oxidative stress. Besides the well-studied antioxidant systems consisting of low-molecular antioxidants and specific enzymes, recent works have begun to highlight the potential role of flavonoids, phenylopropanoids and phenolic acids as effective antioxidants. During heavy metal stress phenolic compounds can act as metal chelators and on the other hand phenolics can directly scavenge molecular species of active oxygen. Phenolics, especially flavonoids and phenylopropanoids, are oxidized by peroxidase, and act in H2O2-scavenging, phenolic/ASC/POX system. Their antioxidant action resides mainly in their chemical structure. There is some evidence of induction of phenolic metabolism in plants as a response to multiple stresses (including heavy metal stress).
Chapter
This chapter presents the important tea-growing areas, the manufacture of different types of tea (green, white, oolong, and black), and the chemical constituents of tea. Flavonoids and other polyphenols are the most important constituents of tea, the foremost being catechins (flavanols), which are the most abundant flavanoids in fresh tea leaves. The conversion of catechins to theaflavins and thearubigins during the manufacture of oolong and black teas, and the chemistry of other flavonoids, such as flavonol and flavone glycosides, proanthocyanidins, and bisflavanols, are presented. Data on the content of these compounds are given and concepts for the differentiation of different types of tea (e.g., green and black tea) and the detection of geographic origin are discussed. The chemistry of non-flavonoids in tea, such as alkaloids, carotenoids, minerals, amino acids (especially l-theanine), carbohydrates, lipids, and volatiles/aroma compounds, is discussed. The brewing of tea and tea products (decaffeinated and instant teas, tea-based ready-to-drink beverages) is explained. The potential health benefits (e.g., anticancer effects, protection against cardiovascular heart disease) of tea and its constituents, foremost tea flavonoids, are described. This chapter includes a section on the bioavailability and metabolism of tea constituents. State-of-the-art analytical methods for the determination of selected tea constituents are listed.
Article
Background: Epidemiological studies suggested that consumption of fruit and vegetables may protect against stroke. The hypothesis that dietary antioxidant vitamins and flavonoids account for this observation is investigated in a prospective study. Methods: A cohort of 552 men aged 50 to 69 years was examined in 1970 and followed up for 15 years. Mean nutrient and food intake was calculated from crosscheck dietary histories taken in 1960, 1965, and 1970. The association between antioxidants, selected foods, and stroke incidence was assessed by Cox proportional hazards regression analysis. Adjustment was made for confounding by age, systolic blood pressure, serum cholesterol, cigarette smoking, energy intake, and consumption of fish and alcohol. Results: Forty-two cases of first fatal or nonfatal stroke were documented Dietary flavonoids (mainly quercetin) were inversely associated with stroke incidence after adjustment for potential confounders, including antioxidant vitamins. The relative risk (RR) of the highest vs the lowest quartile of flavonoid intake (greater than or equal to 28.6 mg/d vs <18.3 mg/d) was 0.27 (95% confidence interval [CI], 0.11 to 0.70). A lower stroke risk was also observed for the highest quartile of beta-carotene intake (RR, 0.54; 95% CI, 0.22 to 1.33). The intake of vitamin C and vitamin E was not associated with stroke risk. Black tea contributed about 70% to flavonoid intake. The RR for a daily consumption of 4.7 cups or more of tea vs less than 2.6 cups of tea was 0.31 (95% CI, 0.12 to 0.84). Conclusions: The habitual intake of flavonoids and their major source (tea) may protect against stroke.
Article
Tea Catechins associate with proline rich proteins such as caseins. In this work, the interactions between epigallocatechin-gallate (EGCG), one of the main tea catechins, and caseins at an oil water interface were studied. The association of EGCG with sodium caseinate was quantified by measuring the amount of EGCG adsorbed on the emulsion droplets. In addition, the viscoelastic properties of the protein and EGCG–protein layers formed at the interface were studied using drop tensiometry. Different concentrations of EGCG were added to a model emulsion to obtain final concentrations of 10% soybean oil, 0.5% sodium caseinate and 0–9 mg/mL EGCG. At concentrations < 2 mg/mL EGCG, more than 90% was adsorbed at the interface. At higher concentrations, below 5 mg/mL, about 70% of EGCG was adsorbed. The surface load reached about 1 mg/m2 at 9 mg/mL of EGCG. The dissociation constant for the complex formed, was estimated using schatchard plot, and was 3.7 × 10−5 M for less than 20 mol bound EGCG/protein. The interactions of EGCG with sodium caseinate did not affect the interfacial tension but increased the dilational modulus of the EGCG-protein layer at the interface. This study demonstrated that sodium caseinate emulsions could be employed as carriers for EGCG, and that the complexes formed at the interface are affecting the physico-chemical properties of the emulsions. The results are of significance in the development of dairy based beverages containing tea polyphenols.
Article
The effect of milk on the absorption of polyphenols is still controversial so far. In order to determine the impact of milk addition on green tea catechins bioaccessibility and intestinal absorption an in vitro digestion/Caco-2 cell model was applied. Green tea extract (GTE) was solubilized in distilled water at 23 °C and 100 °C, combined with skimmed milk (GTE + 10% milk and GTE + 25% milk) and subjected to simulated gastric and intestinal digestion, followed by transepithelial absorption in Caco-2 cells monolayers. In the mixture with milk, gallated catechins: ECG and EGCG showed binding to milk proteins while EC and EGC seemed to have weaker affinity. Catechins were stable during gastric incubation and very sensitive to intestinal digestion. Bioaccessibility of green tea catechins brewed at 100 °C was higher than brewed at 23 °C. Catechins from digested GTE with 10% and 25% milk exhibited enhanced intestinal permeability in Caco-2 model in comparison to non-digested GTE and digested GTE without milk. Apparent permeability coefficients (Papp) of EGCG and ECG in digested GTE with 25% milk were significantly higher compared to those in GTE with 10% milk, and amounted to 2.41 × 10− 6 cm/s and 1.39 × 10− 6 cm/s. The recoveries of all catechins in GTE with milk in Caco-2 cells after 2 h incubation were significantly higher than that without milk. To summarize, these data suggest that milk addition may increase catechin bioavailability by enhancing their transepithelial absorption and uptake from green tea extract.
Article
Antioxidant activity of different types of tea (green, oolong, black, pu-erh) were measured using different modern methods. Several types of commercially available teas, from various manufacturers were tested for antioxidant content using the amperometric method, the data is displayed here. Data gathered about antioxidant content of these different tea samples can be used to estimate quality and type of tea. The data collected using this method is also important when trying to account for the normal daily consumable antioxidant of healthy people and also patients using clinical antioxidant therapy.
Article
Fluorescence spectroscopy was used to investigate the interaction between resveratrol and whey proteins. The whey proteins examined were lactoferrin, holo‐lactoferrin, apo‐lactoferrin, whey protein isolate (WPI) and the β‐lactoglobulin‐ and α‐lactalbumin‐rich fractions of WPI. Both an analytical‐grade and food‐grade resveratrol were examined. In all the systems studied, it was found that resveratrol interacted with the whey proteins to form a 1:1 complex. The binding constant, K s, for the protein–resveratrol complex for all the proteins examined varied from 1.7 × 104 to 1.2 × 105 m−1. Furthermore, the interaction between the whey proteins and resveratrol did not affect the secondary structure of the proteins.