1. Introduction
Sour taste is one of the five basic tastes, namely sweetness,
sourness, saltiness, bitterness, and umami, and is often perceived
on the sides of the tongue, towards the rear of the mouth
(Bishop, 1971). Sour taste is one of the independently identifiable
sensory attributes of beer (Clapperton, 1973; Clapperton,
Dalgliesh, & Meilgaard, 1976; Langstaff & Lewis, 1993). Sour taste
has an important effect on beer flavour. Generally speaking, a moderate
sour taste is necessary for beer, as it can reduce the cloying
feeling of the beer. Nevertheless, too strong a sour taste will
destroy the harmony of the beer flavor, and is considered to be a
kind of flavour deficiency except for sour beer. High levels of sour
and acidic flavours used to be thought to be due to a sanitation
problem (Kulka & Walker, 1946). Gram positive bacteria, such as
Lactobacillus and Pediococcus, which are present in dust and saliva,
can infect beer by generating haze or rope, causing unpleasant
flavour changes, such as sourness (Vaughan, O’Sullivan, & Van
Sinderen, 2005) (Menz et al., 2010). Pectinatus and Megasphaera
often spoil beer in the later stages of processing, causing high turbidity
in beer and formation of by-products that cause off-flavours
and sour tastes, making the beer unsuitable for consumption
(Paradh, Mitchell, & Hill, 2011). Beer without spoilage is also perceived
to have a prominent sour taste, especially adjunct beers in
which worts have weak buffering capacity. A consumer is unlikely
to repurchase a sour product. Thus, it is highly important to control
the sour taste of beer.