<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>061400 - Disclosure - MAJOR CUSTOMERS AND CREDIT CONCENTRATION</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

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</LabelSeparator><Level>1</Level><ElementName>us-gaap_RisksAndUncertaintiesAbstract</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>MAJOR CUSTOMERS AND CREDIT CONCENTRATION [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>ztho_MajorCustomersAndCreditConcentrationTextBlock</ElementName><ElementPrefix>ztho_</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>verboseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="c20130101to20130630" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="font-family: 'Times New Roman', Times, serif; font-size: 10pt;"&gt;&lt;div style="font-size: 10pt; font-family: Times New Roman; font-weight: bold; text-align: left; margin-left: 0pt; text-decoration: underline; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;NOTE 14 &amp;#8211; MAJOR CUSTOMERS AND CREDIT CONCENTRATION&lt;/div&gt;&lt;div style="text-align: justify; margin-left: 0pt; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;&amp;#160;&lt;/div&gt;&lt;div style="font-size: 10pt; font-family: Times New Roman; text-align: justify; margin-left: 0pt; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;The Company&amp;#8217;s customers are food manufacturers.&amp;#160;&amp;#160;There were two significant customers that each accounted for greater than 10% of sales for the six months ended June 30, 2013. These two customers accounted for 39% and 26% of total net sales, respectively.&amp;#160;&amp;#160;&amp;#160;In addition, the same two customers each accounted for greater than 10% of accounts receivable for the quarter ended June 30, 2013. These customers accounted for 67% and 13% of the accounts receivable as of June 30, 2013, respectively.&lt;/div&gt;&lt;div style="text-align: justify; margin-left: 0pt; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;&amp;#160;&lt;/div&gt;&lt;div style="font-size: 10pt; font-family: Times New Roman; text-align: justify; margin-left: 0pt; display: block; margin-right: 0pt; text-indent: 0pt;"&gt;The Company maintains cash deposits with major banks, which from time to time may exceed federally insured limits.&amp;#160;&amp;#160;At June 30, 2013 and December 31, 2012, $229,180 and $394,804, respectively, were in excess of federally insured limits.&amp;#160;&amp;#160;The Company periodically assesses the financial condition of the institutions and believes that the risk of any loss is minimal.&lt;/div&gt;&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>The entire disclosure of the extent of the entity's reliance on its major customers, if revenues from transactions with a single external customer amount to 10 percent or more of entity revenues and concentrations existing at the date of the financial statements that make an entity vulnerable to a reasonably possible, near-term, severe impact.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>MAJOR CUSTOMERS AND CREDIT CONCENTRATION</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>MAJOR CUSTOMERS AND CREDIT CONCENTRATION</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://ztrim.com/role/MajorCustomersAndCreditConcentration</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
